Potentiometers

[akis]

Some time ago I bought a bunch of pots from ebay, metal body. Almost all of them, except perhaps one or two, make weird loud noises while being turned. On the most-left position they make huge noises as if the wiper loses contact with the track.

So I thought this was a bad decision to buy pots from ebay, so I bought a bag of new pots from Farnell. I placed 5 on a new pre-amp, and all but one of them (which is really quiet), make similar scratching noises, as well as the master volume pot, when turned fully to the left making huge crackling sounds.

Is it my bad luck with pots, or am I missing something?

[nigelwright7557]

Farnell usually supply decent gear.
Do you smoke or have a dusty environment ?

Scratching is usually foreign matter on the tracks causing the wiper to leave the track momentarily.
Have you tried a contact cleaner ?

Might just be your amp is very sensitive to noise.

[BinaryMike]

Scratchy noise often indicates that DC current is flowing through the pot. Not good. Generally caused by leaky coupling capacitors.

[benb]

LOUD noises are likely caused by DC on the pot, either the "input" side or from the circuit the wiper is connected to. Any slight disconnection of the wiper when sliding causes a voltage change proportional to the DC applied, and it can be many times the signal level. Such DC is usually from a leaky capacitor either on input or output, but it could also be from a bad design such as using the wiper as a current return path, or the full resistance of the pot as an amplifier circuit's load resistance. The "fully to the left" thing makes me suspect whatever's connected to the wiper.

If it's simply a bad pot with no other problems in the circuit, it should make very little or no static with no signal, and static that's no louder than the signal when there is one, or the signal seems to have dropouts when the pot is adjusted. Putting DC through a pot amplifies and exacerbates any 'scratchiness' problem a pot might have.

Can you post a schematic where the pot is used?

[akis]

No smoking. Brand new pots just out of the bag. Only 2 out of about 20 work quietly. On the oscilloscope, while moving the wiper, the screen goes crazy, when you stop moving it, you get a very clear signal at any position.

[akis]

I use them in multiple places. It must be my designs are bad.

Fo example, this one here makes very little scratching noises (if any) but an almightly one on the full left.

[djk]

Try a 1Meg or higher resistor from the wiper to ground, see if it does anything.

[Mooly]

As many say, it's usually DC that's the problem.

In the circuit you posted does it make the noise with C13 removed. It should be silent.
If it is replace C13 and with nothing connected to the input check again... should be silent.

When it make a noise use a DVM on millivolts and see if you can measure any voltage at all across the end pins of the pot... there should be none (0.00volts)

[ontoaba]

The current draw on it just too high. You need bigger wattage pots for your design, or lower the current. If your pots current is too high, your pots will not life long.

[Samuel Groner]

Wiper current relative to the resistance of the potentiometer should be minimised--make the input impedance of the following stage higher and/or reduce the pot resistance. As suggested above a higher-power part might bring some improvement as well, but I'd attempt the other two options first.

As shown the input Z of the following stage is about 13 kOhm. This will significantly alter the gain law of the pot and cause the troubles you observed. I'd say the wiper load should be at least 10x higher than the potentiometer resistance. That's 1 MOhm for that particular case which is not easy to realise if we stick to your basic circuit plan. Probably better to go for a 10k pot.

Samuel