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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!

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closed as off-topic by Andy aka, Leon Heller, PeterJ, Ricardo, nidhin May 18 '15 at 4:15

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Andy aka, Leon Heller, PeterJ, Ricardo, nidhin
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ I haven't asked about how to repair the issue. My quest. was whether or not A or B may be cause to an effect. As I said, I'm new to electronics: my question looks to understand how one component and another works so I can then troubleshoot the device and find out who's the culprit. If you read my question again, you'll see that nowhere do I ask for steps to fix my amp, nor anything of the sort. I was just looking for basic info on what some components do. If this forum is not the place for basic info on electronics, if it's only open to pros, please let me know where an amateur can go. \$\endgroup\$ – QuestionerNo27 May 18 '15 at 15:20
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If you hear noise while turing a pot (for volume, eq, etc) in a traditional fully analog amp, then it's almost certainly a dud pot, and all you've done to try & nurse it back to decent performance so far pretty much confirms this. Replace it, and be happy :). Ensure you replace it with the same resistance, and the same logarithmic type (not linear). (Or if it's under warranty just have the whole thing replaced!)

The relay protection is indeed to prevent 'turn-on pop/thump' as the amp's power-supply & circuitry powers up, which can potentially damage speakers - it's a fairly common feature in traditional amps like this (except for amps designed by golden-eared monkey-beaters who'll claim it negatively impacts sound quality). The signal from the amp to the speakers goes through the relay, such that the amp's signal either gets through the relay to the speakers, or doesn't go anywhere at all. The coil of the relay is powered by the amp's power supply, but with a short delay of a few seconds to provide time for the amp circuitry to stabilise, & disconnects immediately that it senses loss of power, too (some amps can put a decent whack of DC into the speaker as the power-supply fades). This is highly unlikely to be the cause of your issues.

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In addition to a possibly dud pot, it can be caused by DC applied across the pot, possibly from a leaky coupling capacitor. Measure the DC voltage across each channel of the pot - you may find the noisy channel has some DC while the other channel has much less or none.

DC in the presence of slight variations in the track resistance will generate intermittent AC signals as you adjust the volume, which sound like crackles or thumps.

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  • \$\begingroup\$ Hello, Brian, thanks for the answer. Since, I've measured the voltage across each output channel (not exactly each channel of the pot) and have found the crackle channel to output way more voltage than the wanted max. So I'm thinking you may be on to something here. \$\endgroup\$ – QuestionerNo27 Sep 13 '15 at 20:08
  • \$\begingroup\$ ... leaky coupling capacitors was my bet in the answer ... and still is. That's about all the help I can offer. \$\endgroup\$ – Brian Drummond Sep 13 '15 at 23:01

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