Home Theater catch-all


On a different subject, I've had trouble with my DefTech sub pretty much since I got it. That was somewhere around 15 years ago. It likes to give off a hum. Sometimes it is real soft, sometimes it isn't. Sometimes, just touching the Level knob was enough to make it stop or diminish, and sometimes that wouldn't help. This has happened in all 3 houses that I've lived in since I bought it. I recently moved the Amp downstairs as I mentioned earlier and tested it. It didn't take long before it started happening.

I figure it could be a grounding issue, a cable issue, a "you connected it wrong" issue, an interference from something else issue, or just about anything else.

So, before trying to diagnose it, I'd like to know what the correct way is to hook this sub up to my particular amp. Ill put a pic of the back of the amp and the back of the sub below. I'll hook it up that way and then go from there.

The Denon AVR-3806 amp is 7.2 with multiple zones. For this situation, assume 5.1.




I haven't seen a receiver with this particular arrangement of inputs and outputs, where there is no obvious lfe output, but I believe you should be connecting the pre-out labeled SW to the lfe input on the sub.

However you can also put it in the speaker wire path by connecting the amp's speaker outputs to the sub's high level inputs, and then the sub's high level outputs to the front left and right speakers. If you do the latter you just need to make sure your amp's front speaker settings are set to large and if possible disable the lfe output so that it doesn't filter out lfe from the front speakers.

If the hum is produced in both connection scenarios, the sub is likely the issue. If it only occurs in one of the scenarios, the amp is likely the issue.

Yeah, I had to puzzle at it for a bit, but I'm pretty sure you should run a cable from "SW" in the Pre-Out section to the yellow LFE port on the bottom of the sub.

Don't get confused with the SW in of the Ext In section. You want the second SW port, sixth down from the top, fifth up from the bottom, in the second column of red RCA ports.

If it's still humming after that, the sub's amplifier might be at fault.

You could also try the speaker-wire version, but that's intended for amplified sound sources that can't do bass management. The 3806 can definitely do bass management.

Malor wrote:

Yeah, I had to puzzle at it for a bit, but I'm pretty sure you should run a cable from "SW" in the Pre-Out section to the yellow LFE port on the bottom of the sub.

This should be correct, though I cannot ever remember seeing a home theater receiver that leaves the SW output in a separate pre-out section. That's very weird. My very first 5.1 receiver had pre-outs, but also had a dedicated LFE output. That was... yeesh, 20 years ago?

bepnewt wrote:

I figure it could be a grounding issue, a cable issue, a "you connected it wrong" issue, an interference from something else issue, or just about anything else.

A hum like you're describing is typically some kind of ground loop issue, just as you thought. Can be maddening to trace out though.

I'd try Mao's suggestion first if you have the speaker wire for it. Run your front speaker connections to the sub, and then from there to the front speakers, and set the fronts to full range. Then adjust the manual crossover setting on the sub to filter out what gets sent to your front speakers. 80hz is kind of a standard recommendation, but 100hz might be necessary if you're using particularly small satellite speakers as fronts.

I wouldn't do the speaker-wire setup on any kind of permanent basis, because it's inferior to proper bass management by the receiver. Instead of getting all the bass from all the channels, you're getting only the bass from the front L+R. This can still mostly work, and if it fixes the hum I guess it might be worth it, but doing it properly will usually give better results.

Normally, just getting the speaker and sub onto the same outlet will fix a ground loop.

Some amps have a hum to them, no matter what, and that might be what's happening here. It's usually a sign of poor design or cheap construction. I thought DefTech was a high quality outfit, but maybe not?

Not meant as a final solution, just a quick-ish way of testing if it's the Sub's amp on it's own or something in the signal path.

Also: It's possible Definitive Technologies is a perfectly fine sub maker and he just got a lemon with a minor amp issue.

Stuff happens.

I had an amp die in one of my SVS subs. It was roughly six months out of warranty as I recall. They were mystified. I contacted them to ask them if I could order just the amp directly and what it would cost. Instead they shipped me a new amp for free anyway.

If you end up looking for a replacement, all the recommendations for SVS from me.

I did some reading on subwoofers in general and my Denon amp specifically. Now, I understand when to use LFE (or LFE + Main), what to set my speakers to in the amp (large vs. small) for specific situations, and more stuff about subwoofers than I will be able to retain until tomorrow.

After reading, I messed around with the setup. I plugged the sub in and after about 3 seconds, it started the hum. It wasn't loud, but it was there - we'll call this level 2. We have an open floor plan with the living room abutting the kitchen. I went into the kitchen and could barely hear it. Barely, and only because I knew what to listen for.

Then I plugged in the RCA to the sub. One end was in the SW out on the Denon and the other was now in the LFE on the sub. The second the RCA jack came in contact with the RCA connector on the sub, the humming doubled in volume - we'll call this level 4 - and it stayed at level 4 while plugged in. Unplug it - goes back to the lower level 2 volume hum. Back in, it doubles back to 4.

Playing TV or music didn't change anything, it still hummed along while it did it's normal sub thing.

I unplugged the RCA cable then unplugged the sub from power. Then plugged it back in and it started the level 2 hum. THEN, without connecting the sub I touched one of the screws on the sub and it went to level 4. Stop touching - level 2. Touch the back plate - level 4, stop touching - level 2.

So far, when the sub was plugged into a recep, it was a different one than the receiver. The receiver is plugged directly into the wall on a different recep. I then plugged the power cord from the sub into a power strip that is plugged into the second outlet on the same recep that the receiver is in.

This made no difference. I was really hoping that it was a ground loop issue and this would fix it. I got out my recep tester to make sure that the polarity wasn't messed up on the receps and they were all fine. What I didn't do is plug both the receive and the sub into the power strip. I should have tried that.

While the sub was sitting there not connected to the Denon, merrily humming along at level 2, I turned the dial on the sub's Low Pass Crossover. It was sitting around 90hz before I touched it. I did not expect anything to happen but was surprised. When I turned it down to the lowest setting, 40hz, the hum didn't get louder or softer, but there were some soft, audible pops when I turned it. Then, I turned it all the way up to 150hz and the hum got softer. About half as loud as level 2 (which I'll call level 1). I turned it back down, and it went back to level 2. Turned it back up, and the volume went down to level 1 again. At this point, the hum was barely perceptible. In the Kitchen, I couldn't hear/feel it.

While the crossover was sitting at 150hz and the volume of the hum at 0.5, I touched the back plate and it went up to level 2. Stop touching it, back to level 1.

It would take some effort to go the other route and run the speakers through the sub using the sub's crossover. I may still try it, but I don't expect any better results.

At this point, the humming is almost acceptable. It would be even more acceptable if I plugged the sub's power cord into the amp and let the amp power the sub on and off. The amp normally runs 24/7, but I could maybe get Kit to turn it off when she's not using it. The idea of turning it off to stop the humming may convince her.

The whole "old amp downstairs" thing was originally just to test the speakers in the ceiling in preparation for the new amp. That is, until 2 days ago when we found out that Kit has to get a tooth implant. And yesterday, when the guy who started the back porch project found evidence of termites. So, it looks like no new amp and the Denon is staying downstairs.

I've been scouring the 'net trying to find any replacement parts for the sub to no avail. Someone who knew what they were doing could buy some other amp for this thing to make it work, but not this guy.


That definitely sounds like a problem with the electronics in the sub, since it hums just plugged into the wall. That more or less proves it's not the Denon.

Setting the crossover higher won't hurt anything, because you'll be doing the crossover on the receiver.... if you set it at the typical 80Hz, then the sub will only get those signals, and the higher setting won't change anything. (except reduce the hum.) The sub would go higher, but the receiver won't send it anything over 80.

The fact that touching the back plate increases the hum makes me wonder if it's not an internal grounding problem. You could try opening it and see if any wires are touching the metal case or look obviously wrong or shorted.

The popping when you turn the dial usually means some corrosion in that potentiometer. Getting some electronics cleaner in there and turning it repeatedly should eliminate that. It might take care of the hum as well.

Deoxit or Crc 5103 are the ones I tend to use the most.