EH -Big Muff Pi (1977-8)

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EH -Big Muff Pi (1977-8)

Postby Gila_Crisis » 19 Nov 2007, 15:10

i built the opamp big muff and i'm preatty happy about it.
but i got some questions:
first the 8k2 resistor before the 741, i noticed some people changed it to a 33k, cause they had some oscillation problems. i tryed this, and the pedal seams now a bit more quiter. but i read you can put in place of it a 50k pot so that you can controll the bias of the 741, is it true?

then i noticed a schem with a pair of mod about the gain pot (47ohm resistor direct to ground and a 10uF cap between the pot an pin7 of the 4558), what do change if i wire it like that? is it better?
here you'll fine the schem: http://analogguru.100free.com/193/schem ... _OP-77.gif
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Postby Gila_Crisis » 20 Nov 2007, 10:43

can someone help me please???
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Postby analogguru » 20 Nov 2007, 11:23

I thought this:
http://www.diystompboxes.com/smfforum/i ... ic=62901.0
was sufficient enough.

Ok, since I am the originator of the schematic(s):

1.) There exists two version: one ´77 and one ´78 version.

2.) The second has a switchable tone-control.

3.) both versions reflect that what I have personally traced and verified from original units(all together more than 20 units).

4.) Not every commercially available unit is constructed technically perfect, even when it is working. The drawn mods are my recommendation for new builds where one more resistor or capacitor doesn´t count.

5.) Changing the 8k2 (R10) to 33k will reduce the gain of the distortion-stage. It is a possible countermeasure against oszillation, but better is that you look for a better component placement/layout.

6.) with a 50k-pot instead of R10 you cannot control the bias, you will control the gain. If you do it, you should limit the maximum gain by placing a 4k7 in series with the 50k pot.

then i noticed a schem with a pair of mod about the gain pot (47ohm resistor direct to ground and a 10uF cap between the pot an pin7 of the 4558), what do change if i wire it like that? is it better?


Instead of "better" I would use the term: "technical correct".

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Postby Gila_Crisis » 20 Nov 2007, 12:19

ok thank you, another question i noticed some differences between the standart opamp Bm and the one built in the deluxe big muff.
here i noted those:
http://i6.tinypic.com/73jxaid.jpg

a friend, who built an opamp BM too, told me that when he tryed it near an original one, the original was less compressed and had more volume.
what could be??? we both followed your schem.
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Postby analogguru » 20 Nov 2007, 19:33

The differences between the BigMuff-OP and the Deluxe BMP cer. 1 are caused by the fact that Deluxe is using a split supply, so doesn´t need a bias-supply.

What was the "original"....any more info...guts ?
Maybe it was a transitorized BMP ?

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Postby Gila_Crisis » 20 Nov 2007, 23:50

the "original was not a transistor Bm but an opamp one! but i don't know more about that, this friend who tryed told me so.

mh.. so would it make a big difference not having that 220k resistor form +9v to pin 3 of the opamp?

and what about the resistor and the capacitor in parallel with the diodes? how do those affect the sound?
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