New Class D amp making "back-firing" noises. What the heck is going on?

[Vasr]

I bought a multi-channel amp for surrounds recently. Sounds good and seems up to the job. But I have discovered a disturbing problem.

It is using a shared SMPS power supply between all channels.

If the unit has been playing for a while (just two channels) and I turn it off and then turn it on within a minute or two, there is a loud sharp sound or two inside the amp (not through the speakers, coming from inside the amp near the power supply area). It sounds like a sudden discharge of electricity. I will see if I can get an audio recording but it is loud enough for one to think something blew up, does not sound like a relay. But it plays fine after that.

Happens even if I have removed all speaker connections and inputs before turning on. If I wait for several minutes before turning back on, there is no such sound. Not very knowledgeable about electronics but it seems like it is related to whether the power supply capacitors still have certain amount of charge from previous use when the amp is turned back on.

If I just turn it on and keep it idle and cycle power on and off, no problem. It needs to have played some content for a while for this to occur. Don't know if this is related to heat from the playing. The amp does not seem to get hot at all while playing.

This cannot be normal. Anyone experienced anything like this ever?

 

[Sal1950]

Happens even if I have removed all speaker connections and inputs before turning on.

I could make a couple guesses about the operation of the speaker protection circuit timing but it would be just a guess.

On the other hand, it's for sure not a good idea to do what I have quoted above, depending on the amps design, that's a good way to make it go up in smoke.

 

[SIY]

I bought a multi-channel amp for surrounds recently. Sounds good and seems up to the job. But I have discovered a disturbing problem.

It is using a shared SMPS power supply between all channels.

If the unit has been playing for a while (just two channels) and I turn it off and then turn it on within a minute or two, there is a loud sharp sound or two inside the amp (not through the speakers, coming from inside the amp near the power supply area). It sounds like a sudden discharge of electricity. I will see if I can get an audio recording but it is loud enough for one to think something blew up, does not sound like a relay. But it plays fine after that.

Happens even if I have removed all speaker connections and inputs before turning on. If I wait for several minutes before turning back on, there is no such sound. Not very knowledgeable about electronics but it seems like it is related to whether the power supply capacitors still have certain amount of charge from previous use when the amp is turned back on.

If I just turn it on and keep it idle and cycle power on and off, no problem. It needs to have played some content for a while for this to occur. Don't know if this is related to heat from the playing. The amp does not seem to get hot at all while playing.

This cannot be normal. Anyone experienced anything like this ever?

Return/exchange. You have an arc.

 

[Feargal]

I have a Hypex amp which had a problem with (possibly underrated?) thyristors blowing which protected against inrush current to the power supplies on power up. This resulted in a sharp "crack" sound immediately on power up - and the blown thyristor was a sorry state. Though this was immediate on switch on, not a minute later. Uprated replacement thyristors (one on each PSU) solved it.

 

[Vasr]

The arc seems to be the most plausible explanation since this happens over and over again so not something just blowing up when that sound happens. It also makes sense that this is related to the inrush of current at power on but only when the power supply capacitors have not fully discharged? Since waiting for a while before turning it on works fine. Or is it heat related in that there is a bad pathway when that board is hot for the incoming current to arc?

In any case, I will start the process of return/exchange. Not sure if this is a defective unit (to exchange) or a defective flaw in the design itself (to just return and walk away). It serves my purpose well, otherwise.

I had offered to send this to Amir for measurement but I don't want this to potentially burn his house down. So will assume, a beheaded panther for now.

 

[restorer-john]

Where are you? US (120v) or 220V?

The SMPS likely has an automatic voltage sense with runs the PSU in voltage doubler mode for 120V. That is usually done via a relay. Depending on what charge remains in the main caps (after a short power off period), the unit may start in one mains mode and switch rapidly to the correct one with the resultant arc/pop on the relay contacts.

 

[Vasr]

120V.

This was a while ago but soon after I posted the thread, the amp made a series of firecracker sounds when it was switched on from being on standby and went dead. It was replaced with a new one by the dealer which doesn't have that problem.

Probably a bad relay.

 

[RayDunzl]

That blowed up real good...

 

[watchnerd]

Return/exchange. You have an arc.

I was going to ask if he smelled ozone.

 

[watchnerd]

120V.

This was a while ago but soon after I posted the thread, the amp made a series of firecracker sounds when it was switched on from being on standby and went dead. It was replaced with a new one by the dealer which doesn't have that problem.

Probably a bad relay.

Glad you're safe.

How's your fire insurance?

 

[Chrispy]

So what amp was it?