Snap sound on preamp powerup

trombone · 1st February 2006, 01:40 PM

Unit: tube preamp with tube regulated power supply in outboard psu.

Problem: a snap sound maybe 10-15 seconds after warmup from cold start. There are no relays in the power supply. Generally, it takes maybe 5-7 seven seconds, and then voltage appears on regulator output going up to 330 in several more seconds. Not sure if snap is coming from psu or preamp box. This has happened 3 times, but always from cold start. I'd been doing some work on the unit and if I had it up on a Variac, and later play it, the problem doesn't occur.

I'm thinking that the problem should be in the power supply, but I haven't been working there. I have been putting in new .01 bypasses to the decoupling caps, but they don't look damaged.

I see nothing burnt and have found no shorts. I have several ideas, but wonder if there are some special things to look out for, particular parts of the circuit to check (e.g., parts affected by initial voltages), techniques to try, etc.

Thanks.

pedroskova · 1st February 2006, 02:57 PM

Is this a "physical" snap coming from your preamp with the power amp powered down, or an "electrical snap" heard through your loudspeakers because your power amps are already on(a no-no)?

If it's the first, it may be arcing within or between components in your PSU...if the latter, turn your power amp on only after your pre has warmed up.

trombone · 1st February 2006, 06:56 PM

The former. I'm checking some stuff now. What a pain.

1st February 2006, 01:40 PM	#1
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	Snap sound on preamp powerup Unit: tube preamp with tube regulated power supply in outboard psu. Problem: a snap sound maybe 10-15 seconds after warmup from cold start. There are no relays in the power supply. Generally, it takes maybe 5-7 seven seconds, and then voltage appears on regulator output going up to 330 in several more seconds. Not sure if snap is coming from psu or preamp box. This has happened 3 times, but always from cold start. I'd been doing some work on the unit and if I had it up on a Variac, and later play it, the problem doesn't occur. I'm thinking that the problem should be in the power supply, but I haven't been working there. I have been putting in new .01 bypasses to the decoupling caps, but they don't look damaged. I see nothing burnt and have found no shorts. I have several ideas, but wonder if there are some special things to look out for, particular parts of the circuit to check (e.g., parts affected by initial voltages), techniques to try, etc. Thanks. __________________ Trombone

1st February 2006, 06:56 PM	#3
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	The former. I'm checking some stuff now. What a pain. __________________ Trombone

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1st February 2006, 02:57 PM	#2
pedroskova RIP Join Date: Dec 2002 Location: C'ville VA, USA	Is this a "physical" snap coming from your preamp with the power amp powered down, or an "electrical snap" heard through your loudspeakers because your power amps are already on(a no-no)? If it's the first, it may be arcing within or between components in your PSU...if the latter, turn your power amp on only after your pre has warmed up.

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