KLH HPS 1100 powered subwoofer. as soon as plugged into AC we get a loud hum from spkr. had unit for 8yrs.

Try the following steps to stop subwoofer hum.

1. If the subwoofer is plugged into its own outlet, plug the subwoofer's power cord into an outlet shared by the other components. Use an extension cord if necessary.
2. If the hum persists, and your system includes a cable TV, disconnect the cable line. If the hum stops, install an isolation transformer on the cable.
3. If the hum persists, disconnect the remaining component cables one at a time until the hum stops. Install a coaxial isolation transformer on that cable.
4. If the hum persists, install a line-level ground loop isolator on the subwoofer's line-level feed cable. This usually solves most hum problems.
5. If nothing helps, disconnect all the audio cables from the subwoofer. Leave the subwoofer plugged in and switched on. If the subwoofer hums, it points to a defective unit. Contact the company's customer service to technical support.

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.

Disconnect the input and see if it hums in the absence of an audio source. A bad audio cable shield or unwisely-routed audio cables will allow entrance of unwanted signals from external power sources, magnetic fields, even dimmer-controlled track lights. Sometimes, simply reversing the orientation of the ac power plug can eliminate humming.

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

Did you ever find a solution for this problem? I have the exact same thing happening with my KLH subwoofer and I'd love to know how to fix it!

Sounds like the hum might be mains hum caused by the failure of the power supply capacitor(s) in the sub. It/they will be the largest on the pc board. You will find all the details (needed to replace it) on them. If you find more than one replace both.

check and make sure the ristors are not rusted

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

just try leaving it unplugged for a while thats what i did and it fixed it

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