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.

Are both subs plugged into the same outlet? either on the wall and/or the amp?

I have tried with both on the same outlet and have also tried the subwoofer on another circuit as well. Neither had any affect. They are normally plugged into the same APC surge protector.

What's new? If this is a recent problem, look at all changes made in electrical appliances, light fixtures, plugs, switches, etc. that were made in that same time frame. Any electrical work in the neighborhood?

Actually just moved into a new house, so it is new to me. I find it very odd that it affects one of the subs and not the other. The system works just fine when I unplug one of the subwoofers, which is why I changed out the amp. I have searched everywhere I could think of. Maybe another call to Definitive Technology is in order.

Couldn't hurt. Sometimes these issues can be hard to track down. I've even heard of folks shutting off every breaker, one by one, to see which circuit the problem was on, then following up by inspecting each item on that circuit.

Good luck!

Let's look into this a bit further. Are you actually saying the hum changes when the receiver is turned on and off WITH NO CONNECTION TO THE SPEAKER OR WOOFER OF ANY KIND. I doubt that.

Next, moving the AC feed to a different outlet, may not matter if all the outlets in one room are on the same circuit (very likely).

Finally, simply try a three prong to two prong AC cheater plug on the humming woofer. Don't worry about safety. You're still grounded through the interconnect. If they are out of warranty, proven defective, and Def Tech cannot fix it (Martin Logan, for example, cannot fix some of their woofer amp modules while the "new" Sunfire company cannot fix older Sunfire products), call me. I'll help.
Frank 847-668-4519. Call Def Tech first. I will tell you, the parent company has developed a pretty grim reputation in my industry. Don't let them try to sell you a replacement amp. Almost anything can be repaired.

Chances are you have a "Ground Loop" issue. I had this same problem that took much research to resolve, and is a common problem regarding annoying subwoofer hum issues. Some manufacturers request that you use a 3 to 2 prong plug adapter for the power plug from your subwoofer to the outlet -- this reduced the amount of hum in my case but did not resolve the issue completely. FYI PS audio does make power cords with a screw off ground pin. What does work (at least to rid you of 99% of the issue) is to purchase a ground loop isolator -- this is a small round unit that you use between your main coax lead to your splitter or main line to your cable TV or router cable. This does work -- the one I purchased was from Amazon and ran about $30.
You could also call an electrician to deal directly with the competing grounds but will need to find one who is well versed in ground loop issues. We had our home rewired and our electrician was not aware of the issue and was not up with home theater issues. This is far more expensive, but is a final resolution.
What did work for me with a 100% resolution was unplanned but welcomed. I had reached my breaking point with Comcast and its terrible X1 box, and decided to switch to AT&T Uverse. Comcast uses Coax cabling which when completely removed and replaced with the Cat 5 cabling AT&T completely eliminated the subwoofer hum -- this came with great joy!
I am running 5 subwoofers and 15 speakers in my system so have a rather elaborate setup.
Hopefully the above should resolve your issue. Try the isolator -- an inexpensive solution.

Let me know if this resolves your issue.

Best regards,

Jack

Since you mention that the problem is present when there are no signal input wires connected to the speaker and starts when the AC power cord for the subwoofer's amp is plugged into the wall, there are only 2 possible causes for the humming; (1) electromagnetic interference (EMI) from a source external to the speaker enclosure, or (2) a failing or defective electronic component (probably a capacitor) in the subwoofer's powered amplifier circuit. To rule out possible electromagnetic interference, try physically switching the two speakers, positioning them exactly where the speakers are presently positioned. Has the problem switched speakers? If so, the problem is EMI. If the previously humming speaker still hums, and the previously non-humming speaker still functions properly, the problem is internal to the speaker with the hum and almost certainly is a defective electronic component in the subwoofer's amp circuit. Final thought: you said you recently moved. If the speaker with the hum was accidentally dropped, and if there is some Faraday shielding inside to minimize EMI, the shielding support could have been damaged, allowing the shielding to shift position and no longer function effectively. (If the speaker was dropped onto a carpeted surface, there may be no external evidence of damage to the enclosure.) In any case, it shouldn't be hard to isolate the problem to being either internal or external to the speaker enclosure. Good luck!

Cheater plug maybe to TEST a system, Id' advise against leaving one in place as a permanent solution. Equipment is grounded for a reason, and interconnects can carry lethal voltage.

Also dealing with hum/buzz issues in my system, not sure if it is ground loop related or not, but if you'd like to research further, check this out:

http://www.jensen-transformers.com/application-notes/

Thank you all for the wealth of knowledge. From the above, I may have mistyped. The only time that the receiver has an effect on the hum is when the speaker wires are attached when turning on and off. If I remember correctly, the sub cable did not make a change. I have completely removed my entire system and will just set up the receiver and front speakers to eliminate any noise when I have some free-time.

In regards to the cheater plug that everyone is mentioning, the power cords for the subs do not have a ground plug already. They are only the hot and neutral two prong cables. I didn't think it would matter, but I tried an extension cord to another room. That did not change anything either.

I removed all components including the cable box (which is a known culprit), and that didn't change the hum either.

I am thinking of trying one of the radio shack ground loop isolators.I have heard good things. Although, I am starting to think that it may be something else internal, not sure what as the amp was just changed out. Thank you again everyone for all of your help.

If there's no hum when the receiver is not commented, the receiver is the problem, not your sub section. I repair and upgrade Sunfire Subs and can tell you that if the sub does not hum with nothing connected to the input, it's not the sub. If the caps were failing, a symptom would be hum and it would be there all the time, input or no.

There is a hum when the receiver is not connected. When the receiver is initially turning on, the hum changes pitch. When it "clicks" to completely turn on, the hum goes away. The hum starts again when the receiver is turned off again.

Reverse the hookup. I'll bet the hum moves to the other speaker. If so, it's the receiver.

I have finally deduced that there is something internal that is causing the problem. I tested with nothing else connected besides the power cable. One sub was completely fine. The other still has the hum that gets louder the more I increase the volume of the sub. I even tried plugging both subs into the same outlet via a 3-plug to 1-plug adapter with the same results. I am just going to hope that Def Tech can help, otherwise I will just be down a woofer.

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...

Thank you, Gladwin. That was a very informative post and not something I have read before. I did call Def Tech yesterday and the employee on the phone was helpful and said that although rare, he thought that the transformer went bad so is sending me a replacement out. When I replace that, I will look for potential air leaks.

You've already established that the hum stops when the speaker input wires are shorted; that's step 1 (and in reality, the only evidence required to establish that it is a feedback loop - that is the factory test). Checking the midrange movement on the STS is trivial since the grille pops off easily. I'd recommend trying that before changing the transformer. I can't imagine any circumstance where the transformer could cause this.

You were correct. It was not the transformer. When I did turn up the subwoofer after replacing, I can feel some air between the base and the tower. I also found it rather strange I could feel the air come through where the subwoofer should be connected. Looks like you are correct, and it's time to find all the air leaks.

I sealed up everything with silicone and let it sit overnight. Of course, this did not have any effect on the horrible noise either. When plugged in and turned up high, there is a slight hum and then everything (sub, both radiators, and both speakers) shake violently until turned down or unplugged.

I'm out of ideas, so I'll just cut my losses and use the STS as a speaker only. It was not meant to be... Since when I went to purchase the silicone last night, I got a puncture in my car tire on the way to Home Depot.

All though this is very old, I would like to know if you have found a solution.
I have a similar issue. Thank you.

Not the OP but when I had a similar issue on a CS8060 center, it turned out to be a powerful WiFi router too close to the speaker. They play havoc with (some) subs apparently. I moved the router into a closet and the humming / thumping stopped.

Thank's for your answer, but my problem is exactly like that mentioned in the beginning. If I short the high level input the hum disappears or if slightly touch the 2 mid ranges. It's an air leak between the sub encloser and the high&mid ranges box. There is no other way to transmit the sub's driver vibration to the mids.
The problem is what causes the sub's driver to vibrate! Anyway, when it gets signal, the hum stops, but one can hear, right near the driver, the lower mid range vibrating exactly like the sub's driver!
I bought them new and the other one doesn't receive any low level signal, only high level (speaker wires) but the volume level is weak, even with the sub volume at maximum!
I a word, my joy was shuttered to pieces with this 2 Mythos STS!

Sorry, I was never able to properly troubleshoot the issue. I moved houses, and the problem still persisted.

I ended up purchasing a few SVS subs and using the DefTechs as speakers only. I'm going to sell everything next time I move and go the DIY route.

Hi and thank you for your answer!
My problem, besides the air leek, is that the further I move the transformer from the amplifier, the quieter the driver is, but not completely! So, I think that there is an inductance or capacitance coupling between the transformer and amplifier. I hope that I'll be able to discover where the parasite signal is injected in amplifier, because, unlike you, in my case humming does not change rotating the gain knob, so, I think the problem occurs after the knob.
If I'll have news, I'll post them here.

Can you, please, tell me how you get access to the mids and highs drivers box, in order to seal it? Thank you.