high pitched PSU noise bleeding through RC filter

[deeruss]

Hi,
I did a classic RC filter on a 9V PSU. It is working fine except for a random high pitched noise. It doesn't last long (approx 10s), but I can hear it randomly and regulary.

I can hear it on my sound probe whether I only plug the 9V Vcc+ or Gnd to my gnd pin probe.

Why does the RC filter let it bleed through ?
Thanks

[Rob Strand]

The PSU could be producing an enormous ripple, perhaps due to light loads.  Alternatively it's some other effect.

Try putting a 220 ohm dummy load on the output of the DC adaptor.   A dummy load can stop switch-mode power supplies squealing due to light loads.  There's a few posts about it on the forum.

[ElectricDruid]

What are the RC values and how is it wired up? Have you got a schematic of what you've done?

[deeruss]

All right.I'll try with a dummy load.

The RC is made of a 50ohm resistor with a 220uF polarized cap. I've added different non polarized cap in parallel with the 220 but the noise is still there.
Got inspired by this :


The PSU has a voltage selector (3 to 12v). Should I run a steady 9v DC unit instead?

[antonis]

The PSU has a voltage selector (3 to 12v). Should I run a steady 9v DC unit instead?

IMHO, yes..

Those (cheap) ouput selective voltage devices tend to whine due to unused secondary windings interaction..

[Rob Strand]

I suppose we don't know if the wall-wart is a light-weight switch-mode type (like a phone charger) or a heavier old-school transformer based unit.   Dummy loads only help switch-modes, since they need a minimum-load.

[fowl]

"High-pitched" suggests switch-mode to me.  A power transformer is going to tend to have 50/60/100/120 Hz noise issues.

Not clear to me how this noise is being heard.  Acoustically?  Run through an amp?  My amp will let out random noises sometimes that are related to wifi and cell phone signals.  And aging preamp tubes too.  Seems it would be constant if related to the filter section as depicted.

[deeruss]

Hello,
I did a test with a 9v battery and the noise is still there...?
It might be generated by the transistors? (both of them are : CII222Q 05 7428)

Here is the full schematic (not the best drawing I've done in my life, sorry)



The noise is a 5 sec high-pitched sinewave followed by a very short lower-pitched sinewave. It isn't regular. Kinda random.

[idy]

If you are picking up some random bit of wireless nonsense, the fix is a little ceramic cap on the input to ground, the old "RF shunt." Maybe 470pf?

Also try different guitar/cable. And try same guitar/cable into similar high gain fuzzy circuit.

[antonis]

(both of them are : CII222Q 05 7428)

CIL222 datasheet declares it as P-N-P Si transistor..

[MikeA]

Hello,
...I did a test with a 9v battery and the noise is still there...? The noise is a 5 sec high-pitched sinewave followed by a very short lower-pitched sinewave. It isn't regular. Kinda random...

This sounds suspiciously like the noise a nearby digital device (cell phone, pad, tablet, laptop, etc.) can generate in an audio circuit.  If so, you can make the interference change by moving the device closer or farther away.

[deeruss]

Thank you all, noise can be heard all over my place, but I'll try the pedal in another place. I'll test it out when I'm done with the final assembly, I'm tired of breadboarding it...
I added a ceramic cap at the input. (10nF)

(both of them are : CII222Q 05 7428)

CIL222 datasheet declares it as P-N-P Si transistor..

Are you sure ? I did a test with a LED and a resistor, it acts like a NPN. It's CII not CIL