Volume pot has rustling sound

[Charles Wasserf]

Hi,

Just finished building an RH84 which sounds great, but I get noise from the pot in the middle of its range. The amp is nice and quiet with the control in the min. or max. position but anywhere inbetween produces a not very loud, but annoyingly noticeable type of crackling/rustling sound. Any ideas as to what this could be. I built exactly as per shematic.

Regards
Charles

 

[Mooly]

Is the noise present only as you turn the pot ? or do you mean the amp becomes noisey when the pot is set and left alone.

If the former it's often caused by a slight DC voltage on the pot... even a few milivolts makes them noisey... a coupling cap maybe ?

 

[Charles Wasserf]

Hi Mooly,

The strange noise happens when the pot is left alone in any position other than min. or max. It sounds fine, just with this intrusive ' rustle of leaves'

 

[Mooly]

That sounds strange... the circuit I'm looking at is here,

RH 84 - Tube Audio ...... RH DESIGN

so I'm assuming you have connected the pot wiper "direct" to the grid.

What happens if you replace the pot with two fixed resistors ? equal to the "setting" of the pot in circuit.
What happens if you AC couple the pot?
Stereo... two channels ? Are they both the same ?

 

[Jaap]

bad tube ?
bad solderjoints ?
leaking coupling capacitor ?

 

[chris661]

Sounds to me like a dirty track. Happens to me quite often (says something, doesn't it?)

Switch the amplifier off, and keep turning the volume up and down. When you're sufficiently bored, switch it back on, see if there's any improvement.

 

[tangmonster]

My input:

Get contact cleaner ans spray it in the pot and turn it a few times. IF that does not solve you problem I am willing to bet lots of rands (ZAR) that it's somewhere else in the circuit. Some ground issue.

probably of no use:
I had a problem once when designing a filter circuit for an amp. The amp was a test unit with no input DC blocker cap. The Op amp circuit that I was designing had a design error where the input audio had no ground reference. Things worked fine for some reason , but as soon as you touch the pot on the amp it sounded like rustling of leaves en speaker moved in and out vigorously.

 

[Charles Wasserf]

Hi guys,

Yes the pot. wiper connects to the grid stopper resistor 330 ohms which is soldered on a very short lead to the grid.

I don't think its caused by a dirty or faulty pot. because as I approach min. or max. the problem completely dissappears. I tried some switch cleaner and rotated the knob PLENTY. The noise is at its worst from about the 9 to 3 o'clock position.

I retouched all solder joints with no change. The amp is dead quiet otherwise.

???

Charles

 

[Charles Wasserf]

Hi Mooly,

I have not tried replacing the pot. with fixed resistors as I don't believe it to be the problem. Its new and the problem is common to both channels.

What I failed to mention is the 'scope shows assymetrical clipping on both channels with the top of the wave clipping gracefully, but the bottom looks terrible with a sharp lower edge and then a convex curve before becoming a sine wave again.

 

[el156]

Hi Mooly,

I have not tried replacing the pot. with fixed resistors as I don't believe it to be the problem. Its new and the problem is common to both channels.

What I failed to mention is the 'scope shows assymetrical clipping on both channels with the top of the wave clipping gracefully, but the bottom looks terrible with a sharp lower edge and then a convex curve before becoming a sine wave again.

I Think you need a decoupling capacitor in the input of the amp ,because you may have some DC present in
the input! Test 1MF/100V!
good luck

 

[Mooly]

If you disconnect the "top" of the VC leaving just the wiper and ground connection does the noise still appear as you turn the pot ?

el156's suggestion is good... try it. Does adding the cap alter the clipping behaviour ?

 

[Charles Wasserf]

I will try the points mentioned but will only be able to do so in about a week. I will report back.