Big Muff Pi Op Amp (solved)

[Anonymous]

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Hello everyone!

I am working on the Big Muff Pi Op Amp v4. I have “finished” the build, but the pedal does not sound right.

I have successfully built a couple of clones using PCBs before, and I know some of the basics about electronics, but I consider myself a major newbie.

I have attached a couple of images of the populated circuit board, and an audio clip of the sound I am getting.

I have built a couple of fuzz faces, and I have played around with deliberately mis-biasing Q2 on a fuzz face. The sound I am getting from the Big Muff I am working on reminds me very much of a mis-biased fuzz face. This makes me think I have maybe screwed something up with the bias of the op amps.

So far, I have used an audio probe and traced through the circuit. I am getting audio through the pedal at all expected points on the circuit.

My DC supply voltage is a steady 9.3V.

The voltage coming out of the voltage divider (R15 and R16) is 4.2V.

The voltage I am getting coming out of the output of IC2A (Vb) is 1.4V.

Is Vb supposed to be 1.4V?

A couple of other things to note. I am using JRC4558P for IC1 and IC2.

The sound at the end of the audio clip is my clean sound.

Any suggestions?

THANKS TO EVERYONE!

 

[Billy]

You’re correct Tom VB should be half power around 4.5v ish 4.2  at your voltage divider junction is fine what is the voltage on pin 3 of IC2A with the IC in and out of it’s socket

IC2A is being used as a voltage buffer sending the VB bias voltage to your op amps so I’d expect they aren’t biasing correctly

If everything is as expected at the voltage divider junction there’s not a lot between there and the input pin 3 so I’d think it’ll be either the IC isn’t securely snapped into its socket, you’ve got a dry solder joint / poor connection or solder bridge on the IC socket or the socket or IC is faulty

[Anonymous]

Thank you so much for replying!

Pin 3 of IC2A (with the IC in its socket) is 4.37V. When the IC is out of the socket I get 4.61V.

Does that sound right?

[Billy]

Yes that’s right Tom

You’ve got the correct voltage going into IC2A but not coming out if you’ve only got 1.4v on pin 1

Try reflowing the socket solder joints with the IC out of socket of course,  if that doesn’t make a difference and you have a spare 4558 try that

[Anonymous]

OK I FIXED IT!!!!

Luckily, I knew that I was likely to screw something up, so I actually purchased 2 of the PCBs. Having determined that 1.4v is NOT the right voltage, I decided to investigate the potential cause. I saw on the schematic that there should be continuity between pin 1 and 2 on IC2. I tested for continuity on the populated board. Nothing. Nothing even between the pads. I must have fried one of the pads.

I checked the empty board and there was indeed continuity between the pads. I definitely fried one of the pads on the populated board.

HOWEVER, I decided simply to solder pins 1 and 2 on the IC together and then put it back on the socket, AND IT WORKS!!

Now, Vb is 4.2 volts, like we would expect.

Now this baby is screamin’

THANK YOU THANK YOU THANK YOU THANK YOU

[Billy]

Well done to you Tom glad you got it working

[Author]

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