On Monday 10 April 2006 15:39, default wrote:

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

I have contructed a four channel power amp with a 600 VA torodial myself. I use
two special NTC's in series with the primary coil to limit inrush current. Is
that no solution for you?


Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

As I said, the fuse also blows with nothing connected to the transformer. It's
tjust the tranny itself which is causing it to blow, not even filter caps.


A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.

I thought speaker thumps were the result of the amp's inability to cope with
low voltages. A trick in poweramp design is employing a constant current
source to avoid that, if I'm not mistaken.