Big Muff - some reasons for the creamy flavour

[brett]

Hi
I've been through the process of making my first Big Muff Pi.  At first, I only replaced 2 of the 1uF caps with 0.1uF caps, but it was still very thick with bass.  In the end I went for all 0.1uF caps in the signal path (ie Tonepad's caps C1, C3, C4, C7 and C13).  After this, was happier*, but I still wondered what the frequency response was like.  So I measured it.

The frequency response** with 200mV p-p sine input, and the sustain and tone controls set to "12 o'clock" is roughly:
100 Hz 0 dB, 200 Hz 0 dB, 500 Hz -2 dB, 1 kHz -3 dB, 2 kHz -5 dB, 5 kHz -6 dB, 10 kHz -14 dB, 15 kHz -28 dB

Those high frequencies cuts are really wiped out.  Looking at the schematic, it seems clear that those 500pF feedback caps are totally taming anything above 5 kHz.  No wonder it sounds so creamy!

Any comments?

* the change from 1uF to 0.1uF increases the cutoff frequency in each stage from from about 2 Hz to 20 Hz.  Presumably, the main effect is on the functioning of clipping circuit, which can be affected by these sub-audible frequencies..   
**these are approximate only.  They are based on peak-to peak voltages, not true RMS values.  The waveforms were not very "peaky", so the results are probably close.

[MartyMart]

Hey Brett, that's interesting - after reading through "Yun's" Creamy Dreamer mods etc, I reduced those
500pf's ( 470's in my case ) to 180's and much preferred the slight "highs" increase.
It's still quite "smooth" sounding and I have a mix of 100n and 1uf caps in there.
All 100n's ( like the triangle ) just didn't do it for me !

MM.

[brett]

Hi MartyMart
That's a good idea with the 180 pF caps.
I'm going to try 1 x 470pF (forst stage) plus 2 x 220pF (distortion stages).
cheers

[mac]

What did you use in the tone control?



mac