[Ralph Barone[_3_]]

wrote:
Scott Dorsey wrote:

===================

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.


** The OP never mentioned any " thump" and is not talking about one.

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.


** Nonsense, speaker output electros ( rarely seen in the last 30
years) do not do that, they charge slowly.

I = C.dv/dt

if C = 2000uF and the cap charges to 30V in 0.5 second, I = 120mA.

To " bottom out " a woofer a takes several amps.


Sure. But lets say that the power supply comes up to full voltage in 1/4
cycle, which is perhaps closer to the truth. 1/4 cycle is either 4.33 or 5
ms, depending on where you live. Punch in those numbers and you get dV/dt =
30/0.005 = 6000 V/sec and I = 6000 * 0.002 = 12 A. Thatll make your
woofer move.


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.

** Direct coupled amps sometimes have such relays, a great many do not
and don't need them.

In most cases, a simple muting FET between the pre and power stage does the job.


......... Phil