[net.audio] suppressing turn-on surges

ark@rabbit.UUCP (10/28/83)

My Heathkit power amp (250W/channel) has two 13,300 microfarad filter
capacitors, rated at 125V and run at 100V.  Needless to say, it would
do a good job of blowing fuses if there were not some way to spread the
load at turn-on.  This amplifier uses a temperature-sensitive resistor
in series with the power transformer primary.  I think its resistance
decreases as it gets warm.  In addition, there's a relay that runs off
the DC power supply voltage.  When that voltage gets large enough that
the capacitors can be considered almost all charged, the relay closes.
This (1) shorts out the temperature-sensitive resistor so the power
line goes straight into the transformer primary, and (2) latches the
relay so that later transients won't cause the relay to open even if
they momentarily deplete the capacitors.