[sdiy] VCF caps in modern synths

cheater cheater cheater00social at gmail.com
Sun Mar 28 15:52:37 CEST 2021


> On Mon, Jan 18, 2021 at 11:49 AM ColinMuirDorward <colindorward at gmail.com> wrote:
> In my 4pole 3320 BPF, the polypropylenes sound slightly better. The obvious difference is that at lower frequency settings, they go into self-oscillation more readily than the ceramics. To my biased ear, the ceramics make some small distortion perhaps, or for some reason add a slight edge to the sound. I did do a blind test using a 2pole 2164 SVF and the difference there is more noticeable than in any other filter I've tested. Here, the polys are a clear win. I want to use words like "liquid, juicy, lush" for the polys vs "bite, edge, grit" for the ceramics. However those are too strong words to describe a very subtle difference. I am waiting to find the right modern mono synth that I can do a capacitor "upgrade" on and test if it's possible to perceive a change in the sound in a factory-made unit. I have an uno synth on the way (was on sale for $135cad?!), perhaps I'll be lucky enough to find space inside that tiny case for four WIMAs.

The sound quality you're talking is "roughness" and is measured in Aspers.

Tiny capacitors have a very large voltage gradient across a very short
casing. They're probably self-modulating, i.e higher voltage means
less capacitance. Essentially self-filter-FM.

Look at this article:
https://passive-components.eu/dc-and-ac-bias-dependence-of-mlcc-capacitors-including-temperature-dependence/

Look at Figure 4. They took ten samples of "1uF 0603 16V X5R parts
from Vendor-F" and invariably they lose 40% capacitance at +15V /
-15V. So unless you're running your synth at low voltages, you're
running into exactly that. I believe there's a filter out there that
leverages this, but I forget what it was. There's some filter for sure
where the audio was low voltage AC (maybe 100mVpp or less) and the
cutoff was high voltage DC (0-x volts). Hell if I know what it was...
some EQ maybe? IDK

Figure 5 shows capacitors range from -40% to even -80% capacitance at
+/-15V depending on make and model.

C0G as I understand specifies capacitance retention across temperature
ranges. I don't know that they're specified across voltage ranges. A
quick look at the first C0G datasheet I could find (from AVX) shows
that they don't even specify voltage dependence.

So yeah, in this case, physically larger components are just better if
you want to stay at +/-15V. Or alternatively go to something like mV
range and have a much more linear, tiny, capacitor - but at the cost
of EMI adding a bunch of noise. Your choice. Fully-balanced synths
anyone? I mean you could probably fit two copies of the same circuit
in 0201 in the footprint that you'd otherwise be using for silver mica
or red brick wimas.

One more option is to add self-filter-fm cancellation to your filter,
and have it as part of the tuning or calibration procedure. Probably
for the best anyways.

Well, that should explain the issue hopefully. Cheers



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