I am looking for input regarding a tunable bandpass filter schematic for a diy modular audio synth want to build... CV control is a bonus (else i'll adapt it). Apart from one example in my ref books, it's really hard to come by. Searches on the site haven't been helpful to me unfortunately and google has been real productive.
reqs:
-2nd order filter minimum
-inout voltage and current is whatever makes it work well (vco is open to change)
-Audio frequencies, 100 hz to 20Khz
-independent frequency and resonance controls (pots or cv)
- I presume butterworth is better (for audio, no ripple)
- bandpass, but better if it's possible to also get lp and hp signals.
also:
- filter ic's under a few dollars each can be acceptable
- better if there is no need of matched components, but ok if available on an ic.
- Reliable (does what it's supposed to do all the time, with no fuss).
I've found two quite different circuits and would appreciate discussion regarding the best one for my needs and also their pros / cons. I've not including more schematics regarding moog ladders (or variants using matched transistors (on ic) or switched capacitors but still want to know if those are better than both below.
Schematic #01
I've just built it and need to debug my breadboard: the output level is is proportional to the frequency control pot position, the input signal's frequency not affecting the gain at all when i sweep the frequency from 100 hz to 100khz. Under 1khz, with the 'gain' (via control pot!) higher, the test square wave output is a bit 'slopie', indicating low frequency loss and phase shift, but not much. it's not the coupling cap, i'll recheck later.
**broken link removed**
Schematic #02
This one uses electronic switches to precisely control rc network values. Phase shfit filter.
**broken link removed**
So basically,
1) am i looking at the right type of filters or is there a better designs my audio specs?
2) comparing pros and cons of filters above, and moog ladders + state variable filters?
Thanks!
reqs:
-2nd order filter minimum
-inout voltage and current is whatever makes it work well (vco is open to change)
-Audio frequencies, 100 hz to 20Khz
-independent frequency and resonance controls (pots or cv)
- I presume butterworth is better (for audio, no ripple)
- bandpass, but better if it's possible to also get lp and hp signals.
also:
- filter ic's under a few dollars each can be acceptable
- better if there is no need of matched components, but ok if available on an ic.
- Reliable (does what it's supposed to do all the time, with no fuss).
I've found two quite different circuits and would appreciate discussion regarding the best one for my needs and also their pros / cons. I've not including more schematics regarding moog ladders (or variants using matched transistors (on ic) or switched capacitors but still want to know if those are better than both below.
Schematic #01
I've just built it and need to debug my breadboard: the output level is is proportional to the frequency control pot position, the input signal's frequency not affecting the gain at all when i sweep the frequency from 100 hz to 100khz. Under 1khz, with the 'gain' (via control pot!) higher, the test square wave output is a bit 'slopie', indicating low frequency loss and phase shift, but not much. it's not the coupling cap, i'll recheck later.
**broken link removed**
Schematic #02
This one uses electronic switches to precisely control rc network values. Phase shfit filter.
**broken link removed**
So basically,
1) am i looking at the right type of filters or is there a better designs my audio specs?
2) comparing pros and cons of filters above, and moog ladders + state variable filters?
Thanks!
Last edited:
