noise from the potentiometer

[m0nty]

I have finished building a headphone amplifier:
http://headwize.com/projects/showfil...szeke1_prj.htm
replacing r4 with an lm317 current source, as suggested in the project addendum

now, it has a good sound, but there's noise coming from the volume potentiometer: that is very low or nothing when it is full on or full off, but quite loud when the pot is about half
i tried to wire the case of the pot to ground, but it made it even worse... now I removed it, but it didn't change much
by the way, the noise is present on both channels at equal level

what should I do? i'm really thinking about removing it ...

i wired the first pin to ground, the middle goes to the amplifier circuit, and the last one is from the rca input
it's "reverse" wired, that is you turn the volume up counter-clockwise (but I don't think that's the problem)

any help?
thanks
Nicola

[m0nty]

here is a photo:

[MikeB]

You might consider oscillations, i often had that if i had no inputfilter,
the amp oscillated when turning the inputpot to half volume.
If this is the problem, it goes aways with ~33pf-~100pf cap to gnd
after R1. If it does not change with this cap, the problem is somewhere else...

EDIT: Another possibility: The resistance of the inputpot might be too high,
did you use 100k ? If yes, try a 10k.

Mike

[eds65gto]

Hi:
DC thru the pot will cause noise. Is there any?
Why did you use the little PC board on the pot? It is not necessary and the wiring would be neater without it.
What kind of pot is it? An ohmite pot is a better design.
Hope this helps
Ed

[phase_accurate]

What kind of noise is it ? Is it hum ?

Regards

Charles

[m0nty]

i tried to put a 68 pf capacitor as eds65gto said, but it was better as it was before
also, i tried bypassing the potentiometer and going directly to the amp circuit: no noise at all! sadly the volume is WAY too high, and I can't turn it down from the cdplayer ...

so, i restored the conncection as they were before, and I noticed that when i move my hands near the circuit, the noise gets somewhat louder

i don't think it comes from the power supply (it is a 15V 1.5A switching wall wart), anyway I added a 1mF cap in addition to the 0.1uF (C3) from the power supply.

any other ideas?
meanwhile, i'll try to change the psu to see if something changes

thanks for the help
Nicola

[m0nty]

i forgot, it is hum...

i just tried to run it with another "heavier" 12v power supply.
noise is somewhat lower, but still present

... i think i'll have to build a power supply too

[phase_accurate]

When the wiper is in the middle position the pot represents a source with higher impedance (0.5 times its nominal value) than the stage driving it. So the middle positions of pots are prone to capacitively coupled noise (additionally to the aforementioned oscillation possibilities).
How long are your connections ? Are they properly shielded ? What is the input impedance of the stage "driven" by the pot ?

Regards

Charles

[Black Heart]

The problem is most likely that you don't have a very good ground return on the input of the circuit since you're apparently relying on the pot and it's wiring. I'd recommend an input termination resistor be installed in the circuit from R1 to the circuit ground to assure a good solid termination without any leads involved. Something around 50K is a standard value, but it would depend on what value you're using for the pot and what kind of load impedance you were trying for. I'm kind of surprised that was left off the schematic because it's fairly standard practice. You do have C1 in the circuit so that the input impedance doesn't effect the bias, right?

Dave

[m0nty]

Phase_accurate:
i made the connections as short as I could, but using standard red/black wires ...
about the impedance of the amp stage, i don't know, however it's a mosfet source follower, so input impedance should be quite high

Black Heart
yes C1 is present
i'll try with a resistor from r1 to ground

thank you both for the replies
Nicola