[sdiy] VCF caps in modern synths

Brian Willoughby brianw at audiobanshee.com
Wed Jan 6 10:36:03 CET 2021

On Jan 5, 2021, at 23:56, ColinMuirDorward wrote:
> What kind of caps are the grownups using for the VCFs in their synths, especially their polysynths? 
> I've checked out pictures of voice boards in the DM12, minilogue, and prologue, and I don't see anything sticking out past the SMDs except for some electrolytics, so I assume NP0/C0G is the norm? I don't have much modern stuff to peer inside. In most of the vintage stuff that I own and have opened, I see polystyrenes. I'd love to know what some of the current, more upscale manufactures are using in their polys.
> In my own filters, I've noticed polypropylenes often make a sweeter sound than C0Gs, but not always. For eg in high distortion types like my MS20 version, I can use just about anything and it sounds more or less the same. But if I wanted to clone something like an SH-101 vcf, I'd choose polystyrenes and work from there. The behri version must have C0G though, yeah? They aren't stuffing TH parts, right?
> In the world of hifi/mastering gear, attractive looking WIMA packages, non-polar electrolytics, and the like, seem to add credibility to a product. I don't see this trend perpetuate in the commercial-synth market, despite VCF-fetishes. At least I haven't noticed any big manufacturers bragging about their caps. Can the pros design for C0G and make it sound just as good as would something fancier? Or would my DM12 sound a wee bit better if I could somehow bodge in 48 polypropylenes?

Poly caps like WIMA are going to be worthwhile for your sample-and-hold circuits, because they don't self-discharge like just about every other cap. You can get by with SMD, but poly is better - assuming that you still need fast charge times and long hold times without frequent refresh.

I don't know that poly caps have any benefit for audio filters. Of course, that doesn't mean there aren't benefits.

As for Behri, I would except they're too cheap to even pay for NP0/C0G. Just knock on one of their synths to test whether any microphonic noise appears in the output. Then again, such a test might not work because Behri build quality is so poor that knocking the synth might break it long before you have time to gather reliable data on microphonics.

Other film caps might be good for filters, but I suspect there's a strong design to use SMD to meet the price points that consumers are willing to spend for.

Sadly, I've not done any independent research on the topic.


p.s. I can, however, quote some other folks...

Paul J. Stamler says:
polystyrene best, or polypropylene, for sound quality.

Ceramic discs were among the first small capacitors available, and are microphonic. Stacked film or polypropylene can replace ceramic disc power supply bypass, but keep the ceramic in case the new caps are not as effective. Mica capacitors are older than ceramics, and behave like them. Paper should be replaced with polypropylene, except in guitar amps where the coloration is desired. Polyester "Mylar" (DuPont) replaced paper, "film" usually means polyester, but should be replaced with polypropylene or (if small value) polystyrene. Stacked film is not as good as polypropylene, but stacked polypropylene is expensive when available.

Polypropylene: Panasonic 50V ECQ-P to 0.47 pF. They're larger because they're inefficient. Available in two types: film and foil, and metallized. Some listeners say the smaller metallized don't sound quite as good (the author says he's agnostic). Avoid metallized in high-voltage tube circuits.

Polystyrene are best, Teflon if you can find them in small quantities. Only to 0.05F (is that pF or 50,000 µF?). Mallory and Mial available from Newark. Parts Connection and Welborne Labs carry other polystyrenes.

X7R are nonlinear and have microphony

Scott "Old Crow" Rider says:
Avoid mylar and tantalum. Replace mylar with polystyrene or polypropylene. Use polypropylene in the most tolerance-critical applications: oscillator timing, tuner filter elements, et cetera. Polystyrene can be used most everywhere else for things such as replacement S/H caps, op-amp circuits, et cetera. Replace tantalum (e.g. Prophet-5) with aluminum electrolytic parts of the same value. Tantalum have very low WVDC and are the first to die if voltage spikes make it past the power supply defenses. In general, replace old aluminum electrolytic with new ones. Replace the usually 0.1uF ceramic disc capacitors with monolithic dipped-ceramic parts.

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