FET + opamp preamp

[El Heisenberg]

[El Heisenberg]

I forgot o put in the schematic, there is a 100 ohm resistor before any power gets to the op amp. But only on the op amp


I just went ahead and ripped out the second fetzer valve I had in there and tried an opamp as the gain recovery stage. It doesn't work. I dont really know what I'm doing, I just tried what Id seen in other schematics.

First off, the volume control works weird. When it's all the way up, the sound cuts off. Turn it down a little and you get pretty good volume, but only for a small part of the rotation. Turn it down a little further and the sound dissappears. Turn itdown a little futher and it comes back a LITTLE, keep turning it down (past 40%) and theres a loud pop and it goes dead.

The tone controls work funny too. I can't turn them down without the sound dissappearing. I'm pretty sure its cos I dunno how to keep the opamp happy and working correctly.

I also put a switch so I could switch between a jumper and a 1meg resistor between pins 1 and 2.  I thought this would bring the volume up in case there wasnt enough. Butinstead it just get farty and I still have the same problems.

Could anyone point out whats wrong with my schematic??? I thought an FET/ opamp config would be much simpler to impliment than two FETs....

[JKowalski]

First thing I noticed was that you need a decoupling capacitor between the volume pot output and the op amp + input.

Right now the resistor to Vref and the volume pot make a voltage divider that changes the bias voltage as you change the volume. Changing the tone controls can also mess with this problem. Whenever you are switching between different grounds (in your case true ground to Vref) you need a decoupling capacitor.

Second, you don't need the decoupling capacitor on the output if the GGG reverb has one already.

Third, I don't think the 4k7 and the 220n capacitor are really doing anything at all. You can take those right out. In fact, you may not even need the op amp at all because the GGG reverb will pick up the signal just fine without buffering. In that case, take the op amp section out completely and just hook the volume pot up directly into the GGG reverb.

[El Heisenberg]

Thanks. I thought it might be something to do with the different ground. I was just messin with it takin voltages and clipped the 1 meg resistor to vr. Now it dont work at all. But before I did this, pins 1 and 3 were measuring 1.5v. The same voltage that the second FET stage that I ripped out biased at. I didn't mess with the volume pot tho. And pin 3 was obscured by a resistor so I couldn't measure, but I know something was wrong from those readings. I measured pin 5 too which isn't connected to anything and it was .47v.

I realized I didn't need the output cap after Id already installed the stage. I was planning on removing it. But i knew this wasnt the source of my problems.

Another strange thing, the mid control functions sorta. It doesn't act as a level control at all tho. I turn down the treble and bass all the way and there's nothing. I have to turn the other controls almost all the way up to get any response from it.

I prolly will resort to just omitting the 2nd stage altogether.



EDIT:

I just bypassed the 2nd stage. After the volume pot it goes straight to the reverb circuit. It works now, but its quieter than when I had the weird-functioning JFET stage that I had ripped out. I can only use the volume control from about after 7 o'clck before the signal dissapears.  I was looking for at least the same, if not more volume and something that I could be sure was working correctly. Before, my second stage wouldn't work unlss the MPF102 measured 1.5v at the drain. Which makes no sense. Since it wasn't working like it should, I thought it could only get better if I fixed the 2nd stage.

[El Heisenberg]

I put the buffer in, but it doesnt act like a gain recovery stage. It was dumb to tjink it would. Would putting a resistor between pins 1 and 2 raise the volume? Or do i need to add more stuff connecting to "vr" in addition to that resistor?

[Hupla]

I put the buffer in, but it doesnt act like a gain recovery stage. It was dumb to tjink it would. Would putting a resistor between pins 1 and 2 raise the volume? Or do i need to add more stuff connecting to "vr" in addition to that resistor?

I dont understand why the 68p cap is bypassing the volume control?

[jacobyjd]

I put the buffer in, but it doesnt act like a gain recovery stage. It was dumb to tjink it would. Would putting a resistor between pins 1 and 2 raise the volume? Or do i need to add more stuff connecting to "vr" in addition to that resistor?

I dont understand why the 68p cap is bypassing the volume control?

If my first impression is correct, it's to allow treble to bypass the volume control (i.e. treble is not attenuated, even at low volume)

[Hupla]

I put the buffer in, but it doesnt act like a gain recovery stage. It was dumb to tjink it would. Would putting a resistor between pins 1 and 2 raise the volume? Or do i need to add more stuff connecting to "vr" in addition to that resistor?

I dont understand why the 68p cap is bypassing the volume control?

If my first impression is correct, it's to allow treble to bypass the volume control (i.e. treble is not attenuated, even at low volume)

Thats what I thought, but why would you want to do that on a volume control?

[caspercody]

Take out the 4.7, and 220n and just add a 1k between pins 2 and 1. Like what is on the TS808.

[jacobyjd]

I put the buffer in, but it doesnt act like a gain recovery stage. It was dumb to tjink it would. Would putting a resistor between pins 1 and 2 raise the volume? Or do i need to add more stuff connecting to "vr" in addition to that resistor?

I dont understand why the 68p cap is bypassing the volume control?

If my first impression is correct, it's to allow treble to bypass the volume control (i.e. treble is not attenuated, even at low volume)

Thats what I thought, but why would you want to do that on a volume control?

It's a treble bleed similar to what you can do on a guitar volume pot...I'm not sure of the merit of it in this circuit, but generally, it's to keep things from getting muddy as you turn them down--basically, removing the 'body' while retaining the detail.

[El Heisenberg]

The treble bleed cap is in most fender amp schematics ive seen.

A 1k resistor wont do anything. Ive already tried 250k - 1 meg and it just stops functioning.

[El Heisenberg]

The tube screamer has a 1k between - and output on the 2nd opamp but the first has a 4k7 resistor and a 47n cap to "vr"??

[El Heisenberg]

I didn't know how many ways there were to do this. I looked this up to see what it looks like.

http://www.generalguitargadgets.com/richardo/practiceamp/practicemaneq.gif

How many other ways are there to do this

[El Heisenberg]

I tried 1k between pins 1 and 2 and couldnt tell any dfference. I tried 47k and I got garbage. Fizzy and static. Not even a guitar signal, just a constant loud zipping tearing noise. So i tried also running a 4k7 resistor and a 47n cap in series from pin 2 to vr in addition to the 47k resistor, and its sorta the same. More ripping and tearing.

I just left it like that. It's switchable with an spdt. I have pin wired to the top lug and pin 1 wired to the middle lug. I put the 47k from the top lug to the bottom lug, and the resistor and cap from the top lug to vr. I wanted to be able to have a boost switch with maybe a little grit or overdrive. I dunno how to use these things.

[JKowalski]

The tube screamer has a 1k between - and output on the 2nd opamp but the first has a 4k7 resistor and a 47n cap to "vr"??

The resistor and cap to Vr on the first stage determines the gain of the stage since it is a noninverting gain stage. The resistor/diodes/cap (just pretend it is one resistor for now) across the (-) and output is the upper half of a voltage divider going from the output to Vref, the 4.7k and the 220n (again pretend it is just one resistor, no cap) is the second bottom half of the voltage divider. Since the op amp wants to keep the (-) input and (+) inputs the same, then the output has to be large enough to go through the voltage divider and get attenuated but still equal the (+) at the (-).


For example consider this:


Ignore Rz. Rf and Rin form a voltage divider from the output of the op amp to ground. The (-) input is connected to the middle of the voltage divider. Say they are both 10k resistors. That means that at (-), the signal is half of the output because it is  in a 50% voltage divider with the same value resistors.

That means that (-) is half the value at (+)! The op amp tries to keep both inputs at the same voltage. So what it does is it doubles the output voltage, so that now as it goes through the voltage divider it is equal to (+). Then it is happy. You have a gain of x2 at the output pin, since the op amp is compensating for the voltage divider going back into its (-) input. Thats a simple view of whats going on.

Here is a java sim you can play around with and change the values of the resistors to get different gains



Here's a buffer:



A buffer works the way it does because the (-) equals the output always since they are directly connected to each other. There is no voltage divider (actually you could say the voltage divider is made of a 0 ohm resistor from the output to the (-) and a infinite ohm resistor from the (-) to ground) so it doesn't have to compensate for anything and it just spits out a replica of the same signal.






You don't need that 4.7k and 220nf, they won't do anything. Consider where I mentioned the 0 ohm / infinite ohm divider. If you make the infinite ohm a value say 4.7k, what does that make the voltage divider? A 0 ohm over a 4.7k resistor will not change the gain or do anything at all. The op amp will just happily feed the output back into the (-) pin and pay no heed to the 4.7k and 220n cap. It's not a voltage divider anymore its just a resistor to ground.



The second stage is too complicated for me to explain simply...

I didn't know how many ways there were to do this. I looked this up to see what it looks like.

http://www.generalguitargadgets.com/richardo/practiceamp/practicemaneq.gif

How many other ways are there to do this

There's a lot of ways. They all have different response characteristics, so you just got to find one that you like. Treble mid and bass on different schematics can often do very different things to the frequency response. Also, you got to decide if you want it active (op amps or transistors in the mix) or passive (just the resistors and caps... and maybe inductors)

[El Heisenberg]

thanks, that was really clear. And easier to understand than what I've read about op amps before.

What i've been going for is a fender sound. I orignally had two fetzer stages with a fender stack in between. The 2nd stage wouldn't function correctly not matter what, so Im now trying to get an opamp to do what that stage was supposed to. Id rather not start all over with the preamp since im low on components, and theres a lot of hot glue and stuff in there. I went into this thinking id already built the "prototype" so it was going to be neat, and Id never have to go back in a rip stuff out/add stuff in. Ugh when am i gunna get this thing straight so I can start biulding more of these...

so how much gain do you think I should try to make up for the loss in the tone stack? Im gunna start experimenting with it, maybe tonite. I'll start with x2

[El Heisenberg]

wait, but what does the cap after the resistor to ground do when it's in a circuit? Does it send more high end - voltage to ground for extra highs at the output or something?

[JKowalski]

wait, but what does the cap after the resistor to ground do when it's in a circuit? Does it send more high end - voltage to ground for extra highs at the output or something?

Yes, exactly! Not in a buffer for the reasons explained above but in a gain stage yes. This is because you now have a capacitor on the bottom half of the voltage divider. Capacitors pass high frequencies with lower "resistance" (not resistance in a true sense, frequency dependant resistance is actually called reactance) so that means that for high frequencies the lower part of the voltage divider is smaller, making it attenuate the output more and the output of the op amp needs to compensate alot more for that! At DC and low frequencies this capacitor becomes more "resistive" and reduces the attenuation, so the op amp doesn't have to compensate as much. So you get a high frequency boost.

[El Heisenberg]

great thanks. I still havent gotten around to it. Stupid friends. Gotta hang out with them. Instead of do this stuff.


tho I'm thinking if i remember correctly, x2 gain doesnt mean x2 decibles. Is it x10 gain to get x2 db?? Or did I jus pull that number outta my butt...

[El Heisenberg]

Well i tried x10 and x2. x10 was totally unusable. I did 100k and 10k resistors for x10. x2 is a tiny bit louder than . I notice more clipping or something tho, and i don't like it. a little bit of hiss is introduced too. The cap to vr is a 47n. I kept it at x2 for now, since I can switch back to just bufer mode.

Thing is, i kinda don't like the way the op amp sounds. Its a little buzzy at x2 gain. I liked the FET stage that wasn't working correctly better. I still havent gotten any ideas on why the second FET after the tonestack would always work at aroun 1.5v only. I couldn't get it to 6 volts at all (Im running it on 12v). The problem with the sound was that I couldn't play too hard or it would fart out. Like if I had my vox wah with an OD before it, in the toe down position it would fart out. The design worked on breadboard when i first tried it out. I've built the circuit 5 times now, and rebuilt the 2nd FET stage more than I can remember to try and fix it and it always worked out the same. No matter if I used mpf102, J201, or 2N5458. No matter if I used a 1k5 resistor, or a 2k trimmer at the S. I tried 1 meg to ground at G. But every time I put a second fetzer stage after the tonestack it wouldn't bias right at D!! Could it be the transformer im running the TDA2003 and the preamp from? Its rated at 1.2A. Is it a current problem? I wish I could create another thread about this but don't wanna make the same threada 4th time.

I'm thinkin of just giving up and ordering a Dr Boogy PCB or something and running it on 9v as a preamp. Or maybe I'll make my first PCB.




EDIT:

with the op amp in gain stage mode, when i turn the treble all the way up I get moterboating and can't get a guitar signal through. It stops when treble is at about 3 o clock, but when i turn the volume up with the treble at 3oclock, i get the motorboating. But this is only when the resistor that sets the gain of the tda2003 is high (4k7). When its 1k8 I just get fizz, but its still unsable. This doesn't happen at all when I run the preamp out through a marshal cab sim into my PA. It works great. And I get a tiny tiny volume boost from switching the gain resistors on the tda2003 from high to low when playing out through the sim. The amps input is grounded when the line out switch is engaged. Anyway, all this makes me think it might be a current problem?

Another thing Im noticing about the way Im using the op amp as the recovery stage is that there is less bass. It sorta bleeds into the mid control, response-wise more than it used to when there was a [functioning?] FET stage after it.. When I switch in the 22 uf cap to ground at the send of the first stage FET, the bass volume goes up more than the treble. Im using this setting right now, the gain of the power amp low, and the gain of the first FET high. But in the fetzer article, the new fetzer design is more fender-like, which is what im goin for. So i'd prefer to get the volume from the amp and a better tone from the superior fetzer set up.



this thread about the dr boogey: http://www.diystompboxes.com/smfforum/index.php?topic=80166.0

It talkes about SRPP. I dunno what that is, but he says its self biasing. Maybe I could do something like this for the second stage. It would have more gain too. I should've been done with this long ago, but this 2nd stage had given me so much greif. I thought it'd be simpler than this.

any other suggestions would be welcome too. Thanks for being patient wit me