I’m building the Kassutronics Transistor Ladder Filter and it sounds very good, other than one issue (that I’m not really sure if it’s an issue or not). With no audio input, the self oscillation range is from ~4kHz to the ~40kHz.
I’ve read that transistor ladder filters self oscillation begins around 180Hz so I’m guessing there’s something I did wrong here, but am unsure what it is since I followed the schematic to a tee and everything else seems to work fine. With audio input, the resonance is audible and sounds very good.
The transistors were hand matched. I don’t have a high-resolution mode on my MM, but the resonance peaks are symmetrical when feeding the filter a square wave, so it appears good enough.
Maybe matching the transistors even closer could fix this issue? I replaced the 1.5k resistors from U1B and U1C with 1k ones as it drove the resonance louder but it still cuts off abruptly below ~4kHz. Adjusting the trimpots does their intended purpose, but nothing to affect the self oscillation frequency.
Any help is appreciated! Thanks! Here’s a video of the issue:
Oscillation typically occurs in system that has a feedback with a gain that exceeds 1. So in your setup there must be a frequency depending factor in the feedback systems that makes the gain fall below 1 when you lower the cut off frequency below 4kHz.
Stumped as to what’s causing the gain to drop but perhaps I’ll try replacing some of the transistors to see if that solves the problem. I watched some demos of the filter and people seemed to be making the self resonance sing, and the transistors seem to be the most ‘bespoke’ component of the circuit.
I doubt it. If it was that critical then hardly anyone would succeed in getting the thing to work. Just to make sure you could check the passive components by measuring their values (instead of trusting what is printed on them).
I’ve built this filter, in the Kosmo version by @sebastian, and can verify it self oscillates down to about 135 Hz. Not very large amplitude at that frequency, but it’s certainly there.
The Kassutronics instructions point to these directions for doing the transistor matching. I wrote up the process and a stripboard layout for the matching circuit here. But it seems to me unlikely transistor matching is the problem here. More likely a component or connection problem in the feedback section, I’d guess. And it’s conceivable the breadboard connections might be somehow limiting the gain at low frequency.
