Faint hum in Steiner VCF - is this normal?

Scot Solida · Joined: Oct 24, 2009 Posts: 100 Location: Hutchinson Kansas

So I've built the improved version of the Steiner filter and it works very well indeed, except for one slight nagging issue. There is the faintest of sine waves (around 210Hz-250Hz at a guess) coming through, even when no signal input is present and resonance is all the way down - assuming that the cutoff is wide open. This hum is constant in pitch and is not affected by incoming CV.

So I am wondering - is this normal for this filter? I can't see it it going unnoticed - though it is very faint. Anyone game to check their Synthacon filter for me? Does it make a faint sound with the cutoff full even when no input is routed in and resonance all the way down? I've got it calibrated pretty well, i think. I've tried the various options I've seen here in the hopes that one of them would eliminate this sound, but to no avail (and the only other thread describing anything like a "hum" in the signal isn't describing the same issue, I think).

Scot Solida · Joined: Oct 24, 2009 Posts: 100 Location: Hutchinson Kansas

Ahhh. My son has just pointed out that the Synthesizers.com QPS2 power supply that was powering this module is a switching supply. When I plug the module into my big modular system, the problem vanishes. However, this module needs to work with the QPS2, so something will need to be done. Ferrite beads instead of the 10 Ohm resistors on the module's power supply seems a good start, but we maybe increasing the value of the caps?

blue hell · Posted: Wed Jan 14, 2015 9:49 am Post subject:

Scot Solida · Joined: Oct 24, 2009 Posts: 100 Location: Hutchinson Kansas

Thanks. Yeah, that's likely our next step. However, in this case, we swapped out the power supply for a linear job and the problem went away.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

You may double the value of C7 and C8 and use 100uF for C1 and C2, and add a small 100nF cap in parallel.
Using ferrite beads instead of R1 and R2 would also greatly improve the rejection of the HF noise of your switching PSU.

Using a switching PSU is OK in a modular if it is followed by low drop linear regulators which dampen the power fluctuations.

Scot Solida · Joined: Oct 24, 2009 Posts: 100 Location: Hutchinson Kansas

Thanks Yves, I'll give that a try!

JRC4558D · Joined: Jun 29, 2014 Posts: 14 Location: Portugal

JRC4558D · Joined: Jun 29, 2014 Posts: 14 Location: Portugal

romaindegode · Joined: Feb 28, 2019 Posts: 26 Location: France.

Hello

I got the same problem except that it appeared later. Suddenly this low frequency noise is here only if the freq pot is open.

I already replaced a lot of capacitors, TL074 and both freq and res pot.

I'll try the tips here but i dont understand why it came suddenly Question

I don't have any problem with the arp, ems and minimoog filter so i don't think this is a psu problem

romaindegode · Joined: Feb 28, 2019 Posts: 26 Location: France.

Ok, i replaced C7 and C8 by 100uF cap and its way much better !
Wanted to replace C1 and C2 but i think its good enough.
Don't know why this "hum" noise when almost 6 months later after the building but i m happy anyway.

I replaced also matched transistors by new one before changing C8/C7 and i think there are better matched because the vcf sound better now Smile

Love this filter !

blue hell · Posted: Sat Dec 14, 2019 10:22 am Post subject:

romaindegode · Joined: Feb 28, 2019 Posts: 26 Location: France.

Ok, thanks for the information

I use Panasonic cap and yes i'm using 50 v cap for 15v.

I don't think they had a problem because i replaced them with the same capacitance and I didnt noticed any change.

I'm very happy to have read this thread because i was about to change everything on the board .....

Anyway i'm already not understanding much simpler things, so I'm ok with my working Steiner Laughing