ac to dc conversions

Will probably work if you connect the 9.6 vdc to the output of the power supply. This will often be between ground and an electrolytic filter capacitor following the rectifiers.

Some more info is needed here. The stereo might be using a half-wave rectifier running off the AC to give about +/-9V DC inside. Do you have a model number, a schematic or a picture of the insides of the stereo?

try connecting the battery to the input any way (polarity is not important as the input is ac)

the battery power will run through one side of the (probable) bridge rectifier inside. so long as the stereo will run at that level and has no specific requirement for AC all should be fine - you won't kill it whatever happens.

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A comment. A little off topic but might raise a question?

Was in a RS many years ago where an employee was openly 'butchering' the power supply section of an AC mains operated audio amplifier wiring in a small 12 volt battery charger/power supply of the type then used for 'desk' operation of mobile 12 volt CB sets.

I politely asked what he was trying to do and if I could look at the circuit schematic on the counter! He said he was trying to get the amplifier to run on 12 volts in his car!

Whereupon I noted that the 60 hertz AC to DC power supply in the amplifier had +12 volt and -12 volt outputs!

His single 12 volt power supply (only 3 amps maximum) wasn't suitable and possibly too small anyway! Often wonder what happened to that piece of equipment!