In article ,
Paul Dorman wrote:
This was used for a guitar amp that was continually
blowing slow-blow fuses instantly upon turning the amp
on.
https://www.youtube.com/watch?v=k5oNQ1etN2c
In my case, the bulb would briefly light up, and then
fade out within about a second. I read this meant the
amp was normal, and didn't have a short to ground, and
when I plugged the amp into the wall normally, I was able
to trouble-shoot it normally.
But I would assume the bulb lighting up initially, is due
to the initial in-rush current, that charges up the electrolytic
filtering caps, on the outputs of the rectifiers?
No. This is a "power-on thump" which is caused by the coupling capacitors
charging up, not the power supply.
It's made much worse with amplifiers that run on a single supply rail, so
the output of the power amp stage is sitting halfway between the supply
rail and ground during normal operation. This means there is a huge
coupling capacitor from the output stage to the speaker and that has to
charge up. While it is charging up, the woofer coil will bottom out.
Well-designed amplifiers have a protection relay that cuts the speaker off
when there is any appreciable DC offset. It will sometimes take a little
time to stabilize because of the turn-on thump.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."