I have a fairly old amp, a Sanyo. I made the mistake of being lazy and messing about with input connections when the power was on. Needless to say something blew in the amp and it stopped working.

I took it apart and found several fuses inside had blown, which I have now replaced.

However, now only one speaker channel works correctly. The left-hand channel is fine, but when I plug a speaker into the left-hand channel, the speak emits a low, loud hun, one of the speaker cones gets sucked right in, and after even jsut a few seconds of the amp powered up like this, a smell of burning comes from the speaker...luckily the speaker doesn't appear to have been damaged by this and works fine through the other channel.

Any ideas what could be causing this?

It sounds as if one of the amplifier voltage rails is not suppling one side or the other of the push-pull driver or output stage of the affected channel. This could be caused by numerous faults in the output circuitry. IE./ destroyed resistor(s) in the driver or output stages affected, damaged transistor components (shorted or otherwise).
I would first perform visual inspection, noting and burnt components. Measurement of voltages at transistor leads of both drivers and outputs. Transistors should show fairly even plus and minus voltages. Measurement of voltage drop across emitter resistors on outputs. A high voltage drop would indicate an open or high restistance at that resistor caused by too high of current across resistor due to out device short circuit condition.
These would seem to be the most likely failure areas although their are various other possible conditions that could cause similar symtoms.
A few minutes checking voltages in these areas could be very fruitful.
If you can give me any other relivant details or possibly voltages that very different than normal, I may be able to get you closer to the problem!
Steven E. Brenner
stevenbrenne@hotmail.com

P S = Good Luck !