In article , Eiron
wrote:
Have we all stopped arguing? May I ask a question? What's the best way
to check if a power amp is likely to be stable into a capacitive load
such as an ESL? I could put a 2uF cap on the output and look at the
output on a scope while feeding it with a square wave. But is it likely
to damage anything if the amp fails the test?

The old test used to be something like a 2.2uF cap. But TBH you would be
better experimenting by trying various capacitances, and some inductances.
I've know amps that were stable with 2.2uF but oscillated with 10,000pF.

What happens will depend entirely on the amp design. So unless you have
other info the only way to know is to experiment.

If in doubt you can try using a small fuse in the output. Or, better, to
put smaller dc fuses inside on the rails. They may blow needlessly, but
might also save the amp if it would otherwise damage itself. However if it
is OK, you may then need to re-test without the output fuse as that can
alter the loading it saw!

One old Rotel amp has a warning in the manual against capacitive loads,
which is a pity as it is a nice, low-power amp and not likely to make
sparks if the kids turn it up to 11.

If in doubt, put a low-value resistor in series with each output. Say 1.5
Ohms. Alternatively, if you know the amp lacks an output network, add one.
These changes may alter the frequency response, but will help avoid
instability!

Slainte,

Jim

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