Steiner VCF and noisy res pots

[jumunius]

I've recently "finished" a build of the Synthacon VCF, and have been doing a little tweaking over the weeks. Amongst my issues was the scratchy resonance pot that some have reported. I've seen the issue of scratchy pots on the Synthacon tributes reported a bit, but never really explained with much certainty here or elsewhere, so I did a little investigation. For those unfamiliar, this is the static sound that occurs when turning the res pot on some Synthacon builds -- some might mistake this for a dirty pot, but it's far too consistent to be a pot issue.

As I happened to be emailing with David Ingebretsen about other things, I thought to ask him if he had any knowledge, since he is intimately familiar with a later Nyle Steiner VCF. He sent me his thoughts along with the attached article and schematic (with blessings to post). He happened to talk to Nyle, and sent further notes after that.

I probably can't do the electronic theory justice as to why this issue crops up in the Synthacon builds, but in a nutshell it is related to DC voltage across the pot. The original design has a 10uf capacitor from CW to wiper on the res pot (CW is positive), which blocks DC. This cap was added in later CGS boards (but not in the Rev 1 that I built). This is the most important fix, and indeed, I tried a variety of caps here and found 10uf to be the best value.

One other aspect of the original design, and one that is not represented in Ken's boards, is a 1.5k resistor parallel to the pot (in other words, from CW to CCW). This allows a path around the pot for DC. Since I've never seen this one mentioned here or elsewhere I thought I'd bring it up.

I tested the resistor fix a bit and found it improved matters slightly. My somewhat unscientific test suggested it dropped the static noise level about 2db. More subjectively it appeared to me that the static sound was also slightly less sustained when it occurred.

Now, the cap fix alone drops the res pot noise level a good 10db or so, so for most people this is pretty satisfactory. Nevertheless if you are looking for further improvement you might give the 1.5k a try for the cleanest possible result. Do note that this effectively changes the value of your pot, which might prompt other changes around the pot. In particular, for me, I further lowered the value of 2.2k resistor -- the one to ground that attaches to the junction of the 2n2222 emitter and the 1k resistor that precedes the res pot. (To optimize your resonance, you may have already lowered this per Ken's notes in one of the Steiner threads in this forum -- if you haven't already you may want something more in the 1.7-1.9k range here. I installed a trim pot as a variable resistor.)

I hope this is helpful to anyone else troubleshooting this issue. Excellent filter by the way, and so nice to have the individual and simultaneously accessible HP/BP/LP inputs.

---

N-Steiner-VCF-1974.pdf
Description:	Nyle Steiner original filter design article	Download
Filename:	N-Steiner-VCF-1974.pdf
Filesize:	722.62 KB
Downloaded:	217 Time(s)

---

---

VCF Schematic.pdf
Description:	Synthacon VCF schematic	Download
Filename:	VCF Schematic.pdf
Filesize:	274.69 KB
Downloaded:	227 Time(s)

---

[funkyfarm]

This is great ! at least for me and this old project. i will try.

[jumunius]

[unterbit]

Someone may like that nosy action of reso pot, mine I like it..) it couldn't be forgiven to cutoff!!

[jumunius]

[RSFC]

I am repairing a rev 1 board that has this scratch pot issue. I just put a 10uf electrolytic as suggested here and as is implemented on the later revision schematics on kens site. The noise is greatly reduced, however it now seems the resonance knob does very little if anything aside from adding the much reduced noise when turned. I can get it to self resonate . anyony have an update or suggestions on this mod?

[haxster]

I can vouch for the Noisy Res Pot not being a dirty or bad pot. Yes the 10uf greatly improves the noise level. Also changing those Capacitors near the Transistors (47uf) to 100uf-220uf will clean the audio signal path from hum. I just found this out this month messing around.

[RSFC]

FWIW I realized the wiring diagram on Kens site is wrong. Wire it per the schematics and implement the other noise reduction suggestions and it works out pretty nice.

[haxster]

Did anyone try the Capacitor increase mod like I suggested? It cleans up the audio path. Don't increase the 10uf on the res pot, I tried that and it does not make it better.

[jumunius]

[haxster]

Change both 47uf to 100uf.