DIY Stompboxes > Building your own stompbox

Boss SD-1: R7 / C4 influence

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mstoppini:
Hi,
please, does anybody know what is the cut frequency of  R7 and C4  (the couple after the first opamp)  in the SD-1 circuit (or the 1K and .22 cap in the Tube Screamer, same position)?
Marcello

Mark Hammer:
Believe it or not, this produces a 6db/octave rolloff starting at around 723hz.

The rolloff of any RC combination like this can be calculated using the formula:

f = 1 / (2pi*R*C)

R is in megohms (1k = .001Meg) and C is in microfarads.

The same formula is used to calculate rolloff when you have:

a) a resistor and cap in the feedback loop of an op-amp

b) an RC circuit, but reversed so that it makes a highpass filter (R to ground)

c) a cap and resistor in series from the inverting pin to ground in a non-invertig op-amp

...and many many more.

mstoppini:
thanks.
Wow. Does it mean sd-1 cut frequencies under circa 740 Hz with R6/C6 in the feedback loop and over 732 with 10K e .018 uF at the  opamp output?
It sounds a bit weird... I'm going to work around it with different values.
M.

Mark Hammer:
Read the paper on "The Technology of the Tube Screamer" at www.geofex.com  It will explain the role of the various lowpass and highpass filter schemes.

One of the things it will address is the logic behind having such a low lowpass rolloff.  The purpose is to complement the highpass filter used in another part of the circuit.