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?
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?
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?
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?
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)
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)
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
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
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Hi, I the diagram is not correct, there is 15K resistor attached to the wiper, to make the pot behave logarithmically, although the next stage is also 15K-ish.
I just tried it again. Moving the wiper is now fine except, when you are reaching full left, there is a "disconnect" sound as if the wiper is off the track and if you turn it fully left then there is a 100Hz hum (the PSU). The most quiet is when it is a little bit turned on, never fully turned off.
The same symptom happens on the ebay pots used in a very similar configuration like the one below: as you turn it fully left, a big clicking noise and then the hum.
I just tried it again. Moving the wiper is now fine except, when you are reaching full left, there is a "disconnect" sound as if the wiper is off the track and if you turn it fully left then there is a 100Hz hum (the PSU). The most quiet is when it is a little bit turned on, never fully turned off.
The same symptom happens on the ebay pots used in a very similar configuration like the one below: as you turn it fully left, a big clicking noise and then the hum.
Attachments
No signal is necessary to cause the pops.
On ther scope, there is also a +/-5mV 100Hz (or 50Hz, can't remember) at all times. When you pass the wiper towards the left side of the pot the scope goes crazy for a while.
In addition, the DC voltage at the base of the transistor is constant at any position, except the left-most, then the voltage drops by about 0.5-1V.
On ther scope, there is also a +/-5mV 100Hz (or 50Hz, can't remember) at all times. When you pass the wiper towards the left side of the pot the scope goes crazy for a while.
In addition, the DC voltage at the base of the transistor is constant at any position, except the left-most, then the voltage drops by about 0.5-1V.
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