Adding load changing output

I recently bought a car audio kit based upon the LM1875. The kit works ok at the 100KHz I use but I have noticed something I want to understand.

I connect the kit to a +/- 25V supply through a mains transformer and whether I use a load at 4 ohms or no load at all the voltage output stays at 12Vpp

I connect the kit to two PP3 batteries supplying dual +/-9V and this time if I connect a 4 ohm load the voltage drops from 12Vpp to 7Vpp.

Why is this?

Thanks.

Hi, the 9V batteries probably can't supply enough current there for you are getting a voltage drop!

Yes that makes sense. Thanks.

9 volt batteries are horrible for hobby projects, their pretty much only useful for smoke and C02 alarms.