I picked up an inexpensive 2x120W amplifier to drive my 8-ohm desktop speakers, and it produces a bit of a hiss, particularly when the treble is mixed further up which is what it seems to need. Is there a product or technique to put something between the amp and speakers that will filter this out?
No chance whatsoever. At the output of a power amp, "hiss" is nothing you can get rid of without getting rid of the signal in the high frequencies as well.
However, you may want to check that the amp is receiving a signal at the proper level it expects. If the signal level is too low and you compensate by turning up the amp, you'll automatically turn up preexisting hiss as well as hiss from the preamp stages. This is nothing you can fix at power stage levels.
If the amp is the source of the hiss there is nothing you can do. Some computer headphone jacks have weird noises going through them when devices are connected. But if it’s not that and the hiss is before the amp you can try this:
Make sure that the signal going into the amp is balanced and at line level or at least shielded.
Make sure your connections are proper and wire gauge is right, and that the computer and power amp have the same ground.
Is it a 50Hz humming, or just a hiss? If it is the former, see http://www.electronic-products-design.com/geek-area/electronics/audio/solving-audio-hum-problems.
Is your system grounded properly? Do you use shielded cable?
On my system, I reduce volume to a point where hissing/humming is inaudible, and increase volume at the audio source (TV, radio). I am certain some component in my amplifier (the switching mode power supply comes to mind) is misbehaving and emitting some highfrequency noise, but I don’t have a good enough oscilloscope to find the source.
Another cause can be the amplifier chip itself receiving a lower voltage than it can handle. I have some off-the-shell TDA7297 amplifier boards which hiss horribly at 12V, but this goes away when they receive 18V.
The good news is that you can play an audio file through this amplifier, and make a recording of it with Audacity, then compare it against the original. With some effort, you can learn the frequency of the hissing.
My best guess is the switched mode power supply - you will need a real linear power supply. Ask on the hamradio.SE. My second guess is too low a voltage to the amplifier chip.
Hope it helps.
It seems the most prudent approach is to go through the amplifier with an oscilloscope that can show waveforms down to 5mV.
It’s also recommended to have a central grounding point (to avoid ground loops I presume). Ground is just 0V, I would run shielded wire from all the boards in the amplifier directly to 0V at the DC voltage input.