OP AMP big Muff: FIXED

[mongo]

I have an original BIGMUFF PI, the one with the opamps from 1977,and  I thought it was a good idea to make a clone out of it,take it  onstage and leave the original Home.


 AnalogGuru  made a schematic out of my clone wich is here:




And a pic of my ORIGINAL muff with the components marked:




 I made a PCB out of the original (wich I can post if anyone is interested), and well, it ALMOST works, unfortunately, it produces a high pitched Note  (or oscillation??)wich gets louder when I turn the TONE  and SUSTAIN pots, but I can hear the distorsion from the big muff as well, I can feel that I'm getting very very close  of making it work but...  it has become out of my knowledge to debug it.

 according to the "What to do when it doesn't work" section,  I've made some voltage readings,comparing my original's  with the clone.

 I changed  C12: 120nf to 150nf.

                  C9: 16Pf was changed to 22pf (in my original the cap is marked 160J but in the schem analogguru says  that it can be either a 150Pf or a 330P, go figure)

                   R9  is 39 ohms insted of 41ohms in my original.




   the readings I'm getting in my clone are:

IC2 RC4558:

P1 4.21
P2 4.27  
P3 4.22  
P4 ZERO  
P5 4.20
P6 4.25  
P7 4.24  
P8.51

LM741:

P1 .003  
P2 4.59
P3 4.40
P4 zero  
P5.003
P6 4.63
P7 8.51
P8 zero


DIODES:

D1 -- K 4.60   A 4.46
D2 -- K 4.46   A 4.65
D3 -- K 4.65   A 4.63
D4 -- K 4.37   A 4.60
D5 -- K 4.16   A 4.37
D6 -- K 4.63   A 4.16


the IC's  and diodes readings  are in fact quite similar to the original, so I dont where to look for the bug now, I hope this message makes sense and that  there is someone skilled enough willing to help me!!

Thanx!!

Andy

[nelson]

Ii would be really interested in the PCB layout.

If the voltages are the same as your clone, I suspect it is a problem with the PCB layout, atleast thats my limited knowledge. It could also be wires running too close together, have you tried using shielded wire? This is a problem with many high gain circuits.

[mongo]

Ii would be really interested in the PCB layout.

If the voltages are the same as your clone, I suspect it is a problem with the PCB layout, atleast thats my limited knowledge. It could also be wires running too close together, have you tried using shielded wire? This is a problem with many high gain circuits.

   The voltages are SIMILAR, for instance

pins in IC2 in the original are               P1 0.22, P2 4.64 P3 4.45 P4 Zero


Pins on the same IC on the clone are   P1 .003 P2 4.59 P3 4.40  P4 ZERO


  I wonder what's wrong... I'm sure  I dont see the bug because it is something very simple...

  A+

[analogguru]

Last week I received my Big Muff Pi Op PCB which i bought on ebay.

R9 is a 47 Ohm.

R17 is definitely a 47 Ohm.

P3 is a 100k linear taper in the original-unit.

C9 is a 150p ceramic.

Your value for C9 is much too low and probably the cause for the oscillation.

@mongo
please exchange the schematic to the newer version (1.1) which you will find in your email.
sorry about the delay, but i have a lot of stress at the moment.

[petemoore]

PM = php error...DAFTIT.
 so here's the boiled down version...
 OA Voltages look well within operational conditions  
 Oscillations...hmmm...layout...offboard wiring may cause/prevent this.
 EZ to try, may help...I tried...[what do you guys think...?]: Connect wires to a small-ish cap [like ~ .0068uf], one side of the cap connect to ground, the other side of the cap connect to different points in the circuit, along the signal path [shunting HF Content to ground, and possibly squelching the oscillation], maybe you find a place where this cap does the trick, then reduce it's value to the minimum value that does it...this will cause a touch of High end content being lost...something to try anyway...might just end the osci's and let plenty of high end through..
 Before I did that, I'd double check the LP filter cap values in the circuit. [C2, C4, C8...]
  C3 and C9, also worth looking at.

[PRR]

Welcome, Andre.

The thread was long-dead. You added --

> It may remove capacitor C3, then vysokočastotnaâ Generation collapse.

If you can post in English the discussion will be better.