I'm slowly building a very basic distortion pedal from the ground up, trying to learn some basic electronics as I go. I'm a programmer by trade, so my electronics skill level compared to learning how to bake, is like knowing where the kitchen is.
I have a basic gain stage as shown in the diagram, with a gain of 11 and a power supply of +9V DC. I verified the input signal from my guitar and the output signal from the op amp (TL082) using my oscilloscope. The input from my guitar is about 250mV p2p and the output signal is boosted as expected. I didn't configure enough gain to distort the signal but I will do that later (and add transistors to clip the output) once I'm happy with a basic boost stage.
I plugged the output into my small Peavey practice amp's clean channel and set the volume to around 2. When I played my guitar, I could hear the signal through the Peavey, but the signal broke up when the guitar volume knob was set turned up. I guess it was overloading the amp and it sounded terrible, but not distorted. Perhaps the bass frequencies are too strong. I will learn how to add tone control in a bit.
Do I need a resistor on the output, after the capacitor to lower the amplitude? Is there a max voltage the output should be, say less than 1V? I only have a 10k potentiometer (need to order some) so I could put that at the output to control the volume. Is this the correct way to go or is there something fundamental I'm missing? I'm not even sure I'm asking the right questions here.
I've looked at some schematics for existing pedals and they have a volume pot at the end of the tone stage or output buffer. Any help is appreciated. Thanks
(https://i.postimg.cc/2VqMbYQc/gain-stage.png) (https://postimg.cc/2VqMbYQc)
such a 10k pot is perfect for an output volume control. just hang one outer lug on the output capacitor, the other outer lug on ground and the middle lug (wiper) goes to the output jack.
there are pedals called 'boost pedals' that provide an amp with up to 9v of p2p signal.
most solid state amps react badly to being boosted in such a manner as you are probably overdriving the internal chips, which is obviously different than overdriving tubes (although there are plenty chips which sound good when overdriving and ''sounds good'' is subjective).
Quotebut the signal broke up when the guitar volume knob was set turned up ... and it sounded terrible, but not distorted
a bit of a contradictory statement here, does it sound blown out or like its starving?
welcome to the forum,
cheers
Welcome to the forum!
When this is in context, followed by other stages, requirements may change. Generally:
Yes, some small (maybe 1k?) resistor in series with the output is good practice just to avoid a possible short of the output. That will limit current. To lower voltage you need two resistors or a pot. (look up voltage divider.)
Yes a volume control will give you a voltage divider and allow you to keep your amp from being overwhelmed.
Tone controls are usually treble cut (low pass). The basic idea is bass cut early, before you drive the boost stage into cut-out, and treble cut after the distortion stage to smooth things out.
So first try changing the input cap from 1uf to something like .022uf, like a tube screamer. Try to find a value that thins things out right for you.
10uf output cap is also overkill. Maybe 1uf there.
Best level for your amp depends... Tube amps are generally happy with pretty big signals (some guys like pedals that run on higher voltage, up to 27v like the Klon.) Transistor amps are not famous for loving high voltage boosters.
Hi & Welcome, also..
23k (56k//56k) input impedance isn't "ideal" for such a configuration,, :icon_wink:
Quote from: iainpunk on November 12, 2021, 05:57:35 PM
such a 10k pot is perfect for an output volume control. just hang one outer lug on the output capacitor, the other outer lug on ground and the middle lug (wiper) goes to the output jack.
there are pedals called 'boost pedals' that provide an amp with up to 9v of p2p signal.
most solid state amps react badly to being boosted in such a manner as you are probably overdriving the internal chips, which is obviously different than overdriving tubes (although there are plenty chips which sound good when overdriving and ''sounds good'' is subjective).
Quotebut the signal broke up when the guitar volume knob was set turned up ... and it sounded terrible, but not distorted
a bit of a contradictory statement here, does it sound blown out or like its starving?
welcome to the forum,
cheers
Hmm, I'm not sure of the auditory difference, but perhaps blown out? Mushy? :)
Quote from: antonis on November 12, 2021, 06:09:41 PM
Hi & Welcome, also..
23k (56k//56k) input impedance isn't "ideal" for such a configuration,, :icon_wink:
What do you recommend? I will look at sample circuits again to see what they use. Thanks.
Actually, with your relatively low input impedance (those 56k bias Rs) you may not need as small as 22n. But try it.
The lower the input impedance, the bigger that cap needs to be. See fuzz face (2.2uf), rangemaster (5n and cuts lots of bass) vs high impedance MOSFET boost like SHO (1n and full range).
Quote from: YurkshireLad on November 12, 2021, 06:11:15 PM
What do you recommend?
Either 10 to 20 times higher value or a 1M extra resistor going from 56k/56k junction to non-inverting input..
(for the later, voltage divider junction should be set out of signal path, of course..) :icon_wink: :icon_wink:
I'd recc. trying the suggestions above...smaller input cap. We find these "AC AMPLIFIER SCHEMATIC" things online, and have to tinker with them. The easy way is to steal use work that has already been done for us.
The 2 56k resistors are providing the opamp with bias - so the AC signal can work in the DC-operated device...to increase the input impedance you could use a 'bias network' with say, 2 10k resistors and a bypass cap to get the 4.5v you need for bias, and send it to the opamp thru a 220k to 1Meg resistor, thus improving the input impedance.
Sample Circuit: An example from the MXR Distortion Plus...schematic HERE https://scfxguide.wordpress.com/tag/distortion/ (https://scfxguide.wordpress.com/tag/distortion/) You would want R2 and R3 to be 10 to 22k or so, though. The 1Meg feeding the opamp can be lower, that is common with this 'starved' design... Just an example of what the 'norm' is with biasing an opamp (if there is one). good work so far! There are many ways to skin this cat.
Quote from: antonis on November 12, 2021, 06:15:37 PM
Quote from: YurkshireLad on November 12, 2021, 06:11:15 PM
What do you recommend?
Either 10 to 20 times higher value or a 1M extra resistor going from 56k/56k junction to non-inverting input..
(for the later, voltage divider junction should be set out of signal path, of course..) :icon_wink: :icon_wink:
Oh, so connect the 4.5V rail to the input signal path, instead of running the input signal through a voltage divider? Like in the Boss DS-1?
http://www.electrosmash.com/images/tech/ds1/boss-ds1-distortion-schematic-parts.jpg
Yes, R10! :)
(https://i.imgur.com/1Vg2ORk.png)
MS Draw sukcs.. :icon_mrgreen:
I managed to put this together in LTSpice - I may have screwed up some of the values, but it's a start. It's actually fun simulating the guitar signal and then probing around the circuit. :)
(https://i.postimg.cc/Dm03LZtF/pedal1.png) (https://postimg.cc/Dm03LZtF)
Nice! Just a couple of fixes...Spice doesn't care what you do, it's stupid and will give happy returns even if we mess up, so...
There are rules for bias supplies (I don't have at hand, I have had a few pints! 10 times, blardy blar)...100k is too high a value for the divider. Try 22k or 10k, as a rule. 10k (well, 20k total) is only a fraction of a mA anyway, so no worry!
You obtained 4.5v bias, great! Now...to feed it to the opamp, look at Distortion Plus or the FULL DS-1 schematic...where you have 4.5V input to the opamp, there is a resistor. It is there cuz direct connection would cause AC to ground thru the CAP in the divider! AC and DC are separate and different issues. The resistor keeps AC from 'flowing backwards' up the pipe to the divider. You will build up to understanding as you do this. Add a 1M, or 470k, etc, resistor between the tap of the divider and the opamp input. It is a big player in the input impedance mentioned earlier, so between those 2 values will put us in a good place. Pick something, lol. See what you get, and we'll visit this more.
Good first steps!
Some light reading that helped me out...Pictures of Lily for the opamp enthusiast! Can't do 'the thing' we want to do until we know how to use the device(s) that we need to do them with, and all that. FYI, we're mostly "split supply", so check that stuff out early...
https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf (https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf)
1m is 1 milliohm in SPICE
1Megs is 1 Mega ohm
Suppose you have gain 11
And input 1vp
You expect 11vp output
Which is not possible with 9v supply
But you won't see that in spice if you use model of ideal or generic Opamp which has no connections for Vcc
So please use model for TL072 etc where you need to connect Vcc to the Opamp model.
Quote from: Vivek on November 13, 2021, 03:39:34 AM
1m is 1 milliohm in SPICE
1Megs is 1 Mega ohm
Suppose you have gain 11
And input 1vp
You expect 11vp output
Which is not possible with 9v supply
But you won't see that in spice if you use model of ideal or generic Opamp which has no connections for Vcc
So please use model for TL072 etc where you need to connect Vcc to the Opamp model.
Thanks. I didn't know how to specify mega ohms but now I know it's "1 meg".
Hmmm. I thought I looked for a TL072 but couldn't find it. I'll look again.
LTSPICE model for TL072
http://ltwiki.org/index.php?title=File:TL072.sub
Quote from: Vivek on November 13, 2021, 09:18:08 AM
LTSPICE model for TL072
http://ltwiki.org/index.php?title=File:TL072.sub
Thanks I'll, install it later.
Just take into account what Sir Mike said.. :icon_wink: :icon_wink:
As it is, input impedance is about 50k (100k//100k) and in case of a VBias cap, it simply is zero..
(OK.. not absolute zero but very close to nothingness..)
P.S.1
You tend to place input electro caps with wrong polarity.. :icon_wink:
P.S.2
You set circuit ground one diode voltage drop above PS ground..
Thanks again - some excellent information I need to try to understand. :)
You'll get it in time. Do a lot of reading, and look at the common schematics - you'll start to notice things that occur constantly. I don't have a lot of the deep theory on semiconductors, but I'm pretty good at copying! he he.
I borrowed the power supply from the DS-1 and I made various changes that were suggested. I installed the "sub" and '"sym" files for the TL072 into LTSpice. The guitar input signal is about 200mV p2p and the output of GAIN_OUTPUT is just under 400mV p2p. The input impedance is still low I think, and I've also lost a lot of gain for some reason.
(https://i.postimg.cc/bddZV849/pedal2.png) (https://postimg.cc/bddZV849)
If I revert to this, then I get the gain of 10 as I would expect. I'm not sure why there's a difference.
(https://i.postimg.cc/mz4C0qfy/temp-pedal.png) (https://postimg.cc/mz4C0qfy)
You need to wire the ground side of the 1M to the 4.5V point, not the opamp + input to 4.5V.
This form is preferred compared to your second circuit because it picks up less noise from the power rails.
Quote from: YurkshireLad on November 13, 2021, 05:58:44 PM
I borrowed the power supply from the DS-1 and I made various changes that were suggested. I installed the "sub" and '"sym" files for the TL072 into LTSpice. The guitar input signal is about 200mV p2p and the output of GAIN_OUTPUT is just under 400mV p2p. The input impedance is still low I think, and I've also lost a lot of gain for some reason.
(https://i.postimg.cc/bddZV849/pedal2.png) (https://postimg.cc/bddZV849)
Also you're shorting all your signal to ground through C6 otherwise.
At C6, add a 1 Meg resistor from 4.5 bias point to the opamp input. Remove the 1Meg to ground there now. Without some additional resistance feeding the opamp, the signal just goes back to the power supply and shorts to ground thru C6, like 5/7 is saying.
Thanks all. I'll try to update it later today.
Quote from: GibsonGM on November 14, 2021, 09:26:01 AM
At C6, add a 1 Meg resistor from 4.5 bias point to the opamp input. Remove the 1Meg to ground there now. Without some additional resistance feeding the opamp, the signal just goes back to the power supply and shorts to ground thru C6, like 5/7 is saying.
It sounds so obvious when explained like this. I wouldn't have thought of this, but it's something you gain with experience and knowledge. I think FiveseveN was also saying this and I didn't understand what he meant, but now I do. The output from GAIN_OUTPUT now has a gain of 11 once again. Thanks!
Fantastic progress !
Do post your latest schematic
With frequency response graph (some of your caps could be too big)
And wave shape for various amplitudes of input signal
Did you try CTRL R and CTRL E with Opamp selected, to change its orientation ?
You could be right. I need to understand how to view a frequency response graph. I'll check other pedals for examples of capacitor values.
All of them are 1st order High-Pass Filters and obey the general formula f = 1/2*π*R*C..
e.g. 1μF & 1M from a HPF of 0.16Hz cut-off frequency (subsonic..) :icon_wink:
An optional Low-Pass Filter should be formed by a cap in parallel with R4..
I haven't modified any values yet, but this is what I have so far.
(https://i.postimg.cc/k66vkzpD/pedal2.png) (https://postimg.cc/k66vkzpD)
The simulation results are:
(https://i.postimg.cc/BXgHwJnM/pedal2-sim.png) (https://postimg.cc/BXgHwJnM)
Though I'm not sure why the output signal is "doubled". You can see the output signal is rendered twice.
Why doubled beats me
Can you post your ASC file here ?
It is done by opening up the ASC with a text editor, copying it
using CODE button here that looks like a #
and pasting into the CODE area
Alternatively you could email it to me.
I can add commands for frequency response etc for you and sent it back to you.
QuoteThough I'm not sure why the output signal is "doubled". You can see the output signal is rendered twice.
QuoteWhy doubled beats me
I was going to suggest setting the time step to something smaller like 0.1ms.
I believe you can (manually) load previous waveforms and compare them against the current simulation but I doubt it's happening here. If you can exit LTspice, do sim, and still see the double lines then it's not previously saved stuff.
So far, I only know of STEP command to be able to get a retrace of same data (but with different parameters). But I could not see a STEP command in the graphic
Feature to compare last waveform to current waveform would be great. How does that one work ?
For time between Data, I like to begin with 100 data points per cycle as the minimum, and sometimes going up much higher.
Quote from: YurkshireLad on November 14, 2021, 06:47:40 PM
I haven't modified any values yet, but this is what I have so far.
(https://i.postimg.cc/k66vkzpD/pedal2.png) (https://postimg.cc/k66vkzpD)
No need for 3 X 1M bias configuration resistors.. :icon_wink:
(make R7 & R8 10k to 47k, say..)
Quote from: antonis on November 15, 2021, 05:03:54 AM
No need for 3 X 1M bias configuration resistors.. :icon_wink:
(make R7 & R8 10k to 47k, say..)
Excellent suggestion on R7 and R8 !
Any comments on C4 = 47uF ?
and C3 = 10uF ?
C3 : Anywhere between 10μF (16Hz) and 1μF (160Hz) should be fine..
(clearly it's a matter of taste..)
C4's value is strongly dependent on what comes next.. (volume control, next effect input impedance, etc)
10μF should be a safe value for following impedances down to 1K or so..
One last suggestion.. :icon_wink:
Either move D3 Cathode to +9V or make V3 +9.55V unless you intentionally aim to reduced headroom..
C2 is polarity backwards, even though it's only 100nF, a usually non-polar size for caps.
QuoteSo far, I only know of STEP command to be able to get a retrace of same data (but with different parameters). But I could not see a STEP command in the graphic
Feature to compare last waveform to current waveform would be great. How does that one work ?
For time between Data, I like to begin with 100 data points per cycle as the minimum, and sometimes going up much higher.
I only use STEP but I thought there was a way to load in results of other simulations. I can't actually see how to do it ???.
In the past I've seen spice produce incorrect simulations when the step size is too large. Imagine a 1MHz waveform feeding a circuit for 1s stepped at 0.1 sec. The errors are more than undersampling the 1MHz waveform at 0.1sec the solver gets lost numerically by solving the differential equations with large steps.
I think the step we specify in transient analysis is the Max step
And spice uses some min step by using its own brains.
QuoteI think the step we specify in transient analysis is the Max step
And spice uses some min step by using its own brains.
Spice tries to automatically determine the step size but it judges its success based on some convergence criteria. What can happen is it jumps too far ahead but numerically it still see the solution converging at that distant point. The initial step might have skipped a lot of history require to converge at the *correct* solution. Imagine a step waveform going into a circuit which has a 10ns rise time. If the effect of the rise-time affects something important in the simulation the automated stepping could jump 1us and completely miss the 10ns feature. For example a circuit which measures the signal slope by differentiating the signal and feeding it into a peak detector. All automated numerical processes on a computer fail at some point. If you want to guarantee a good solution really need to force the steps to at least represent the known features of the waveforms in the circuit, that might mean the steps need to be somewhere in the 1ns to 5ns region. The downside is if there are large regions of time where the circuit is doing nothing the simulation will be slowed down by the smaller steps. Nonetheless there's no point having quick simulations with wrong answers.
In AI you have a similar issues when selecting the learning rates. For finite element simulations you have similar problems with the mesh size.
Quote from: antonis on November 15, 2021, 05:03:54 AM
Quote from: YurkshireLad on November 14, 2021, 06:47:40 PM
I haven't modified any values yet, but this is what I have so far.
(https://i.postimg.cc/k66vkzpD/pedal2.png) (https://postimg.cc/k66vkzpD)
No need for 3 X 1M bias configuration resistors.. :icon_wink:
(make R7 & R8 10k to 47k, say..)
If I take out R1, then (as pointed out by a couple of people here) the signal bleeds to ground through C6?
Quote from: antonis on November 15, 2021, 08:18:16 AM
One last suggestion.. :icon_wink:
Either move D3 Cathode to +9V or make V3 +9.55V unless you intentionally aim to reduced headroom..
Interesting; I borrowed the power circuit from the Boss DS-1. Perhaps the DS-1 does something special to compensate for this?
Quote from: duck_arse on November 15, 2021, 08:26:15 AM
C2 is polarity backwards, even though it's only 100nF, a usually non-polar size for caps.
I used to have it reversed, but someone suggested I reverse it to what I have now. :)
Thanks all - awesome help, which is much appreciated. Even if I don't always understand what you're talking about! ;D
Quote from: YurkshireLad on November 15, 2021, 10:53:23 AM
Quote from: antonis on November 15, 2021, 05:03:54 AM
Quote from: YurkshireLad on November 14, 2021, 06:47:40 PM
I haven't modified any values yet, but this is what I have so far.
(https://i.postimg.cc/k66vkzpD/pedal2.png) (https://postimg.cc/k66vkzpD)
No need for 3 X 1M bias configuration resistors.. :icon_wink:
(make R7 & R8 10k to 47k, say..)
If I take out R1, then (as pointed out by a couple of people here) the signal bleeds to ground through C6?
Antonis meant
Keep R1 as 1 Megs
Reduce R7 and R8 to something between 10 k to 47 k each
Quote from: Vivek on November 15, 2021, 11:10:48 AM
Quote from: YurkshireLad on November 15, 2021, 10:53:23 AM
Quote from: antonis on November 15, 2021, 05:03:54 AM
Quote from: YurkshireLad on November 14, 2021, 06:47:40 PM
I haven't modified any values yet, but this is what I have so far.
(https://i.postimg.cc/k66vkzpD/pedal2.png) (https://postimg.cc/k66vkzpD)
No need for 3 X 1M bias configuration resistors.. :icon_wink:
(make R7 & R8 10k to 47k, say..)
If I take out R1, then (as pointed out by a couple of people here) the signal bleeds to ground through C6?
Antonis meant
Keep R1 as 1 Megs
Reduce R7 and R8 to something between 10 k to 47 k each
Ahhhhh! My apologies Antonis! :)
I'm not getting a double signal on the output, so I probably made a stupid mistake in configuring the simulation. Though I'm getting a gain of less than 8 now.
Re. the "Megs" and 10ks...did you go look at the link(s) I posted of other circuits? Just copy the power filter/bias network! Distortion plus is good, but as Antonis said, make the 1 megs in the network (R7, R8) 10k, 22k or something like that. Keep R1, the 1M, to the opamp input. Done in one.
Don't worry about the ultimate 'performance' at this moment...we need to get your circuit configured well, and your LT running right :) Don't forget, it's a sine wave, there is averaging involved (120V house voltage is more like 170 peak)
Thanks. I did look at them, but I just revisited them now that I'm a little bit more comfortable looking at simple (parts of) circuits. The Proco Rat power circuit looks interesting.
Quote from: YurkshireLad on November 15, 2021, 10:54:55 AM
Quote from: antonis on November 15, 2021, 08:18:16 AM
One last suggestion.. :icon_wink:
Either move D3 Cathode to +9V or make V3 +9.55V unless you intentionally aim to reduced headroom..
Interesting; I borrowed the power circuit from the Boss DS-1. Perhaps the DS-1 does something special to compensate for this?
(https://i.imgur.com/s8WZhAz.png)
Hmm, this is the schematic I found - https://www.electrosmash.com/images/tech/ds1/boss-ds1-distortion-schematic-parts.jpg
Which schematic is identical to many other similar power supplies.. :icon_wink:
That diode is placed between +(9V) and -(GND) and serves as reverse polarity protection diode..
(it shorts the reverse polarity supply..)
Safer for power supply should be a SERIES Schottky diode (right after +9V input) in the cost of 250 - 350 mV drop..
P.S.
Better implement a 1N400X diode (1A) in case of PS shunt diode unless you wish for 1N4148 signal diode to serve as a fuse also.. :icon_wink:
The diode won't conduct if everything is 'normal'...it is reverse-biased in operation. If the power adapter is plugged in in the right polarity (it's not shown here), and if the battery is connected the right way, it's like it's not there. If either is reversed, the diode shunts the WRONG polarity supply to ground and hopefully protects the semiconductors down the line. It should be 'beefier', to conduct the reversed current without burning itself out. 1N4148 (1N914) are too low rated to do this for very long! But we see them in many designs. They're only good for a quick WOOPS when putting in the battery.
Ignore the diode in your simulations, it's not doing anything in that regard.
The Rat has a neat supply, yes. Big cap / little cap for noise and reservoir, the low-value R there to work with caps as a filter. A bit 'fancy', but works very well. I try to keep it simple! That is the basic idea, the DS-1 or the Rat.
I stumbled across this video from Wampler of the Rat's circuit, which gives me an excellent explanation of what each component does. Fascinating.
https://m.youtube.com/watch?v=zBGKdnlh4Ws
The interesting thing about this is
When all quirks have been righted
All mistakes corrected
Best practices adopted,
Your circuit will become exactly same as 1000 others
Who started at random designs
And converged to almost same final point.
For example, everyone has voltage divider with 10k, and 1M going to Opamp as Vref.
Quote from: Vivek on November 16, 2021, 01:51:57 AM
The interesting thing about this is
When all quirks have been righted
All mistakes corrected
Best practices adopted,
Your circuit will become exactly same as 1000 others
Who started at random designs
And converged to almost same final point.
For example, everyone has voltage divider with 10k, and 1M going to Opamp as Vref.
I'm ok with that - as long as I get there understanding what I did (with all your help!). :)
Quote from: YurkshireLad on November 15, 2021, 10:55:24 AM
Quote from: duck_arse on November 15, 2021, 08:26:15 AM
C2 is polarity backwards, even though it's only 100nF, a usually non-polar size for caps.
I used to have it reversed, but someone suggested I reverse it to what I have now. :)
the outside world sits at ground, signals at input and output, to maintain sanity, swing about ground, 0V. so what goes on inside the black box of circuit needs to be isolated from the outside, keep those damn volts inside the box. use a cap at the input and at the output, and add a pulldown resistor [in some form - often the volume pot does the same purpose] to reference the outer end of the cap as the world wants, 0V.
now your cap has 0V on one end, and some volts [positive - you only have a single positive 9V supply inside to divide bias and stuff] on the other end, the inside side, the side connected to the doing-things circuit. so, now, which way do you face the (+) on a polarised cap, at input and/or output, under these circumstances?
also, you can just bung in a cap, non-polarised, of ANY value, and then measure each end, referenced to ground. one end will be most likely more positive than the other, giving you the correct orientation. this applies in all decouple cases.
Quote from: antonis on November 15, 2021, 03:20:56 PM
.... the perverse polarity supply.... :icon_wink:
someone should use this as a .sig.
Quote from: duck_arse on November 16, 2021, 08:24:41 AM
Quote from: antonis on November 15, 2021, 03:20:56 PM
.... the peverse polarity supply.... :icon_wink:
someone should use this as a .sig.
Not familiar with Greek keyboard, are you..?? :icon_mrgreen:
edit: culprit edited.. :icon_wink:
I thought I'd quickly try to design a simple PCB board in KiCad for the power module from the Proco Rat. It's not easy laying it out. It doesn't look that neat, but it's cool that I can do this and visualise it. I may not have the right footprints for the capacitors yet.
(https://i.postimg.cc/WDpTLNZs/pcb1.png) (https://postimg.cc/WDpTLNZs)
I wired up the gain circuit tonight but it doesn't look like the op amp is being biased, as the output has no negative component. I'll investigate tomorrow and rebuild.
My multimeter shows the Op Amp is being biased with 20mV (measured at R1, where it connects to the input guitar signal). LTSpice shows it should be the value of the bias voltage, in my case it's actually around 4.3V. I've triple checked my wiring and I'm sure it's correct.
Track it back...make sure it is 4.5V at the divider and so on. Make sure the opamp pin orientation is correct!! (be sure it's not 180 degrees turned around). Make sure R1 really is going to the opamp even if you have to use continuity check on your meter (I know, but it happens...). Make sure R1 is the right value (1 meg). If the divider is ok, problem is between R1 and opamp, a short perhaps, so look closely. Last thing of all would be a dead opamp, that's not usually the case tho, they are pretty tough.
Now, check that the input cap is in place, and oriented correctly if polarized (not looking at the schematic right now). If still a problem, post the opamp voltages and the current 'version' schematic you're working from (you've updated a few things, so that is probably a good idea anyway!)
Thanks. I updated the schematic with the readings from my multimeter. The voltage divider is definitely providing 4.5V (approx)...
(https://i.postimg.cc/XXJNxZ9b/pedal2-Copy.png) (https://postimg.cc/XXJNxZ9b)
Yoy realise that it can't be unless non-inverting input is somehow grounded.. :icon_wink:
Quote from: antonis on November 21, 2021, 04:07:21 PM
Yoy realise that it can't be unless non-inverting input is somehow grounded.. :icon_wink:
The non-inverting input is grounded after C3?
It definately shouldn't be..
p.s.
Non-inverting input is the one marked with (+) and should (ideally) sit at +4.5V (or at whatever voltage level sits R1 upper leg) due to (ideally) infinite input impedance..
(no current flowing into op-amp hence no voltage drop across R1..)
Geez, it's the weekend and my brain isn't engaged. You wrote non inverting and my brain read inverting. :icon_rolleyes:
It's ok, opamps are a little weird to get used to. It helps to go look at some popular circuits like a Tube Screamer now and then, to see how they did it. It can help jar you out of a mistake like that :) I have a basic inverting and non-inverting gain stage tacked up on my wall for this reason!
Just move C3's grounded leg to 4.5v, see what you get!
a small point - the single pin voltage is much less useful than all the pins voltages. if your supply pin volts is wrong, we need to know why. if the output pin is good, but an input pin "bad", we can tell you another story aboot opamp behaviour. if both input pins is rong, that might tell us something as well. and the ground pin, if you don't tell us it's reading 0V00, we get suspicious.
ok, some success. I rebuilt the circuit from scratch and I can get gain from the op amp. I played my guitar through my amp (minimal tone circuitry) and it worked. However, the output was around 7V (relative to ground), so I put in a 1k resistor and a potentiometer in series on the output. I also replaced the gain resistor R4 with a 50k potentiometer. Now I'm getting around 0.7V (relative to ground) on the output but it's saturating my amp, even when I turn the volume pot and gain pot down. Is this just because the low frequencies are too strong?
I added a high pass filter with a cutoff around 80Hz and that improved things. However, if I roll back the guitar's volume control, the output signal from my "pedal" loses all of the positive signal. That's weird. I probed around and it's the op amp. Time for some more reading.
Woah, bud :) You had me at "7V relative to ground". AC, or DC volts? If AC, are you inputting some defined (AC) voltage? What point are you measuring from? Are you clear in understanding the difference between AC and DC, when using your meter (tell the truth) ha ha. Do you know how to use a DMM?
It seems that you have some 'holes' in your basic learning, just like I did (and probably still have some to fill)...nothing to feel bad about, but - let's cut to the chase and save some time - for you.
1. post the schematic you are right now 'doing' with your guitar and amp
2. post the voltages on each opamp pin.
3. Don't get discouraged or feel stupid, this is complicated stuff if you're new. It's hard to keep track of what you changed or maybe made a mistake somewhere...so, look at the circuit, draw it, post it, and we'll guide you in. It's most likely a very small error stopping you from success, which we all have been thru :) Hang in there and you'll get it going, and will ALSO have learned the pitfalls that we all face every time we start building...you're doing OK, and I bet with a few corrections you'll be on your way...
Quote2. post the voltages on each opamp pin.
DC voltages ;D
Quote from: Rob Strand on November 26, 2021, 01:04:18 AM
Quote2. post the voltages on each opamp pin.
DC voltages ;D
Firstly, without IC in place.. :icon_wink:
I'm embarrassed every time I reply to this thread now; it's a "simple" circuit, which I understand sufficiently to model and build, but something must be going awry in the translation from schematic to breadboard. But that has always been my problem to be honest. At the moment, if I turn the guitar's volume knob, the bias of the input signal changes, not the amplitude!
I'm going to look for an example image of breadboarding an op amp for now to see if I can get a visual on how things are laid out and connected. I do find it hard to keep track of all of the component connections and layout on a breadboard.
I do understand about measuring AC vs DC; I can see the input signal with a bias/offset in DC mode, and the input signal with no bias/offset in AC mode.
The latest schematic (with a unity? gain of 1) is linked.
(https://i.postimg.cc/YhkP6X2f/simple-schematic.png) (https://postimg.cc/YhkP6X2f)
I rebuilt the circuit again, and have:
OUT = 4.62V DC
-ve = 4.62V DC
+ve 2.27V DC
This time it might be right, though I'll have to double check later.
Now that I've rebuilt it again, most of my problems seem to come from wiring up the gain loop and biasing the input, i.e. the ordering of pins in a breadboard column:
https://cdn.shopify.com/s/files/1/0176/3274/files/Breadboard_Remarked_grande.png?15033584625641436291
It's traditional to go thru the pins, in order, Yurk...from the top looking down, left side down, across and up the right side. So if we know you're using a TL071, you say "Pin 7 = 9V" and we know it's ok :) If you measure DC voltage with it in-circuit, don't short the pins together.
The 4.62 is quite encouraging, tho if "-ve" and "+ve" are the inverting and non-inverting inputs, they should be nearly the same...can you post the voltages in this order? This is the pinout for MOST single opamps, disregard if you're using something else, only posted for PIN NUMBERING, see data sheet for what each pin is on YOUR opamp (tho likely the same ins/outs/power).
Watch those breadboard 'rows and columns' :icon_lol:
EDIT: Here is how the pins run, looking down from top, just to be clear:
NOTCH
1 8
2 7
3 6
4 5
Quote
The 4.62 is quite encouraging, tho if "-ve" and "+ve" are the inverting and non-inverting inputs, they should be nearly the same...can you post the voltages in this order?
Multimeter loading can cause the +input to read low. If the -input is a Vcc/2 and the opamp output is at Vcc/2 then when you see a low voltage on the +input you would have some confidence it's caused by DMM loading. After that you could analyse the circuit with an imagined 1M resistor to ground representing the DMM (assuming a 1M input for a cheaper DMM). For example if the circuit has a 1M input resistor to Vcc/2 and you put a 1M DMM on the +input: the 1M and DMM form a divide by 2 voltage divider, so you would expect 4.5/2 = 2.25V. Which looks very much like what is happening here.
Thanks again.
I must apologize as I forgot to point out that the schematic uses a TL071 but I'm actually using a TL082. The TL071 was the only op amp symbol I had with the power supply pins and simulation data.
Ok. If we account for DMM loading, you seem to be ok...I always forget that, my meter is like 7.5Meg. If you're using a dual opamp, you want to configure the 2nd half of the opamp as a voltage follower so it doesn't oscillate, but deal with that after you make this work! You are assembling this in real life to test, right?
I guess my pin numbering picture went away...I'll replace it with a generic description for clarity.
Yes, I'm building as I go to learn.
Does it work now, audibly?
I haven't had time to try it with the guitar amp yet.
Ok, let us know. You may be at the stage where "IT'S TIME" ha ha. I typically assemble things like this on breadboard, no meter, and just plug it in. If it doesn't work, I look at the parts/connections (scratching my head) til I see what's wrong. Meter is LAST in my system, but your measurement may vary ("YMMV")...your method will come in time :)
Once you do a few builds, you'll probably do less 'theory' too, and go for known values, put together the 'blocks' like gain stages, and spend more time thinking about what diodes for clipping, what you want for tone control(s) and so on.
So close, yet no guitar solo. The output signal to my guitar amp sounds like a mixture of a clean signal and a distorted signal. But the distorted signal is very quiet while the clean signal is very load. Will re-investigate tomorrow.
Shouldn't be distorted, no...for simplicity you can leave out the 1k/10u cap in the feedback loop, build a basic buffer to start. Then it's just connecting pins 1+2 as the feedback path, input cap to pin 3 and to output cap from 1 as well. Maybe it's the breadboard rows/columns making you mis-place parts :)
clean mixed into a 'processed' signal suggest maybe you have input also going to ' - in' when it shouldn't be...be sure to 'straddle' the breadboard center line with the opamp....
Not sure if you know this, could be just from another topic I was on earlier, but chips go like this:
(https://i.imgur.com/w6kXP6e.jpg)
*Edit: modified for pinout, OP using TL082 NOT TL071.
What, the pins go down!!?? :icon_twisted:
Yes, Down!! :) ha ha. Sometimes the error we're making is that simple! Earlier someone was discussing confusion about what rows/columns are connected, and I didn't want to go back thru this post to make sure it wasn't you ;) Since of course, there isn't connection across the center strip, but elsewhere there is.
Note I DID go back to remind myself that you have a TL082, so I updated my last pin-post there to be correct for this dual opamp! :icon_lol:
I'm taking this to be the current schematic, pin #'s for the dual opamp. Try building this and see if it works for you; it does on this end. I took out the filter network in the feedback loop (1k/10u), and put the 10u as output cap - 47u is quite large. Added 100k load resistor to the end so the output's not floating. Note only C1's right side is connected, with the 1M "Vr" resistor, to pin 3 node.
(https://i.imgur.com/bKngFu5.png)
I don't know if you can make out the connections here, but hopefully it shows what I have for the op amp. You can't see the connection from pin 4 to ground in the picture.
The orange wire on the left is the input, the orange wire on the right is the output.
(https://i.postimg.cc/4YzvZDqD/PXL-20211201-181414683.jpg) (https://postimg.cc/4YzvZDqD)
Edit....where is the bias voltage that should be going to pin 5, the "+ in" pin? And, is there an input cap we can't see on the orange 'input' wire? Output cap after the output wire?? BAD distortion without the caps! C1 and C3 in my schematic.
(https://i.imgur.com/ROes4WX.jpg)
There are input and output caps but I couldn't get them all in the picture; it was impossible to really see any detail when I pulled the camera (phone) further out.
The resistor you identified is supposed to be R4 in my schematic at https://postimg.cc/XXJNxZ9b. Isn't the gain loop basically a voltage divider?
Yes, the feedback loop is a divider. What I'm trying to get you to do is make a voltage follower first off the schematic I posted above, with the bias voltage there and all....then you can insert the resistors needed to add gain :) I don't see the 1meg bias resistor in your photo, arrow points to the "+" input, no bias voltage that I can see...can't read those d@mn blue resistors, either, he he.
Yeah the bias resistor is off the top of the picture. I'll try to take a picture of the whole shebang but it won't be pretty!
Ok. I don't see it connected, perhaps you have things like " input > Cap > bias resistor > over to chip which would be fine. Provided the parts are in the right places, it WILL work - laws of physics!!
It's probably really hard to see what's going on. I haven't had time to change anything today.
(https://i.postimg.cc/yWZP95WW/PXL-20211202-202501598.jpg) (https://postimg.cc/yWZP95WW)
Quote from: YurkshireLad on December 02, 2021, 03:39:31 PM
It's probably really hard to see what's going on. I haven't had time to change anything today.
(https://i.postimg.cc/yWZP95WW/PXL-20211202-202501598.jpg) (https://postimg.cc/yWZP95WW)
hmm in most breadboards I know, the power rails are splitted halfway. Can you confirm that yours is different?
It's possible that what niek said is the case. But looks like you have gnd and 9V on the top left side...if you are getting 9V at the IC pin and gnd on pin 4, then that's not the case. Quick meter check will show that.
OK - I see the bias divider and line going to the "2in+" pin (5), it looks like. Is the blue wire there (left side) your guitar input, going thru the small ceramic cap?
That seems ok.
What is the R and cap to ground just to the top right of the IC? I'd omit that and go for unity gain buffer at the moment...same with the output stuff, those 2 caps in series (?) and pot. I'd send the output out thru 1 cap only, to the amp (start at volume ZERO!), to see it's working. Appears the pot has wiper going to ground anyway, which would be wrong...have to eat dinner, mess around and I/we will be back in a bit!
It's definitely not a split breadboard, I just checked. I haven't had time to remove the gain loop yet, so I put this picture together.
(https://i.postimg.cc/jLQSFzMt/breadboard.jpg) (https://postimg.cc/jLQSFzMt)
(https://i.postimg.cc/FfqrLGHg/breadboard-circuit.png) (https://postimg.cc/FfqrLGHg)
I don't remember where I bought the TL082s from, but there's always a chance they're fake. :o
Yurk? If you just follow the schematic I posted before, you'll confirm if it works or not, and set up a foundation to start doing cool things like adding gain, volume pots and such :) most of it is already built. A buffer will test your opamp. I don't like your pic, it over complicates things...series caps and all...nah!
Please just rewire the opamp, even going to the other side and use my pin numbers (1, 2, 3 etc). Then we can talk about any problems from a common viewpoint. Seems you're making something that I'm not aware of, lol. You are close (and what you've drawn SHOULD work, tho parts are not needed), but something ain't right. I promise, if you wire it correctly, it WILL work, OR the opamp is junk (or battery if flat, or breadboard is shot or...he he).
(https://i.imgur.com/bKngFu5.png)
I'll try it tomorrow night. Thanks! Much appreciated.
No worries. Hope to hear that it works :) Stick with it and it will.
I retire in about 12 years, will that be enough time to get this working? ;D
If one just takes it step by step...check the voltage divider, check the power supply and ground at the IC...then hook up +, - and output...one should be able to make it work in say, 10 minutes :) But that's provided the opamp isn't damaged or something. And one has to literally do EXACTLY what is on the schematic.
Once you get the buffer working, it shows that the IC is ok. Then you can add 2 resistors and get some gain going. Then a pot on the output for volume. Then a cap in the feedback loop to tailor frequency content. Then you'll have gone thru about 90% of what you do with opamps in DIY! That's why I suggest you scrap what you're doing and use my schematic to make a buffer. If it works, I/we will show you how to add the 2 resistors to give it gain. :)
Those series caps may be something seen elsewhere in the audio world - where they use bipolar power supplies. With common electrolytic capacitors that are polarized, they shouldn't get a reverse current flowing. With bipolar supply, the signal sits halfway at 0v so it swings above and below that. This means there isn't actually a right way around to fit an electrolytic cap. You either use 2 caps (twice the value because series caps divide and your 10uF are worth 5uF) with the negative ends joined in the middle, or else buy nonpolar electro caps that are already 2 in one can.
An advantage of the single unipolar power is the signal is biased to sit at a positive voltage (4.5v) so where one side of the cap is 4.5v (amp out or in), and the other side is going to 0v (jack in or out via what's plugged in or a pot or pulldown resistor) you can use one ordinary polar electro cap and you can work out which way the cap faces.
Ok, I rebuilt using GibsonGM's circuit and I think that works. I'm getting a clean sound of the guitar amp, with the same peak to peak as the input, (approx 300mV, depending on how I hit the strings). so the gain is definitely 1. Also, the amp isn't being overloaded in any particular frequency range, which is good.
great! Now I would connect the volume pot. (1 leg to output cap, wiper to output jack, other leg to ground) so you can control the level.
Then, you get to add your gain resistors! Go ahead and do what you showed with the 20k and 1k resistors, but I would take them to "Vr", your voltage reference/bias point, instead of ground...that point is AC ground. You don't go thru and R to get there, the cap will connect directly.
This is the bit I always have a problem with. Now I get no output from the op amp. This is what I wired up (ignore the pot values, I forgot to change them in the diagram).
Edit: I get output if I add a cap indicated by the red arrow. I get gain and distortion but it has the same problem as before, it sounds like the clean signal is amplified and the gain distortion is really quiet and hissy.
(https://i.postimg.cc/YjHqqNfW/attempt1.png) (https://postimg.cc/YjHqqNfW)
I may have to record a quick clip of me playing so you can hear it.
I don't have a logarithmic pot so I may have to reduce the gain.
Quote from: YurkshireLad on December 03, 2021, 07:20:07 PM
I get output if I add a cap indicated by the red arrow.
You also need an input cap and the biasing network. Like he said,
Quote from: GibsonGM on December 03, 2021, 08:11:26 AM
one has to literally do EXACTLY what is on the schematic.
and that's not what you're showing us.
Remove the 1 Meg on the output. Bring the 1k in the FB loop to VR, not ground. The bias network is the center that the AC signal changes around; it's AC ground. A linear pot will work for volume, just not as well.
This is what you need to build. I put a .1u cap on the feedback leg, that's quite trebly, you can use what you like for right now, such as the 10u you had there before; 1u is probably going to be a good value for cutting some bass but not too much. That cap sets the bass cut when it's in the FB leg. If you want less gain, make R2 smaller, say 10k (more appropriate for a single stage).
(https://i.imgur.com/EwKdbxv.png)
Success!
Thanks to a statement by GibsonGM that I read but didn't catch the magnitude of.
Bring the 1k in the FB loop to VR, not ground. The bias network is the center that the AC signal changes around; it's AC ground.
This fixed it for me, and now I understand it, it seems really obvious. I guess the distorted signal was bleeding to ground, or words to that effect.
Anyway thanks!
I had to put a low pass filter (1k + 1nF for now) in before the op amp as I started getting horrendous RF interference. But I get much better distortion now, although I need to play around with tone next.
Awesome! I'm glad you have success; now the rest of it will just be learning the different tricks to make them do what you want!
Yes - the signal has to ride around Vr, the bias point, at ~4.5v....by grounding that part of the opamp to 'gnd', we're actually disturbing the bias pretty badly (that's how I look at it...wants to be AC riding on 4.5VDC bias, and you're grounding it there) - that's the distortion! And not the good kind ;)
Textbook opamps circuits mostly show everything based on a DUAL POLARITY SUPPLY (where such points WOULD be tied to zero volts)...since we don't often use them, and have to use that Vr bias network...we have to keep in mind where the ground as far as AC SIGNAL VOLTAGE is located...not DC...any time we see those circuits and want to adapt them for a single polarity supply.
* Another thing that may cause distortion here is simply too much gain. If you replace the '20k' (22k?) with something smaller, even 5k...you may find you get a boost without distortion. Any time we ask the opamp to deliver more output than it is limited in doing by the power supply, it will cut off and distort. Mess around with values! Even 10x is going to distort part of the input signal.
Have fun :)
I haven't had time to work on the pedal for a while, but I plugged my guitar in last night. I remembered that the distortion didn't sound right, and it didn't, so I spent time checking and rebuilding the circuit. Later on in the evening, it finally hit me like a quadruple face palm.
My power circuit is using a simple voltage divider, instead of a more complicated schematic I posted earlier. So I don't need R5 shown in - https://i.imgur.com/EwKdbxv.png. There's no short circuit to ground with the voltage divider.
This extra resistor was biasing the op amp to 2.2V instead of 4.5V. I haven't tried removing the fix yet as the realisation hit me when I was staring a voltage divider schematic on a web page later on.
Doh! :o
Hey Yurk,
Sudden realizations are great, ha ha! In the pic you linked, C2 is the path to ground for the signal if the bias R isn't there...at low frequencies, the audio (AC) signal will like that cap more than it likes the opamp input (infinite Z), so bye-bye audio.
Not sure what you mean by a 'simple voltage divider' eliminating this (show a schematic if you can)...unless there's no cap there. There should be; it's good practice to filter your power, even from a battery, which is very stable. It is for 'decoupling' and has several purposes - don't leave home without it. R5 isn't an extra resistor, it's a bias resistor, and is typically required. Lecture over, I'm not the most 'theory oriented' person on here! I mostly copy the conventions of other designers (since they already did all the math!), and they like that R to be there...something about what the opamp pin wants to see for a bias current and so on, out of a textbook I once read but forgot the 'why' about...
Simple voltage divider like this:
(https://i.postimg.cc/xqPZNJ62/simple-power.png) (https://postimg.cc/xqPZNJ62)
Ok, yes, sure, in theory, there isn't a path to ground for AC there, so your assumption was correct. But, for a good, well-made pedal, you'd really want the cap, so the bias resistor is still in play, as I 'related' above. Your instinct on that was right, tho!
They came up with these things to make audio equip. more stable...clean power means less hiss/hash/crap/radio coming into your device, and feeding opamps thru resistors can protect them as well as provide a 'stiff voltage' for the input to see. When you get into using LFOs (low frequency oscillators, perhaps to drive a chorus or trem circuit etc), that will become far more important, too - in keeping LFO clicks and noise out of the audio. Just good practice :)
Using your circuit from https://i.imgur.com/bKngFu5.png, I'm seeing 2.3V DC on the non-inverting input.
Does it work? I'm going to remember this time...that could be due to meter loading :)
Sorry, had to run to make dinner.
As an aside, what do you have for a DMM? Sometimes a meter can draw enough current from the circuit under test that it causes the voltage to sag (called "loading"). I'm curious what the impedance of your meter is, to see if that's the culprit.
If the circuit is operating fine, then that's no concern...esp. if you read about 4.5v back at the divider, and KNOW the 1Meg IS actually running right to that input, and the cap is there to keep DC from flowing back to your guitar pickups etc...
This is my DMM - https://www.canadiantire.ca/en/pdp/autoranging-digital-multimeter-0520052p.html#srp.
I'm definitely reading 4.5V back at the divider.
Maybe it is working; I put in a gain loop with a gain of 11 (10k + 1k resistors) and I'm getting a clipped output signal of about 7.8vpp. I'll plug the guitar into it tomorrow.
With a gain of 1, I'm getting an output signal of about 3vpp, which is the same as the input signal.
Thanks for your help.
Hi Yurk,
Ok...at some point approaching V+, you're going to see clipping if you have an input signal...the thing can only amplify up to a certain point, then power supply limitations cause it to run out of headroom. To avoid that, some people run on 18V - but the vast majority don't seem to need to (why would we ever need to have a signal at over 7V???? 18V is just for super-cleanness).
Your meter is most likely loading...I couldn't find a spec for its INPUT IMPEDANCE (sometimes, Zin). That will determine if it is loading the divider. If you lift the left of the 1M resistor feeding the opamp, at the opamp end...see what it reads for voltage (for curiosity). I'd say it's just loading, esp. if you get clean signal over any reasonable range (and it seems you are given your results!).
Any reason for 3V P-P input? A guitar would be more like 1.4V p-p on average (this is debatable). I say that because the lower input signal would mean you have more headroom than you think. But coming from ANOTHER gain stage, not as much.
I got tired of strumming the guitar every time I wanted to check a signal, so I uploaded a simple SIN generator to an ESP32. It's fixed at 3V, hence the high input signal. I don't have a proper function generator.
If you can find R.G. Keene's "quick and dirty oscillator", on the Geofex site linked at the forum main page, that is a very useful thing to make, and is easy. I use that a lot.
OR, maybe you could add a 100k output pot to your generator and dial in 1v. Not strictly necessary tho, only to have a 'standard' of 1V which makes things easy.
At one time I made a CD of myself strumming and noodling guitar for 10 minutes, and would use that as input :) Same deal tho, it's not 'life like'.
I originally had a resistor on the signal input to bring the 3V down, but I took it out so I could work with the original signal. I'll put a volume pot on the output before I plug the guitar in.
Yes, just put signal in from one end, take the wiper out to your circuit, and the other end gets grounded. (you probably already know that!)
The op amp is still only biased at about 2.3V and the output signal doesn't sound right. I think I'll put this aside for now and move onto something else.
There's only 1 thing I can think of that would cause that - bad opamp. Not likely tho. Other than that, the bias should be fine. If the power is flowing thru the R divider (you confirm this), and the 1M resistor is connected to the Vr point...it is there.
Lift one leg of the 1M bias resistor, the one AT the opamp. Now measure DC voltage right from that resistor leg. Now measure the voltage on the OTHER leg of the 1M resistor. The high input impedance (AC resistance) of the opamp is working with the 1M resistor AND your meter to fool you, I do believe. If your meter isn't mroe than say, 3M input impedance, this could well be the case
EDIT:
Ok, I found specs for your meter, and it says you have 10M input impedance ("Z"). That's a lot, so should be minimal in terms of loading. But there's little reason the bias voltage would be 'off' over such a short run. I mean, there literally are only like 2 places a mistake could be. While at it, measure the resistance of that bias R to be sure it really is 1Meg.
It's definitely 1M - I measured it and I also tried a different 1M resistor.
The leg attached to Vr measures 4.5V DC, the other measures around 2.3V DC. The op amp is definitely being biased by about 2V DC, which means any gain clips the bottom of the signal, but not the positive.
I know what you're getting at.
Maybe it's the TL082, vs a 'standard' TL072 (?). You do have the other half of the opamp set up like a buffer, right? So it's not just floating.
If you've done that...try lowering the 1Meg bias resistor to 100k, see what you get at the chip; if the '82 has lower input z than we're used to, that should increase the bias at no real cost. And lower R2, from 20k to 10k, see if a gain of 21 sounds better....do you have any other opamps?
Hmmm no its floating, I'll try to address that.
I thought I had some LM358s but I can't find them. I might have some LM386s but those have a different pin configuration, so I need to read the datasheet.
(or another '82, just to be sure this one isn't kaput). That opamp should have sufficient input impedance to NOT drag down the bias voltage, which is what seems to be going on. It should be more than 1 meg...which wouldn't load that bias R very much. If it's all built right, that should be the case.
Trust me - once you get this going, you'll make 1 or 2 more, and then it won't be hard! You forget about the 'physics' of it, and just build them (that's why I have to think about WHY yours isn't working right).
Take the unused opamp and terminate it. If we get the gain stage working, we'll go back and make that a buffer before it, or another gain after :) For now, park it so it's not a factor...
https://paulinthelab.blogspot.com/2015/04/proper-op-amp-termination.html (https://paulinthelab.blogspot.com/2015/04/proper-op-amp-termination.html)
I tried another TL082 and terminated the unused opamp with no change.
I changed the bias resistor to 100k and I'm reading 3V DC at the non inverting input and output pins. So an increase of around 0.7V.
Says something, but it's still not right. Is C1, the input cap, polarized? If so, the "+" leg should be going to the opamp. If that was backwards or defective, it could allow DC to pass back to the input jack (draining it away). You might measure for DC across the jack just for grins. There should not be DC there *
What is the inverting input pin reading with 3V on the others? Should be quite similar (same).
You do have a cap from 'Vr' to ground in the divider? What does your battery read?
* as an aside, that cap is way big for guitar work...something 1/10th that value would be more suited and thus non electrolytic..like, .1u or so. If you can avoid using an electro it's good to do so, they are kind of - crappy.
"Is C1, the input cap, polarized? If so, the "+" leg should be going to the opamp."
Ah no, the cathode is connected to the opamp. This will be the first thing I try later.
Thanks again.
That COULD be bleeding off bias voltage...and perhaps the cap is wasted now...so if you have another just replace it. Another of those things one may ASSUME someone knows, but they don't.
Since the DC is biasing the opamp, it's going to be at the "+" potential; a quick look at things like transistors show the same condition...many schematics show the "+" on the caps if polarized, or have the straight line on the + side, showing us which is at a higher potential.
Are you kidding me? I swapped the polarity of C1 and the op amp is biased at 4.5V now. >:(
The distortion doesn't sound right; it kind of fizzles out, but there's a lot of noise in the signal and I'm also picking up radio from somewhere. Still it's progress, right!? :o
If C1 was ceramic, instead of electrolytic, would that block DC in both directions?
Yes but - Ceramic caps suck tho :) If it was a POLY type, like .1u, it would be better...better quality caps, and the lower value will mean less bass input to the stage - more crisp-sounding. So less distortion (often caused by too much bass content).
It'll pick up RF and junk if it's not in an enclosure.....you're actually using a lot of gain there, so that is normal. There are tricks for if it does that once it's assembled properly (like a .1u cap to ground from input etc)...don't worry about that now tho...
If you're TRYING to get distortion...using the opamp to do it probably isn't going to be a nice sound...you'd want to use clipping diodes with it (look up "MXR Distortion Plus Schematic" to see that). I figured you just want a clean boost for right now - so maybe the feedback resistor needs to be a lower value (like 10k instead of 20k?). What we want to do for the 1st stage is raise the tiny guitar signal above the noise floor and get a nice, clean, solidly amplified signal to work with.
Also - what amp are you using? Is it a tube amp? Feeding highish level signals into a solid state amp can sound like crap, where with a tube amp it would sound very nice!
I have a Peavey Blazer 158.
They SAY it behaves just like a tube amp - not sure about that, but we'll go with it. I'll just leave it at - if the amp sounds good when you add more and more drive to the input with your boost circuit, then great! If not, the cause may well not be the boost, but the transistors in the preamp.
Some solid state amps will sound good over a small increase in input, and then turn to BLAHHHHH after that. The amount of bass content you feed the amp can be a MAJOR player in how the drive will sound. If takes way more power from the amp transformer to amplify bass than higher frequencies, which can over-tax it. Why I suggest a smaller cap (and that means doesn't have to be electrolytic). Just something to be aware of, there are limits to how much boost you can get away with. Even with tube amps; eventually, they'll just turn to a solid wall of non-dynamic fuzz.
Anything non-polarized will block DC in both directions...ceramic, a poly type (favored for stomp circuits), mylar, or non-polarized electrolytic (abbrev. "NP" in their documentation).
I have no doubt it's an average amp, because it was pretty cheap when I bought it.
Do you mean use a 0.1uF for C1? I just rebuilt the circuit so I'll try it as is and see if 0.1uF makes a difference.
If we're still going by the schematic that has the 10u input cap...yes, .1 would improve it I think. Esp. in terms of noise.
Also try, as was mentioned, lowering the feedback resistor from 20k to like 10k. You don't need massive gain out of one stage, LOL. If your guitar input signal is say, 1V on average...asking it for a gain of 21 WILL make the chip distort, because the 9V supply can't GIVE 20V out. It clips.
At 10k, still kind of a lot of gain but possibly you won't notice distortion, or it'll be less nasty.
Can try a 6.8K in there too, see if that dials it back a bit....or a 10k or 20k (etc) pot wired as variable resistor, adjust to taste...