[Photon46]

I've got a condition that's got me stumped. When our central HVAC ac compressor is running, my ACI Talisman speakers hum (the right speaker hums much louder than the left channel.)  I have turned off the speaker's internal subwoofer amps, disconnected their power cords, turned off amp & preamp, turned off the power conditioner powering the cd player and phono preamp, and they still hum! I also tried letting a good period of time go by with everything powered off to let amp capacitors discharge, still hums with everything turned off. I'm not sure how long this has been going on, as it's a low level hum that you have to listen for. HVAC system is on it's own circuit in the breaker box shared with other ac circuits in the house.

[avahifi]

Disconnect the speaker wires at the speakers.  If the hum vanishes then, the issue is pickup from the speaker wires.  Try twisting them about three turns per foot for some self shielding.

Regards,

Frank Van Alstine

[Photon46]

Well, as they say, just when you think you've seen it all...I disconnected the speaker cables as Frank suggested, and the hum remained. The speaker cones were never emitting any vibration and so I'd thought the subwoofer plate amp in the speaker was the culprit. However, with no conduit for any kind of electrical energy into the speaker, I realized I probably had to have a mechanical hum issue rather than an electrical hum. I took the speakers off their spikes and now the humming is gone. Apparently, when the compressor in the attic space cuts on, it's vibrations are transmitted through the ceiling joists to the concrete walls. The walls then transmit the vibrations to the floor the speaker spikes coupled to. I'd spent a fair bit of time moving the speakers small distances one way and the other to get them to couple to the floor really well (four coupling spikes meant I had to work at it to get a flat floor plane.)  So this was my reward for efforts to get a well coupled speaker!

[avahifi]

I always have been a bit suspicious about hard coupling speakers with spiked feet into wood floors.

My gut feeling was this was more apt to turn the floor into a sounding board than in improving speaker performance. In this case the external vibrations from the AC unit were being hard coupled back into the speakers.  Humm. 

I have had demonstrated that significant mass loading the speaker with a heavy top load, such as a padded concrete slab, does make a pretty nice audible sonic improvement.  This works by preventing the linear speaker motors from moving the cabinet as much.  Just hard coupling into the floor is probably a crap shoot.

Regards,

Frank Van Alstine

[Photon46]

Yes Frank, coupling speakers to the floor is apparently a crap shoot. By the way, my flooring is carpeting over a concrete slab. Apparently even a big, seemingly inert mass of concrete isn't immune to transmitting vibrational energy at certain frequencies.

[Mike Dzurko]

Interesting situation! Glad it has been resolved, thanks Frank for helping out!