a big muff pi mod I don't quite get

[mordechai]

I was looking at an Premiere Guitar article on good mods for the BMP, and I came across this schematic:

http://www.premierguitar.com/Stream/StreamImage.aspx?Image_ID=D49C8785-E727-4F33-8051-9B01155F0683&Image_Type=image

At the far right, there is an additional cap included (C16) placed in parallel with the 10K emitter resistor on Q4; in the article, it states that this cap should be paired with a removal of the 39K resistor (R1) at the input of the circuit.  What is this cap doing that makes it beneficial to the mod?  How is it interacting with the removal of R1?

[John Lyons]

The cap will cut a little hi end.
Taking out the 39k will add a little gain.
So taking out the resistor will add a bit more
noise and adding the cap takes a little noise out. 

[mordechai]

Thanks John.  Why would the cap have to go THERE, though?  What is most effective about it straddling the 10K resistor at that point in terms of cutting some high end?  Couldn't high end also be cut by increasing the 100pF resistor at the input of the circuit to something higher, like 150pF or even 220pF?

[Kesh]

i think they want want to cut the high frequency harmonics introduced by the effect, not the highs of the input.

[John Lyons]

There are a few ways to cut high end but the hiss at the end of the circuit is the most noticeable
since the gain is the highest there since it has passed through the most stages.
There is very little his as the input of the circuit and the muff isn't that prone to radio and
noise pickup at the input. Try it at the input and see what you get vs at the end of the circuit.
100pf isn't going to be noticeable though.