Loud POP at turn off, fixed by replacing filter caps. How?

[z-adamson]

Carver M200T.

Had some serious pop at turn off. No noise at turn on.

I replaced the filter caps and the problem went away.

What I don't understand is that after I removed the filter caps, I tested them for capacitance and they tested good.

So, what could have gone wrong in said filter caps that caused this problem yet without drifting any in value?

 

[jdurbin1]

You don't mention how many caps, but likely one (or one side) was leaking down much faster than the other side, leading to DC offset in the signal path that the amp's protection wasn't able to suppress quickly enough.

Capacitance is only part of the performance spectrum, have to also look at leakage, ESR etc.

John

 

[Goldie99]

Since a turn off 'pop' is specifically discussed in the service manual, and is one the service tests (it even has a spec. max = +/- 2V peak, per channel), I would assume that it's more related to the amp design than failed filter cap(s).

 

[BinaryMike]

Capacitance is only part of the performance spectrum, have to also look at leakage, ESR etc.

Right. High ESR or inductance could contribute to a burst of spurious oscillation under certain conditions, especially if the amp is known to be susceptible.

 

[z-adamson]

Makes sense.

Thanks.

 

[Pio1980]

Could also be imbalanced aged elyticaps.

 

[avionic]

Or maybe a poor solder connection.

 

[Pio1980]

I had a Carver amp with strong tired on/off thumps, they issued a mod to deal with it I never implemented as the amp sorta walked off.
Imbalanced rails from mismatched old reservoir elyticaps seems possible.

 

[kaplang]

I had that same issue with Marantz 2250. Replaced all the caps in the power supply and the problem went away.

 

[mbz]

Imbalanced rails from mismatched old reservoir elyticaps seems possible.

Yeah, strongly agree, but would have thought that would have caused a thump at p-on and p-off.
Has me puzzled somewhat, Leaning towards Goldie99's line of thought.

 

[ConradH]

Some of those Carver amps hammer the filter caps. I'm guessing "good" didn't cover all the necessary tests at full working voltage.

 

[triode17]

Excessive DC leakage on one cap. could have caused it to discharge faster than the other, causing an imbalance in the bi-polar circuits, which in turn causes a pop. By renewing the caps, you probably have re-balanced the discharge rates so the pop is eliminated.I always check caps, for value, ESR and leakage.

 

[jdurbin1]

Certainly possible the design has inherent pop problems (I don't know Carver designs at all), but going back to the possible issue with rail imbalance at power down (a more elegant summary than my post contained) but not at power on, some amps do a soft start which prevents it happening at start-up. Also if there's a protection relay it will often be set to not open until offset gets into manageable range. At turn-off, if one of the rails collapses fast enough the transient can get amplified and sent to the speakers before the relay opens.

I've also seen service bulletins tying pop and turn-off tied to poor grounding in the signal path between the power amp and preamp. This particular one (from the Tandberg TR 2075) cites flimsy, oxidized connections where the twisted signal pairs plugged in using a less than robust connector. They revised the connectors in later production so you either switch to those or get rid of the connectors and solder the cables in place. Apparently that failing connection allows the zero-crossing point in the PA to motor around away from zero V which leads to offset on turn-off that the relay can't completely suppress.

John

Yeah, strongly agree, but would have thought that would have caused a thump at p-on and p-off.
Has me puzzled somewhat, Leaning towards Goldie99's line of thought.