EH Big Muff Pi (Op-Amp) schematic (preliminary) posted

[analogguru]

PRELIMINARY VERSION, BUT WORKING.
(I don´t believe what I haven´t seen with my own eyes.)

You can find it at :

www.anubics.com

enjoy it, analogguru

[ErikMiller]

Gottfried, can you tell us how yours differs from J. Philpott's and Tobias Karlsson's?

I'll give it a shot as soon as I clear a breadboard! Thanks!

[petemoore]

I've been waiting to compare the transistor BMP with the opamp Big Muff!!! Clone Transistor version works...I dunno maybe kinda mukkey...didn't really remind me that much of the old EH unit
 Built a 'OA Big Muff' [don't remem zackli which one] but it was a nogo...
been a while, my debugging techniques have improved...lol
 I remem the BM [which one I'm clue less] as being super compressusstainney, with a lotta 'velveeta' Fuzz...fun as heck to play on, maybe too 'signaturey' to use as a staple sound.

[analogguru]

My intention is to provide as most authentical information as possible.
What everybody does with this information is up to him.

This is a preliminary version until i get mor informations (maybe the promised pictures of inside views)

The information I have are:

Justin Philpott´s schematic
Tobias Karlsson´s schematic and
a picture found on the web (2, I added the components in pink)

As you can see in the picture (2), the REAL unit does NOT use IC1a as a buffer.  (Does not make sense to have a buffer with a gain of 6 (~15 dB))
The input is fed directly to the footswitch via the pcb.

IC1a works as a gain stage, gain = 6.
IC1b works as a resonant 12dB LPF at a corner frequency ~ 1kHz.
you can find a frequency plot under: (LPF).
This stage makes also an amplification of about 18 dB.
For this reason I would not recommend to use a TL 072 to avoid oscillation.
EH saved a 10µF cap.  To change to my recommendation also increases stability.  Another effect is that C7 in th original configuration can be reverse polarized, depending on component tolerances (e.g. offset voltage).  Theoretically C7 sees on both sides about 4,5 V. But it can happen, that you have 4,6 V at pin 7 of IC1b or 4,4 V at pin 2 of IC 2.
Then the cap is reverse polarized, what is not very good (especially if you use tantals   )   If you change to my recommendation, this cannot happen.

R11 was chosen at EH to 820k because a 741 draws some input current.
For this reason R11 should have a lower value to get 4.5 V at pin 6/IC2.
If you use a FET Op-Amp (e.g. noise considerations) R11 should be 1M to obtain the 4,5 V.

On the original unit is also no Tone Bybass like shown in Justin Philpot´s schematic.  Also missing on this scheme a link between Pin6/741 and the connection 0.1µF/1µF of the Tone Control.

I hope this information helps you.

analogguru

[petemoore]

Im not worried about saving one cap.
 From what I gather ya put a 10uf between ground and the 47E, [none of which would be connected then to the 62k.
 And splice/insert 10uf cap in dotted line J1....I think that's it...

[analogguru]

I recommend only the 10µ instead of J1, this gives DC-decoupling.  and a conection between the 47R and gnd, a second additional 10µ would not be necessary, even contraproductive.

analogguru