Correctly applying filter formulae

[Mark Hammer]

I'm trying to put the finishing touches on a heavily modded Small Clone, using Francisco Pena's layout at Tonepad.

One of the mods is the variable LFO width mod posted there.  Another is variable blend of wet (20k resistor at point D replaced with 15k resistor in series with 100k pot).

Toggle-switch mods include dry lift for vibrato, and delay-range switch for thicker-sounding and more Leslie-sounding chorus.  The third mod switch is the focus of this posting.

I'd like to incorporate a lo-cut function that would restrict the delay/wet signal to mids and above.  This sort of mod can be found in some commercial products and is a nice feature for using it with bass or simply for making the chorus sound less in-your-face.

There are essentially two points of intervention that I can identify in the Small Clone circuit for doing this, each involving the value of a cap in the signal path.  One is the value of the cap on the output of IC1a (currently 1uf) and the other is the value of the cap on the output of Q2, prior to blending wet and dry (also 1uf).

In theory, reducing the value of either or both of these DC-blocking caps will restrict low end response.  What I'd like to know is what the low end is currently set to in each instance, so that I can figure out a priori what cap value would be a good bet for producing the rolloff I want. (somewhere in the 200-400hz zone is probably pretty good but I'll need to hear it to know for sure).

If I assume that the cap just after Q2, and the 220k resistor to Vref (4.5v) form a simple highpass filter, then the low-end rolloff at that point is set to just under 1hz., or full bandwidth, including stuff we aren't even interested in.  If I have assumed correctly, then a 3300pf cap in place of the 1uf one there now would get me a rolloff around 220hz.

So, the musical question is: Do that cap and resistor form a highpass filter to which the standard formula can be applied?

A related question is this: Does anyone out there who has either carried out a similar mod or has a product (hardware or software-based) that includes this function have any suggestions with regard to the "ideal" low-end rolloff?

[Nasse]

:? Dont have much knowledge about what you asked and dont have looked the schem (maybe later after some sleep) but I have some very fuzzy thoughts about how that rolloff or rolloff freq works "musically"

One thing confusing me with filters is that steepness or db/octave thing. Guess what you get is 6dB/okt or 12db/oktave if you fiddle with input or/and output coupling cap values.

I read Graig Andertons "Mudguard" project from some old magazine few days ago. It is just a very steep highpass filter (two opamps) that you can stick in front of cheapo digital reverb. Graig Anderton suggests that nothing but steep filter works, when you are processing "full mix" and want to put on huge amount of reverb but not for bass guitar and bass drum. Maybe phasing and chorus is made with so short delay time so more gentle filter characteristic works just fine. ANd for single instrument like guitar or bass maybe simple filtering works just fine.

6 dB/octave filter "corner freq" is maybe not very well defined. If I am right in PA speakers and bass and guitar amps they talk about -8 dB point from corossover point, like if lowest bass freqs are 8db down you can still hear them quite well. Guess you have sometimes tune low order filter stage very much "over" or "under" if you want to suppress quiet harmonics at critical audio freqs