Problem with subwoofer

[Jim H B]

My subwoofer producing a hum during use:

If I just plug it in and turn it on (with nothing hooked into it) - no hum

If I attach speaker wire from the receiver to the high level input (but not hook up wire from the high level output to my speakers) - no hum

If I then attach the wires that run to my speakers into the high level outputs - hum starts. I've tried 3 different sets of speakers - same result.

The hum varies with the volume on the sub - turn up the volume knob on the sub, hum gets louder.

I'm running a 2.1 system - H/K 670 receiver, two speakers and the sub.

Thanks for any thoughts!

Jim

 

[faber12]

Are your speakers passive or powered? Sounds like a ground loop to me.

 

[JoeESP9]

If the switching allows for running two pair of speakers, try connecting the sub to the B speaker connectors. Leave the main speakers connected to the A speaker connectors. The sub has a very high input impedance and will not have any appreciable effect on the load presented to the receiver.

 

[Jim H B]

Thanks Joe. I just tried it, worked fine. I had been hesitant to try that - I didn't want to put a big load on the amp - thanks for clearing that up. I also wanted to use the high level out on the subwoofer, because it was only passing 120k hz and above to the speakers. I thought I may be getting better performance out of the speakers by doing that. But it sounds just as good running them full spectrum right from the receiver.

Any theories on the cause of the hum? It's a fairly old sub, could be the capacitors are getting old, or their is ground issue somewhere in the amp.

 

[Jim H B]

Are your speakers passive or powered? Sounds like a ground loop to me.

Passive - I hear you on the ground loop - but it doesn't hum when powered on without any high level ins or outs connected. But maybe that doesn't rule out a ground loop. I have tried plugging the subwoofer into a couple of different outlets, and that didn't make any difference.

 

[LHorn]

I couldnt find my system hum either until I unscrewed the coax cable to the cable box. After that I read that the cable needed a ground and mine did not. I have a splitter in the basement that runs a line to my office and one to the living room TV. I run a ground wire from the ground screw on the splitter to a screw on the breaker panel cover since it was 1 foot away...my system..including sub has been absolutely silent since. If you have cable anywhere in the house give it a try.

 

[OnTheBlitz]

If your sub has a three pronge AC plug, try putting a "cheater" plug on it to defeat the ground.

 

[orsen]

try running a wire from a screw on the sub amp
to the ground on the receiver

 

[Jim H B]

Good ideas - I'll give them a try. FYI - now that I'm running the speakers straight from the receiver, the receiver seems to be running much cooler. IT was getting pretty hot.

 

[JoeESP9]

IME running a subwoofer as high as 120Hz makes the subwoofer's placement heard. Above ~100Hz sound becomes highly directional. The last thing you want from a subwoofer is for it to be directional.

Ideally the only time you should know a subwoofer is being used is when it's turned off. The lack of low bass should be easily audible.

 

[Jim H B]

IME running a subwoofer as high as 120Hz makes the subwoofer's placement heard. Above ~100Hz sound becomes highly directional. The last thing you want from a subwoofer is for it to be directional..

Thanks. But running a test cd, my main speakers did really drop off considerably under 120hz, so that turned out to be a good cut off. 40 year old receiver and 40 year old speakers aren't cutting the bass like they use to apparently.

 

[JoeESP9]

What are your mains?

 

[Jim H B]

DLK Model 2 - 12 inch woofer - what the heck, right, why so weak under 120hz? Might be my amp, but I'm suspecting the capacitors on the DLKs are getting old. Mids and highs are great though - you can see why I need the subwoofer.

Another thing I noticed - I was holding my cell phone near the subwoofer (I usually don't have my cell in the area with the stereo) - and static started coming out of the sub. Move the phone away, static subsides. Sub must not be shielded well.

 

[JoeESP9]

I've never heard your speakers. However, I find it hard to believe those DLK's don't go down to at least the low 50Hz area. Are you sure they are functioning properly? A 120Hz high pass on a sub is usually for very small (<4"?) satellite speakers.

You could download some test tones, burn them to a CD-R and use a free SPL meter app to actually measure levels.

OTOH, I'm fairly certain there are test tones playable by streaming from the web. You would still need an SPL app.

 

[Jim H B]

That's baffling me too. I did switch out the receiver to a new receiver, and played some test tone tracks (streaming). The DLK's had a little more bass with the new receiver, but again significant drop in volume around 120hz, picked back up a little around 90hz (test tones descending). You could certainly hear the test tones all the way down to around 40hz, but much quieter. Just using my ears. I'm going to swap out some other speakers run the same test tones.

 

[Grenadeslio]

Plug all into a power strip, this way all are sharing a common ground.

 

[Jim H B]

I've tried it both ways - with the sub plugged in the same power strip, and into a separate plug. No change in the hum. I'm beginning to think the sub amp has an issue.

 

[JoeESP9]

Get a free SPL Meter app for your phone. Use that with test tones.

The drop off in response you're hearing my be because of a room node that has a suck out at ~100Hz. OTOH, it may be your hearing. Only measurements will reveal what's actually happening.

 

[Jim H B]

Get a free SPL Meter app for your phone. Use that with test tones.

The drop off in response you're hearing my be because of a room node that has a suck out at ~100Hz. OTOH, it may be your hearing. Only measurements will reveal what's actually happening.

Good idea, I downloaded an SPL meter app to my Android phone. Put on my test tone - goes from 225hz to 25hz. Pretty steady at 70db until it got to around 120hz, at which point it dropped pretty quickly to 55db, drifted slowly down from there to about 40db at 35hz.

These measurements are on the mains only, no sub.

Yes, could be my room. I'll try different speakers to see how they compare.

 

[JoeESP9]

IME properly working speakers with 12": woofers do not suffer severe roil off at 120Hz. I would expect roll off at around 60Hz.