Author Topic: Anybody feel like helping me troubleshoot an op amp big muff build?  (Read 517 times)

doof

Hi everyone, I don't post here much, but i've been building pedals for a few years now, probably have about 40 or so under my belt, but I've had just a heck of a time with op amp big muffs.  i've tried two on Aion Electronics PCBs that failed miserably, one on vero that worked just fine, and now i'm using a pedalpcb board to build another one.  Build docs and schematic available here as a pdf: https://www.pedalpcb.com/docs/DreamFuzz.pdf

So the pedal basically works, but it's just a really thin, weak sounding fuzz or distortion, nothing at all like the real thing.

I've got an EHX op amp muff reissue, and I used an audio probe to trace the audio path on both pedals side by side, and they each sound identical all the way up to pin 2 of the 2nd IC. When i touch the audio probe to pin 6 of the reissue's ua741cp, it basically sounds like the opamp muff should. When I touch it to the clone's LM071 pin 6, it sounds very quiet and weak. I thought, that's good, I found the problem. I tried a different LM071, and same thing. I thought maybe I got some bad ICs from Tayda. I ordered some UA741CP ICs from Digikey, they showed up today, and... same crappy sound.

I tested each and every component before soldering to make sure they measured ok. I have reflowed all solder joints twice now. I'm using sockets for the ICs. Voltages are as follows:

4558:
1: 4.5v
2: 4.4v
3: 4.6v
4: 0.0v
5: 4.5v
6: 4.5v
7: 4.5v
8: 8.9v

UA741CP:
1: 0.0v (0.0v with IC removed)
2: 4.8v (starts around 4v then decreases steadily with IC removed)
3: 4.6v (4.6v with IC removed)
4: 0.0v (0.0v with IC removed)
5: 0.0v (0.0v with IC removed)
6: 4.8v (starts around 4v then decreases steadily with IC removed)
7: 8.9v (8.9v with IC removed)
8: 0.0v (0.0v with IC removed)

I've replaced the 150pf C7 cap just in case, as well as the 4.7uf C6 cap.

Between the power supply and the 1n5817, it measures 9.44v. after the diode it's 9.19v. After R17 it falls to 8.87v, then after R13 it's 4.64v, and from there it connects to pin 3 where it measures the same 4.64v.

Anyone have any ideas?  Much thanks in advance!


SOLVED:  I put a 470 ohm resistor in the negative feedback loop of the UA741CP instead of a 470k.  I'm dumb. 
« Last Edit: August 28, 2018, 12:32:23 AM by doof »

PRR

Re: Anybody feel like helping me troubleshoot an op amp big muff build?
« Reply #1 on: August 27, 2018, 10:45:51 PM »
You have a good meter.

I would expect the signal AT pin 2 of '740/'071 to be about "zero". Negative feedback through 470K cancels the signal there. The '071 may cancel a bit better then '741, but neither should have much signal here.

doof

Re: Anybody feel like helping me troubleshoot an op amp big muff build?
« Reply #2 on: August 28, 2018, 12:15:11 AM »
You have a good meter.

I would expect the signal AT pin 2 of '740/'071 to be about "zero". Negative feedback through 470K cancels the signal there. The '071 may cancel a bit better then '741, but neither should have much signal here.

Well, I am an idiot. 

Turns out i put a 470 ohm resistor in the negative feedback loop instead of a 470k.  I'll let this be a lesson to myself to never trust myself when I swear up and down i checked every single component.  Thank you very much for your help sir, it is appreciated!

GibsonGM

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Re: Anybody feel like helping me troubleshoot an op amp big muff build?
« Reply #3 on: August 28, 2018, 07:16:25 AM »
That's one of the most common kinds of error we do, Doof, so don't feel bad!  That, and solder problems, wiring switches incorrectly...you'll learn to scope those out fast as you go along!

Now you just learned what placing a 1,000x smaller resistance in the feedback loop does!  :)
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Re: Anybody feel like helping me troubleshoot an op amp big muff build?
« Reply #4 on: August 28, 2018, 01:12:04 PM »
Turns out i put a 470 ohm resistor in the negative feedback loop instead of a 470k.
Been there, done that. And will do it again :icon_lol:

Well, I am an idiot.
We all are idiots. Welcome home :icon_mrgreen: