The thump probably comes because the preamp outputs are
AC-coupled through capacitors to the power amp. (There
are probably also caps in the power amp input.) Thumps
are especially likely if the preamp runs off a single supply
so its internal output is at half the supply, and relies on
the cap to block this. When you first turn it on, that internal
output point rises quickly to half of the supply, and that
fast rise is essentially AC that passes right through the cap
to appear as a thump. To prevent the thump, you'd need
a special slow-turn-on circuit design.
If the preamp has balanced + and - supplies, this is
still a possible problem if the circuit doesn't come on
in a controlled fashion.
So no, there is no simple switching arrangement to
cure this. A complicated arrangement would be to
have a time-delay such that the preamp is allowed to
power up and charge its output cap into a resistor
to ground simulating the power amp, and then after
the voltage across the cap is reduced to a very low
value the switch connects the power amp.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
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