EHX - Germanium 4 Big Muff Pi EC- D66 revision C  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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mr coffee
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Post by mr coffee »

Images are gone. Someone Please repost.

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Ichabod_Crane
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Post by Ichabod_Crane »

Did somebody verifiy the schematic?
Thanks.

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Matlevo12
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Post by Matlevo12 »

I have one of these on my bench.
If I find some time I'll verify what was traced.

One of the two transistors was dead on the overdrive side, I took both of them away and measured the other one.
They're NKT275, sure, but be warned, they're not the best ones (big surprise, I know !) :)

Measured with RG Keen method :
Q4: HFE 37, leaking current 0.760 A !

Might be a useful "what should I do with my crappy germanium stock" circuit, I sure do have a lot of those lying around :)

EDIT : I couldn't resist measuring the other 2 (from the distortion side), they're better.
Q1 : HFE 79, leaking current 180mA
Q2 : HFE 76, leaking current 640mA
Last edited by Matlevo12 on 23 Oct 2022, 10:53, edited 1 time in total.

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Matlevo12
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Post by Matlevo12 »

There you go. Double checked but not verified, but I'm pretty sure about it, and it does make a lot more sense than the original drawing.

The two transistors are indeed just used as clipping diodes, back to back in the feedback loop (which, according to my measurements is kind of a waste... I'll probably use those in the overdrive section).
The fact that both opamp inputs are biased to half-voltage kind of surprises me, it doesn't look very common, but I don't necessarily know much about these kinds of circuits.
The BIAS setting is quite interesting IMO, but it might have already been used somewhere else, so please educate me !
VOLT sets the voltage for everything, including opamp power supply.

Please feel free to ask me to verify anything, I'll keep the pedal for the next couple of weeks.
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