Phuncgnosis again... Filter type? Cv in+signal in?!

[ExpAnonColin]

Just trying to wrap my mind around the brilliant phuncgnosis a bit more, as I sponge up the different filter and envelope follower patterns of the net.



As far as I understand so far is that...

The bottom circuit path is the envelope detector/follower, which is first amplified then rectified by the diodes.  This then (after the 100k resistor) is inserted into a filter...  with the up/down switch providing a reference for the envelope following CV signal (above or below, etc).  What doesn't make sense to me, if the envelope follower is creating the CV signal, is how is the guitar and the CV going into the same "input" of the filter?  I had always looked at filters as the guitar signal splitting off and being turned into a CV, which then controls the filter, while the input is simply being effected by the filter, but in this case there's only one spot they go.  What sort of filter is this?  Are there any other examples of it on the net?

And as far as resonance goes, would this work with this type of filter with a simple active feedback loop from the output of IC1bto the 22k resistor or so?

-Colin

[onboard]

Colin  - afai most if not all edit Mutron III, Seamoon Funk Machine, Dr. Q...edit envelope filters share this topology. Great (sometimes not so great) pains are taken to eliminate or at least minimize the amount of CV that appears in the audio - which shows up as distortion ala the rectified AC.

What gets me about the phunc is the absence of a control transistor or LDR... the 1uf to ground handles all the variable?

[Tim Escobedo]

The CV signal is injected into the signal path as a kind of fluctuating DC component, causing the LED(s) to vary in resistance, which dynamically tunes the filter. The two 680pF caps and the .002uF cap keep that DC from messing with the op amp biasing too much.

The filter itself is a Multiple Feedback Filter design. Pretty simple. These type of filters can cause volume drop problems as they are swept up in frequency, since the voltage divider responsible for doing this causes increased attenuation as tuning goes up. I deal with it by using relatively high gain setting in the filter (which also seems to make for sharper resonance) and using a small input cap (.002uF), which attenuates the bass coming in.

The most unusual thing, which I can't truly explain, is the way the CV is injected into the audio path, before the 22k resistor rather than after (as is more common in tunable filters of this type), which I did because ripple seemed less when putting the CV before the 22k resistor.

[ExpAnonColin]

So it's the LEDs.  That's what I was missing.  VERY interesting design tim.  Hats off to you

-Colin

[Maneco]

Hats off also to the "BAU WAU " envelope filter...nice design,nice sound

[Transmogrifox]

The most unusual thing, which I can't truly explain, is the way the CV is injected into the audio path, before the 22k resistor rather than after (as is more common in tunable filters of this type), which I did because ripple seemed less when putting the CV before the 22k resistor.

I can see the theory here:  The designer anticipated that the output of the previous effect, or of the guitar, is an "ideal voltage source", thus no impedance.  If you used a buffer on the input, it would indeed reduce the ripple in the audio path, as the capacitor/resistor combo would work as a voltage divider for high frequencies.  This is why it ends up being a superior configuration (the way it is). As the output impedance of the previous driving stage increases (such as using a guitar, pot, or transconductance amplifier), then the ripple also increases.  All the same, sticking the CV where it is still is optimal.

EDIT:  oh I get it now.  The designer IS Tim Escebedo.  OK Tim, now you know why you did that

[Paul Perry (Frostwave)]

Thanks for that Transmogrifox, thinking of the left hand end of the .002 cap having effectively zero impedance to ground has done my head in :x

Anyone want to replace the leds with a germanium diode stack and report?