An RC filter comprising a plurality of capacitors connected to provide the capacity elements of the filter and a plurality of bipolar transistors connected with said capacitors to utilize the essentially exponential resistance of the base to emitter diodes of said transistors as voltage controllable resistance elements of the RC filter.
(I don’t know if variable capacitance diodes are practical for audio frequences; iiuc they’re usually really tiny (a handful of picofarads) and the capacitance range is limited (2-10×). Perfectly fine for high-frequency work, but would require huge resistors to get useful RC constants for audio, and won’t cover more than a few octaves. But maybe there are ways to get around that.)
Ha, I think I got the diode capacitance mixed up with the variable resistance that they show when you apply a voltage to them. So I guess the caps and the diode resistors are the RC core of the filter.
Let me know if anyone manages to get a better resonance out of the filter. I’ve kind of finished my project and don’t want to mess with it too much because it’s a bitch to take apart. My resonance pots are only good for the last 5th the pot so it would be nice to get close to self resonance
I’m going to try using the 50k pots for res as that’s what it suggests on the Ken stone version I’ll also lower the res resistance some more as I think I used 680ohm and I read somewhere to use 470ohm. Love the sound of it as it is though but it’ll be nice with a touch more res. Managed to make mine say boobies…
So I found a way to make it self-oscillate but whether it’s useable or not as it’s all or nothing.
Bridge pin 2 to pin 16 with a resistor, I’m not sure what I used I think it was around 100k
It’s always on though so it’s not really useful. Could be good if made switchable maybe.
I was trying to get a nice ping, could be possible with a variable resistor maybe.
I haven’t trued the diodes. Here’s a video of mine with blue leds. (And cv control via vactrols). Sounds great to me. Would be nice to a/b it with a version with diodes.
The bottom pot does not seem to do much, nor does the middle. Is this what one can expect from this module? I may be mistaken, but listening to the effect of the top pot your module responds more like a low pass filter (with a fixed resonance maybe) than a band pass filter. Or am I missing something?
It strikes me that quite a few contributors to this thread have build this filter and did not get it to work. It is unlikely that all of you make the same mistakes, so there must be something wrong then with the published stripboard or schematic.
So I did a search on the net for a bandpass filter and found one on MuffWiggler showing this stripboard:
Both of these are build using 2 dual opamps, but the rest should be more or less the same, give or take a few Ohms.
I think it would be wise for all of you who have built Sam’s variant of this filter and didn’t get it to work to compare them and find out what is wrong with your implementation of Sam’s variant or Sam’s variant it self.The audio examples I hear on this page: Stripboard CGS30 Bandpass Filter sound as what I would sort of expect with a band pass filter. So that is a good reference point for a working filter.
This bypass filter consists of a low pass filter combined with a high pass filter. The output on the left is a buffer with unity gain. Output 1 of the opamp on the RHS supplies the low pass filter output signal and output pin 7 of the opamp on the RHS supplies the high pass filter output.
These outputs could be made usable via e.g. a small serial resistor (1kOhm). Has anybody tried this / added these as extra outputs to their module?
I’ve just looked again at the schematics and the edits I’ve made should make it just the same.
But I would go for that small one you posted up, that looks like a good one. I’ve actually just started building 3
One thing I have done though which has made a hell of a difference to the resonance, I stupidly used ceramic caps, I just changed them to polyester caps and the res is miles better.
https://youtu.be/m1VqcSIEO8o This is it x3 programmed to do formants. It pings as well, so if you put a short sharp gate through it you get a small resonance decay. Great for drums
Not fully, I think the ceramic ones are not ideal for audio paths, they fluctuate in temperature too much and they can also act as a contact mic and pick up signals.
The polyester ones are better suited to audio signals, not sure why, I presume they’re just more stable. I used the polyester box capacitors and it’s really changed it a lot. May be worth experimenting with those caps and seeing what works best.
Oh, there is one thing you might want to consider and that is that even a-polar capacitors have a pole. That is to say, some capacitors are build by winding a 2 layers of conducting material around a layer of isolating material. This means that one side of the capacitor is on the outside and one on the inside of the roll. If you connect the capacitor in a circuit, the outside is susceptible to electromagnetic fields which may induce currents and can become part of the signal that is flowing through the capacitor. When putting such a capacitor in a circuit it is best to connect the pin that is connected to the outside layer to ground or the lowest of both voltages. I saw an interesting video about this at Mr Carlsons’s Lab. The crux is that you need to take care when using them in sensitive audio equipment.
Hey, have someone test it with ping it ? (trig the audio input, with a lot of resonance)
I think that this filter could make really nice drums sounds… But I’m so lazy to build it, I want to ear some sound exemples before…
If i wanted to make this into a dual bandpass would it just be the case of building two putting the output of one into the input of the other? Is there also a way to put a true bypass switch on it?
