I built the Eurorack version of the Mini Moog Filter clone created by Gerbrand Sterrenbur based on the electronic schematic created by Yusynth.
I would like to share my thoughts and ask for help with a specific issue with resonance.
I built a total of three units, two of which are fully functional and one of which the resonance does not work. The resonance potentiometer cuts the sound when actuated.
Comparing the two units I have fully functional the resonance is audibly different between them. Something that both have in common is that the resonance only starts to act after 50% of the potentiometer travel. This project recommends using a 50k anti-log potentiometer for resonance control, but I used a linear one.
This project recommends using 6 low-tolerance (1% if possible) 47nF capacitors, but I used ordinary ceramic disc and polyester film capacitors. This project also recommends 6 transistors matched in pairs, I made the match just by measuring the HFE of each transistor.
Now that I know how this filter sounds with ordinary components I plan to build another one with better quality components, so I ask:
Looking for high accuracy, which type of capacitor should I use, mica, polyester film or propylene?
Has anyone here already built this filter, would you know how to improve the resonance performance, making it responsive throughout the potentiometer travel?
Actually it’s just 4 47 nF capacitors, and YuSynth doesn’t say they have to be 1% tolerance, they say matched to 1%. That is, they could all be off by 5% as long as they’re all off by the same amount.
Looking at DigiKey, for mica caps at 47 nF, I see only two in stock: a 5% one at a price of $58.98, and a 1% one at $65.71. I kind of think I would not go for mica.
The only film caps at 47 nF I see that are anywhere near 1% — 3.5% and under in fact — are polypropylene. No 1% ones in stock, the 2% one (only one variety) is $3.22. Better! 3% ones are $1.15 and up.
Polyester film is 5% and upward. In quantities of 10, or in some cases quantity 1, you can get 5% or 10% polyester caps for under $0.30 each. Then you could go through them and pick out 4 that match at the 1% level. You’d be playing the odds — you can buy four 2% polypropylenes for about $12, you can get about 40 10% polyester ones for the same price, would that be enough to have 4 matched at 1%? Probably, I would guess, and then you’d have enough left over for many projects to come. But no guarantees!
Here’s an easy circuit for doing transistor matching at presumably a better level than HFE measuring:
What a great and detailed answer, it really opened my mind! And, yes, there are only four capacitors. Thank you for sharing the transistor measurement circuit, it will be very useful.
I’ve built two of these as well and have run into similar issues. Maybe we can try and troubleshoot this one together.
First off, the module just sounds really thin for a 4-pole LPF. In addition, the Cutoff & Resonance are very much affected by the Input level attenuation control. As I increase the signal level into the module (AKA decreasing the amount of attenuation), the slope of the filter seems to decrease. Like at below 50% on the Input knob, I get that very obvious carving 4-pole filter sound (even if it’s just carving noise). As I increase the Input knob setting, the filter loses its sharp slope & sounds more like a 2-pole LPF. As a result, I’ve removed both filters from my rack as they were unpredictable to patch and I have 2-pole filters that sound more like a Moog VCF.
On my builds, I used polyester film caps and did match the capacitors to <1% on a cheapo TC-1 Multifunction Tester (although I doubt the tester is really accurate down that far):
For transistors, I used BC547B instead of BC547C (it’s what I had on hand). Maybe this could be part of my issue, does anyone have experience with how hfe affects ladder filter performance? I also used Ian Fritz’s transistor matcher on ~75 transistors from the same batch, and found 3 pairs that were matched in VBE to 0.01mV (although I don’t trust my meter going down that far.)
I also tried a bunch of different CA3086s from several vendors (including a bunch of NOS ones sitting in storage at work). There may have been some difference between their performance, but not enough to make one set solve the issue.
I would love to get this thing figured out as I enjoy the sound at a narrow band of settings, but it’s just kinda useless as-is.
I scratch built this filter from your schematic I must say it is a very nice sounding filter! I built the YuSynth ladder filter after this one and to me it doesn’t sound as nice. I also had noise problems with the YuSynth transistor differential amplifier. I think I’m going to scrap it and make another one of yours - its easier to build and it works a lot better!
Note that you only have to spend about 10 minutes scrolling through posts here before you get promoted past those limits!
I agree with your remarks about filter features, and yet I have use for them, since it’s the first filter I’ve built. I now have three units made.
Unit A works as expected, even with different brands of CA3046, is pictured on the left.
Unit B has the problem in resonance, as described at the beginning of the topic, it is the photo on the right, with blue polyester capacitors (not measured).
The newly built unit C works strangely and intermittently. I ordered a GM328A component tester and will only make changes to the components after it arrives. When I do that I’ll post the results here. The good news is that I have now added antilog potentiometers and this has improved the resonance performance. Previously I had mounted units A and B and C with linear potentiometers instead of antilog as recommended for resonance.
Good to know! I’ve been lurking on here for a long time, but seeing this particular post pushed me to create an account. This place is definitely a treasure trove of DIY knowledge & inspiration.
Definitely post what you find. On a previous breadboard build of the Kassutronics ladder filter, I did notice an increase in Resonance response after matching the transistors, so it’s probably worth your time.
Also, I borrowed an oscilloscope from work, but am at a bit of a loss for what would be the best parts of the circuit to probe. Any suggestions on what to look for are very much appreciated.
This is unit C. Today when I recorded the problem I noticed that the noise is influenced by the LFO that is positioned on the right. After I turned off the LFO the bass noise just disappeared. I bet the filter is vulnerable to interference from other modules in its vicinity. In the second part of the video, from 1’53" (after the video fade), with the LFO off the low noise disappears. (only some artifacts appear in a few moments). The problem is not LFO because it never affected other modules before.
Rectifying.
The filter, unit C, worked perfectly now for about 20 minutes and even with the LFO on. After that time the low noise came back in the same way as in the first video. So it’s not resolved yet but we have good potential I would say.
Hi everyone, the transistor tester has been delivered, now I can select better capacitors and transistors for this filter. More importantly, after I changed my rack power supply I don’t hear any problems with the filters I’ve already mounted. The only thing I would like to tweak here is the resonance performance. In my three built units, the resonance only starts to act in the middle of the potentiometer travel. It would be better if it worked in a larger range.
