Are you sure that's an input? I would say it may be an output for DC 9V speakers that came bundled with the card, but I really cannot say without a closer look to the card.

yes that's an output to power speakers

It was a power output...

jmarsh wrote:

It was a power output...

How was it?

It still is:

I don't see why I would damage anything by supplying the exact power and polarity that the source is outputting. Would boosting a car with another car damage the source car's charging system, or connecting two water hoses together and then having them put out the same flow/pressure into each other cause one to fail? This is just simple DC power, there's no phasing to worry about, nor is there anything drawing any current 😉

TIL the ISA bus is powerful enough to supply power to external speakers. Out of curiosity, how many mA do you guys reckon this kind of output is good for?

Jed118 wrote:

I don't see why I would damage anything by supplying the exact power and polarity that the source is outputting.

If anything, you can be right about 100% certain that the voltage from these two sources wasn't exactly the same.
Sure, the difference might be negligible and the resulting current was probably tiny, but you can't say you didn't get lucky.
I'm guessing the AC adapter output is regulated, which certainly helped. With an unregulated adapter the no-load voltage can be significantly higher than the nominal voltage.

The power adapter is from a KVM switch, I reckon it's regulated - of course it's a sealed unit so I can't check what's inside, and I don't have a scope to see if there's a sawtooth coming out of it.

Some luck (and equipment tolerance) was a factor though. 😁

*edit - I think the take-home for me would be to not assume it's an input and measure first. Also the manufacturer could have put "DC-OUT" on the I/O shield, de-mystifying the port. 😊