Controlling the amplitude of the output signal from a pedal?

[YurkshireLad]

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!). 

[duck_arse]

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.

.... the perverse polarity supply....

someone should use this as a .sig.

[antonis]

.... the peverse polarity supply....

someone should use this as a .sig.

Not familiar with Greek keyboard, are you..?? 

edit: culprit edited..

[YurkshireLad]

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.

[YurkshireLad]

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.

[YurkshireLad]

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.

[GibsonGM]

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!)

[YurkshireLad]

Thanks. I updated the schematic with the readings from my multimeter. The voltage divider is definitely providing 4.5V (approx)...

[antonis]

Yoy realise that it can't be unless non-inverting input is somehow grounded..

[YurkshireLad]

Yoy realise that it can't be unless non-inverting input is somehow grounded..

The non-inverting input is grounded after C3?

[antonis]

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..)

[YurkshireLad]

Geez, it's the weekend and my brain isn't engaged. You wrote non inverting and my brain read inverting. 

[GibsonGM]

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!

[duck_arse]

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.

[YurkshireLad]

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?

[YurkshireLad]

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.

[GibsonGM]

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...

 

[Rob Strand]

2. post the voltages on each opamp pin.

DC voltages 

[antonis]

2. post the voltages on each opamp pin.

DC voltages 

Firstly, without IC in place..

[YurkshireLad]

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.