Ok, so this question has several layers, I admit it.

I recently acquired a Lepai 2020A+ amp. It is, I believe, the latest version PCB because it has the two relays for speaker protection instead of just the one.

I'm new to electronics, so please excuse my total lack of knowledge.

What I understand is that the function of these relays is to delay the power output to the speakers when the device is turned on; to avoid any pop that may damage the former. It may also work when turning off the device, to avoid a thump... but of this I'm not sure. It's a cheap amp with cheap components.

Anyway, I received it and I noticed the left channel had a slight crackle when I turned the vol pot. It started right when I brought it out of 0 percent. But then it started crackling at around 20, then 50. I assumed it was rust or just the protective film flaking off of the cheap pot. So I sprayed it with some contact cleaner. After much caring for it, I've been able to reduce the crackling back to the original crackling I heard when I first turned the pot. If the amp has been off several hours, then it will slightly crackle when turning the pot up from 0. If I turn the pot a couple of times before turning on the device, then there's no audible issue.

All the time, this is only happening on the left channel. The right one is 100 percent clean. Other than this, the amp works as expected.

However, the other day the amp was off and I turned the pot and heard a slight crackle. I did notice, when I first received the amp through the mail, that if turned off and disconnected, the circuit still retains some power: first thing I did; right out of the box, I switched it on without connecting it and that intensely unnerving blue light the Lepai has around the vol control came on and fade off for a couple of seconds.

So I'm still trying to make sense of this issue. Trying to know if it's a faulty, old pot that's seeing its last days on the left channel and, therefore, I should just invest in a better pot and change it (because the rest of the amp modestly rocks) or if it's more complicated than that... perhaps an issue with the left channel relay, etc., and I'd better change the device entirely.

I probably am shooting way off the mark here but the first thing I thought when I heard that crackle when the amp was off was that perhaps the speaker-protection relay for the left channel is not working. But does a relay like this only work during on and off? Or does it still work throughout the process? And, if so, does it have the ability to "filter" these noises?

I'm thinking that even if some "filter" component (not necessarily the relay; if any of you knows the Lepai maybe you know if it has any filter incorporated) is not working like it should, that horrible noise must be coming from the vol pot. Am I wrong to think that? And to think that if the right channel is completely clean, then it must be an issue with the side of the pot that outputs to the left channel?

And finally, could this noise be digital and coming from deeper in the circuitry or does it necessarily have to be analog and coming from the pot?

I know this is a layered question but I've been looking for individual answers all over the net and can't find any, so I'm hoping somebody out there can help me out by guiding me into electronics though this, my first mod project.

Thanks a lot in advance!


Question: Increasing voltage source frequency
Answer: First of all, frequency is not voltage dependent. Secondly, I'm not really sure what you want to achieve. What kind of waveform do you want to have (Pulse, Sinus...)?

If you want to have a pulse voltage output you can use 


  PULSE(0 5 0 0 0 0.5m 1m 100)


which will generate a 5V pulse voltage with a frequency of 1kHz and 50% Duty Cycle. The pulse will be repeated 100 cycles.