EFM 4622 VCF buzzing problem

[kokoon]

building the wildcat, i'm moving on the next module, the VCF1. again, i need help from you guys.

the problem:
the filter works pretty much as it should. VC control of cutoff frequency is okay, the resonance also works as it should. HPF and LPF inputs.

but, there's a constant buzz at 100Hz (i think), and it's very audible.

here's a sample, starting with no input, sweeping the filter (with MOD channel of onboard midi2cv - i think that's responsible for the high frequency noise), adding some resonance, sweeping again... and so on, then with sawtooth LPF input, sweeps, resonance, then with HPF input...
http://www.dontlisten.com/sdiy/wildcat/VCF1_01.wav

and here are some pics of the module:

http://www.dontlisten.com/sdiy/wildcat/vcf1_pics/pic_002.jpg
http://www.dontlisten.com/sdiy/wildcat/vcf1_pics/pic_003.jpg
http://www.dontlisten.com/sdiy/wildcat/vcf1_pics/pic_004.jpg

and here's the schematic:


i have also tried CV controlling the cutoff with a potentiometer, straight off GND and +12V, the low-freq buzz is always there (but the high-pitched noise isn't, so that one's not an issue)

i checked all the resistor values (with the schematics, the BOM list is completely wrong), all the capacitor values, i have replaced the LM13600 and LM3046 and checked all transistor types (using just 2N3904 and 2N3906). everything is as it should be.

one thing i'm not really certain though, are the non-polarized capacitors. i used ceramic and copied values for the 4 caps in the filter stages from the EFM 4622 VCF schematic (now offline) - 0.1uF. this should be okay, right?

so, anyone, any ideas where to look?

thanks!

[RF]

How good is your power supply?

[Peake]

Yves seconds the power supply as a source of that very 100Hz issue:

http://yusynth.net/Wildcat/noise-module.html

"These overtones correspond to residual ripples from the +12V rail."

He evidently had this problem in the noise circuit. Hope this helps, or that someone else has a quick fix.

[yusynth]

Hi

OK I did have those problems either, first of all, forget about the HP input, it is not designed as it should and it is mostly responsible for those humming issues. It also impairs the behaviour of the filter in LP mode.
On my first Wildcat board I tried to solve this problem of humming, I added a 15V zener diode and a 100uF/25V capacitor connected between the ground (0V) track and the power track that feeds in the HP subcircuit, I drilled the PCB as close as possible to the HP subcircuit (see pictures below). This solved the hum issue alright but then I found that the Low-pass mode was not behaving as I had expected. This has to do to the very low impedance at the emiters of the transistors Q1 and Q2. This is tricky issue, I tried to change the value of capacitor C7 as explained by Osamu Oshuyama here ( http://www5b.biglobe.ne.jp/~houshu/synth/Vcfv0212.GIF ) but was never satisfied actually. My definitive solution to the problem was to... GET RID... of the HP section which was causing all the trouble. As soon as I disconnected this by removing C4 and C5 and ... SHAZAM... the Low-pass filter started behaving as expected !!!
Teherefore on my second board I did not populate the HP sub-section and the Low-pass filter behaved nicely with no hum either (no need there to add the extra zener and filtering cap). If I need a HP filtering I use VCF2 which is perfectly efficient!
By the way be sure to use a LM13600 not a LM13700, I obtained much better results with the LM13600.

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2nd-wildcat.jpg
Description:	My second Wildcat board. There I simply left the HP section unpopulated and the Moog VCF behaves nicely, no hum noise, and good dynamics.
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modif-component-side.jpg
Description:	Modification of the HP section of the Moog VCF of the wildcat to cure the humming issue. Added are a 15V 1/2W zener diode and a polarized 100µF capacitor. Note that on this photo C3 and C4 are missing because I simply deactivated the HP section that is t
Filesize:	224.46 KB
Viewed:	249 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

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[yusynth]

By the way do not use ceramic caps for the ladder.

[kokoon]

awesome! Yves, you always have the answer

i'll remove C4 and C5 and see what happens! yeah, i'm using a LM13600, everywhere on the board actually.

ceramic caps - do you maybe know what should change in sound if i change the caps to polyester?

btw - if the power supply is that dirty, shouldn't something be done about that as well?

[yusynth]