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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Steiner VCF and noisy res pots
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jumunius



Joined: Apr 19, 2010
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PostPosted: Fri Aug 05, 2011 1:34 am    Post subject: Steiner VCF and noisy res pots Reply with quote  Mark this post and the followings unread

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

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 Filename:  N-Steiner-VCF-1974.pdf
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VCF Schematic.pdf
 Description:
Synthacon VCF schematic

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 Filename:  VCF Schematic.pdf
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funkyfarm



Joined: Jan 21, 2007
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PostPosted: Sat Nov 19, 2011 6:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Mon Nov 21, 2011 1:21 am    Post subject: Reply with quote  Mark this post and the followings unread

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


Glad to hear it might be useful to someone! Post your experiences, I'm curious if others find similar results.

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unterbit



Joined: Dec 08, 2008
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PostPosted: Sat Dec 03, 2011 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

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



Joined: Apr 19, 2010
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PostPosted: Sat Dec 03, 2011 9:59 am    Post subject: Reply with quote  Mark this post and the followings unread

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


Hmm, it just sounds like a mistake to me, like a dirty pot. But to each their own....

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RSFC



Joined: Sep 02, 2010
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PostPosted: Sun Mar 18, 2012 11:32 am    Post subject: Reply with quote  Mark this post and the followings unread

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?
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haxster



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PostPosted: Sat Apr 14, 2012 9:39 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
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RSFC



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PostPosted: Sat Apr 14, 2012 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
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haxster



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PostPosted: Sat Apr 14, 2012 12:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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jumunius



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PostPosted: Fri Jan 25, 2013 11:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

haxster wrote:
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.


Haxster, I just noticed this suggestion. Do you mean change both 47uf's or just one or the other. If so which one? Thanks!

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haxster



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PostPosted: Sat Jan 26, 2013 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Change both 47uf to 100uf.
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Yekuku



Joined: Feb 07, 2015
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PostPosted: Tue Jan 26, 2016 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

ok here is a reply 3 years later....

I am experiencing the same faint hum both on the original and a prototype that I have build, so I guess it has sth to do with the design or the components.

I did change the 47uf with a 100uf , but the same hum happens.
Did anybody find a solution to this?
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wackelpeter



Joined: May 05, 2013
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PostPosted: Fri Jan 29, 2016 9:41 am    Post subject: Reply with quote  Mark this post and the followings unread

The 10uF cap does the trick for me and greatly improved Performance...

it's described somewhere in this Forum and i propose on the Ken's site too... as far as i remember it was soldering a 10uF cap between Wiper and either the cw or ccw lug of the pot...
but i can't remember right now exactly...

wanted sometime to Change both caps too... but still haven'T any 100uF ordered until now...

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Yekuku



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PostPosted: Fri Jan 29, 2016 11:33 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks for the reply Wink
Sorry it was my mistake , I wasnt talking about the resonance noise issue but about the faint hum that a lot of users are experiencing.
With all the inputs turned off there is a faint hum coming from the output that can be sweeped with the cutoff knob.
I am wondering if the diodes should be matched, although there is no mention about it at Ken's site.
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wackelpeter



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PostPosted: Fri Jan 29, 2016 1:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

could this be noise coming from your PSU???

Did you used ferrite beads or 10 to 15 ohm resistors before the buffering caps at the power connections of the module? Also decoupling caps added to the IC's?
Also a solder pads that should be grounded getting ground?

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Yekuku



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PostPosted: Fri Jan 29, 2016 1:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmm , I can not tell if the noise is coming from the PSU, but I know that my other 20 modules do not suffer from this kind of hum. I always use shielded cables for internal audio connections. The kit was bought from Bridechamber , so everything is there, ferrites, bypass caps etc and everything seems properly grounded.
As I have noted I have also build a protoboard and it has the same behavior as the V1.3 cgs35. Also by searching for a solution I found other users that experience the same problem.( like haxster a few posts above)
This makes me think that there is sth wrong with the components or the design.
I have matched the transistors but not the diodes, do they need matching ?

RSFC wrote:
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.

Can you please clarify which connections are wrong ? does this apply to v1.3 or to a previous version ?
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wackelpeter



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PostPosted: Fri Jan 29, 2016 2:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

the diodes should not be matched as far as i know... didn't matched mine...

so maybe you should also try the solution with the 100uF instead of the 47uF caps...

btw. as you stated everythings seems to be correctly grounded, does it seem or is it? Have you measured all the Points to ground? Could also happen that a trace is broken or faulty on the PCB... this must not be visible...
even on my several stripboard builds i had meanwhile 2 times a broken trace, that was not visible and caught me to tear my hair out, until i found and fixed it... Wink
For that specific PCB i could not get much Information as i build all my modules on stripboard from the schematics...
I have build that one with the separate Inputs for each mode of the filter plus common Input...

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Yekuku



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PostPosted: Fri Jan 29, 2016 2:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

First of all, sorry jumunius for hi-jacking your thread and thanks wackelpeter for your support Smile
Well , I have been hit by the synthacon curse , this project took me years to complete.
I had been swapping transistors/resistors fixing broken traces and finally I got it working but with a light hum, I thought that the pcb might have been damaged so I built a protoboard which exhibits exactly the same behavior.
I have already swapped the 47uf caps with 100 uf but I got no luck.
Let me ask you sth Peter:
When you have nothing connected to the inputs and all the Volume input knobs are set to 0, when you sweep the cutoff do you hear anything ?
If possible , please check it and let me know thanks.

P.s. I will recheck all ground connections just to be 100% sure
Edit: yup, ground is properly connected.
I have just realized that the hum is generated by the first transistor of the LP circuit.
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