High pass / low pass filter like on Novation Circuit

pvision

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I've been playing with a Navation Circuit groovebox - it's a great piece of kit, easy to program and lots of fun. One of the features is a filter sweep knob, described by Novation:

The Filter encompasses both low-pass and high-pass types... Circuit’s Filter knob controls a low-pass filter when you turn it anticlockwise from the centre position and a high-pass filter when you turn it clockwise from the centre position. Note that the control has a detent in the centre – in this position, no filtering takes place

I'm interested in finding a schematic for a filter of similar design. I assume it requires a pot with a centre tap

Thanks!
 

L´Andratté

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What does the knob control, cutoff frequency? Then probably done digitally...
Or do you mean simply something like this (attachment, can be done with 2-pole too)
 

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pvision

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Yes, the knob controls cutoff frequency. It has lots of range so you may be right and it's done in the digital domain

Thanks for the schematic. I imagine that's a "Tone Control". The Circuit filter has a lot more range
 

squarewave

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If you're really serious about replicating it, you should feed white noise through it and look at the spectrum in your daw or using whatever software and see what it's doing. If it's DSP, then you could have trouble coming up with an equivalent circuit because a) it sounds like it only engages the filtering above or below the center position. Meaning when you sweep lows, the highs do nothing. A center tap might indeed help you with that but b) it could be a multi-pole filter. Each pole would require a pot section. You would need to look at the slope of the attenuation in the DAW spectrum. If it's 12dB / octave that would indicate a 2 pole filter. If it's 24dB, that would be 4 poles. Meaning you would need a 4 section potentiometer just for one half of the filter. So in practice you would need an 8 section potentiometer, all with center taps. It is highly unlikely that you will find such a beast. You would have to use switched but that wouldn't be interactive if that's what  you like about it. If you don't care so much about distortion, accuracy and noise and such, you could use voltage controlled filters. That would make things much easier. You could use two moog style ladder filters for example and then just rectify the control voltage for the high and low filters so that they only sweep above / below the center frequency. That's how a synth would do it. But there are VCF ICs that will yield better performance but it will never be hi-fidelity.
 

L´Andratté

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Maybe if one uses 2180 VCAs or Vactrols but would that be worth the hassle for avoiding having one more pot, like in the Drawmer 201 HP+LP (which are more flexible because they could be used simultaneously for bandpass or bandstop filter)...Those are dual pots for each 2pole filter but for 3 or 4pole at least such pots exist-

or use 4 VCAs for 4pole LP and pole mixing to extract 4pole HP, and use a microcontroller for the logistics?
-> hassle ::)
 

pvision

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I was looking at the schematic of the J Haible version of the Wasp filter. It has a LP output and a HP output and was wondering about feeding those to each end of a pot and taking the output signal from the wiper

Then I realised I don't know what I'm doing but it might be good to experiment!

I like the idea of getting a response curve from the filter - I'll see if that's doable
 

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