Originally Posted by TrueFoe
I have run into a real headscratcher. I have a pair of Definitive Technology STS speakers (with built in subs) and am running into troubleshooting issues and running out of ideas. I have been experiencing a hum that I cannot get rid of in one of the speakers. The other is working fine. I thought it was a ground loop hum, so I replaced the amp. The hum is still there after reinstalling.
The hum starts the instant that the subwoofer of the speakers is plugged into the wall outlet even with no speaker wire or subwoofer cable attached. The only time that the subwoofer stops humming is when the Denon receiver is turned on. The instant that the receiver is turned off, the hum reappears. The hum does change frequency when I adjust the volume on the back of the subwoofer. I also found it odd that the hum starts when taking the speaker wire out of the receiver but stopped again when I touched the speaker wires together.
Lastly, the hum changes to a weird frequency when the receiver is turning initially turning on (like a quick wubwubwub hum) which is distinctly different than the hum when the receiver is off or the speaker's subwoofer is plugged in with nothing else attached. The hum stops again when the receiver completely turns on. I am at a loss and don't know what to troubleshoot. I have taken everything else off the system besides receiver and speaker and can't figure it out. Please help. Thanks.
Although rare, this is a potential issue with every self-powered speaker. I've described this on the forums before but it is so infrequent that it is forgotten easily. There is simply an air leak between the subwoofer and the midranges. During shipping or during the service life, one of the seals has opened up. The mids are acting as microphones feeding a small signal back to the amplifier input. The first time I saw this (on a customer's ~10 year old PM700 speaker), I went through tremendous angst. I was convinced that it had to be the bass amp in the speaker. When I screwed the new amplifier in, it would start buzzing again. FYI, The factory does test every
DT power tower for this.
Symptoms: Subwoofer oscillates at a very low frequency (16-30 Hz) with 1) no input connected and/or 2) receiver/amp connected and
off. When a source is connected and
turned on, the problem goes away.
Diagnosis: 1) Short the HL speaker input binding posts (short or <8 ohm resistor). Oscillation will stop. (This is what happened when you touched the speaker wires together) or 2) Lightly touch both midranges by hand, the oscillation will stop. If the hum doesn't stop under these conditions you can disregard the rest of this post because there is another problem.
Fix: Step 1) Find the leak. Remove the lower midrange, then run the sub with bass program material and feel for an air leak. Most effective frequency is ~35Hz. Most common place for an air leak is where the mid/tw wires enter the mid box. The sealing material may fall out if the speaker is old or has been moved. On the ST series, there is also a chance the air leak is at the front bottom of the mid box where the wood box meets the Aluminum extrusion. The glue may separate from the aluminum extrusion with temperature extremes. (I've never heard of this on the ST-L as its mid box is molded with a sealed bulhead connector for the wiring).
Step 2) Plug the leak. Depending on where it is and what you can jam in there. Hot melt, Silicone RTV, Rubber Weatherstrip Adhesive all can work. I prefer to use electrical putty, crack seal, or butyl rubber putty (monkey s**t). These are easy to squish into place and do not harden over time. The putty needs to be sticky to stay in place.
This is a very easy repair. I've never had one leak a second time. Maybe in another 10 years...