Aleph 5 turn-on thump - only the brave should read

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I was hoping to get some guidance in debugging my completed Aleph 5 channel. I've built a total of 4 channels by now, and this is the first time I'm coming across this issue.

When the AC is first applied, there is a turn-on thump (lasting about half a second and sounds like a DC pulse going to the woofer). There is also one of similar duration and amplitude when the amp is turned off.

This has also occured on my other right channel PCB and never on my left channel PCBs. The netlist of both channels are identical and pass the DRC and ERC on the layout software. Any ideas on the possible causes of this thump ?

The new problem is that this same turn-on thump now occurs on large bass transients as well. This is happening at around 10-20W of output power. I suspect that the current limiter may have something to do with it. It seems as though the current source shuts down and then turns back on everytime >10W of power is expected from it. Is possible that the current source limiter can exhibit this symptom under certain conditions ?

The bias in this faulty channel is also lower than the working channels (1.4 compared to 2A). Could an out-of-spec ZTX450 be resposible for this behavior and the thump ? I populated the resistors correctly, but there is a slim possibility that the resistors are out-of-spec. Which are the first transistors/resistors that I should check ?

One other thing I noticed about this faulty channel is that the DC
offset is low at 3.4mV, but rather unstable. It often creeps up or down between a range of +15mV to -10mV. What could be the cause of DC instability ?

My IRF240s are matched to 0.014 (verifed after warming up) and the 9610s are also matched to that spec. Other than the turn-on thump and bass transient thump, the faulty channel is hum-free and sounds (as far as I can tell) like any other channel up until the bass transients are large enough in amplitude to cause the thump.

Any ideas will be appreciated :)
 
The one and only
Joined 2001
Paid Member
Ordinarily the transient is about a volt or so.

If you have a cap in the feedback loop of the amp, it
will store a charge on turn-off if there is not a DC resistance
to the load. The cure for this is to either have some DC
resistance to the load, or place a resistor, say 20-40 Kohm,
in parallel with the cap to bleed it off, and put some time
between turn-off and turn-on.
 
Thanks for your reply, Nelson !

I really don't expect the One and Only to help me debug this circuit, but it's always good to know he is supportive :)

For the other elders on this forum (and Nelson, of course), I'm at a loss on this one and I just need some ideas to start debugging with.

This thump is occurring during normal operation. The clipping behavior is really strange. The working channels clip like any other amp, but this faulty one seems to turn off the current source when it is stressed. When the input is large bass transients, the channel appears to switch off and then switch back on.

I suspect that the channel is unstable. The DC offset is also very low (4mV), but drifts within a 30mV range about GND when measured.

Any help will be *very much* appreciated !
 
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