Capacitor order in Big Muff tone control circuit

[aion]

Does it make a difference in the Big Muff tone control circuit whether the coupling capacitor is located before or after the tone section?

Here is the original Big Muff tone control circuit:



Here is the Colorsound Supa Tone Bender tone & output section, omitting the last transistor stage of the Big Muff:



The coupling capacitor (C12) has been moved in front of the tone control rather than after it.

My specific question is, would the following modification have any electrical difference with the Colorsound one?

[thermionix]

Seems like the Colorsound order is best, it keeps DC off the tone control.  No idea if it sounds different the other way around.

[reddesert]

It's a bit easier to analyze if you consider the extremes of the tone pot position - tone pot all the way up effectively takes the low pass filter of R19-C11 nearly out of the circuit, so you just have to think about the HPF of C10-R18, and tone pot all the way down is vice versa.

It looks like when the tone pot is all the way up, in the Supa Tone Bender the HPF might be slightly higher pass because the 100 nf blocking cap is combined with the 3.9 nf cap C10 of the HPF in series, making the HPF with the 33K R18 (while in the Big Muff the 100 nf cap combines with a 100K resistor), but for these part values that's a pretty small difference.

[PRR]

There's an awful lot of DC flowing through that plan. If you turn the knob real fast it will thump.

This diverts about 10% of the DC flowing in R17 toward Q3, which slightly changes its operating point.

Why do you care which side? It's about the same cap either way.

[highwater]

Another reason why cap in-front is better: it allows you to wire lug 2 of the tone pot directly to lug 3 of the volume pot, rather than having to run all the way to the board just to go through a single cap before it runs back out to another pot.

[Mark Hammer]

The biasing of the Q4 output stage is set by R20/R21.  If the DC-blocking cap  is shifted ahead of the tonestack, then there is a path to ground that is parallel to R21 through the tone pot and R18, messing up the biasing of Q4.

In the other circuits shown, there IS no Q4, just a volume pot, so it is dealer's choice where to stick the DC-blocking cap.

[antonis]

An interesting mode for a Tone_setting_asymmetrical_Q4_clipping, Mark..

[aion]

Thanks, makes sense. I was visualizing it as distinct circuit blocks and wasn't even considering Q4's relationship with the tone control.

So, followup: How do they get away with no blocking cap in the original Big Muff? The pot still has DC voltage present, as shown in PRR's diagram. Is it because the pot isn't actually changing DC levels that it keeps from crackling?

Why do you care which side? It's about the same cap either way.

I'm designing a single PCB that supports either the Big Muff or any of the 3-transistor derivatives, and was hoping that I could get by without needing to put both capacitors in the layout and needing to jumper one or the other no matter which variant is being built. Sounds like that's going to be the only way to accomplish it though.

[antonis]

How do they get away with no blocking cap in the original Big Muff?

They don't...!!

I'm not aware of any "original" Big Muff Π without that cap..

[thermionix]

As a reference:



This has the cap after tone stack like aion's example.  It must have DC across the tone pot, but it's not scratchy, not sure why.  Could use two caps in the BMP, if it was a problem.

[antonis]

Guys, we're talking about a couple tens of μA (micro-amperes) DC current leakage from Collector through 33k+100k+33k to GND..
(maybe less than a mild-leaky capacitor..)

I was talking for C3 absence AFTER Tone pot, due to severe alter of Q1 base bias (as Mark above said)..