I have a KLH ASW12-200B subwoofer and when plugged in and turned on the light is on but no sound comes out. how do i know if the amp is bad?

I would check the caps on the power supply. They are rather large electrolytic capacitors that will look like they burst and/or leaking. When these die, the caps ground and the sub will make a loud buzzing sound. If you are handy with a soldering iron you can replace them yourself. Replace them with the same voltage or a little bigger and same with the size.

If the cap. says 16v 4700pF on it, for example, a 20v 5300pF or a 16v 6200mF cap. will work for this application.

Yes, the 10-120 can use a L & R low level subwoofer input. You can get a short 6" RCA splitter adapter from Radio Shack for a under $10. (1 female input, 2 male outputs) Alternately, just plug the subwoofer output on your receiver to just the low level R channel on the KLH sub.

The transistors at the output stage of the sub amplifier circuit might be shorted so each time you power on, the circuit draws a very high amount of current which causes the fuse to blow. It's definitely a problem with the circuit so using a higher current rated fuse would probably just result in damaging the amp even more. You should ask a qualified electronics technician to take a look at it. I hope you can fix it.

If it has a 3 prong plug, try a 3 prong adaptor - where only 2 prongs are plugged in. Sub's should not be grounded. If it still makes a hum noise, the amp circuitry may be compromised. Replace the amp with an equal at www.partsexpress.com

If you need further help, I’m available over the phone at https://www.6ya.com/expert/dan_77d17b8918f54455

Before you go throwing money at this problem, try to eliminate any external influences like track lighting. I myself am trying to figure out why mine makes a buzz throughout my system when the lights are on. I already replaced the variable light control with no improvement.

Bad cable ground can let airborne electronic noise into the Sub's amp.

Sounds like the grounds are screwed up. Try using a grounded wire for the subwoofer input. If that doesn't work, it sounds like something is internally screwed up, like the power supply doesn't fully power down. I guess besides buying a new amp, just connect it to a power strip where the switch is accesable and shut the sub off with the power supply.

Well, I will try and help. If there was a problem with any of the innards, ie your amp. It's most likely that the red light would come on ar some horrible sound would be emitted from the woofer. reguardless if there was any input. But let's troubleshoot this anyways.

If you are using the low level input (RCA cable) try hooking it up to something else, if you can. This will determin if it is at all the sub or if it's your amp. Then try using the high input. This requires the speaker terminals in the back. If it works, then we know there is something wrong in the path of low level inputs. If it does not, then we know it has to do with either the connection terminal or something further inside.
You can open your amp and check the solder at all the pin contacts in and around the inputs. If they appear to look stable and sound (no breakage or cracks) start looking at the circuit boards for tall tale signs of anything burnt. If you know haw to use a multi-meter, I would suggest using it and start at one end and check everything. Even though they may not look damaged, a resistor can become bad. If the Capasitors are bad, there will most likely be some spillage coming from it. Becareful and unless you have done repair work before. Don't attempt to replace it. If you put it in wrong and get a reverse charge, it could/will explode.
Aside from that, I don't know what else to tell you. Hope it helps.

make sure you have the speaker connection right if you do then it is a ground problem try plugging it in a different outlet

I realize that this is an old problem, but folks are still finding it on searches, so here goes.

Here are the two pages of schematics from KLH. I wrote to their tech department and explicitly asked for permission to share them here.

Here's the lower board, which has the power supply:
KLH ASW 10-120 subwoofer schematic - lower board


Here's the upper board:

KLH ASW 10-120 subwoofer schematic - upper board

Master witch is blown. Also the frequency control swithe is probably gone. No replacement parts available. contact KLH at 818-767-2843