Dreaded Subwoofer Hum

[ACHiPo]

I am getting a hum on my subs that I'm pretty sure is related to signal ground.

It is a low level low frequency hum. It may be 60 Hz, but actually sounds even lower frequency--like 20 - 30 Hz.

I'm using a Parasound JC2 preamp, 2 Rythmik 12SE subs, and a dual 10" DIY sub. The signal goes into one Rythmik sub, and the other subs are daisy chained as slaves.

I hear the hum regardless of which source is selected through the preamp. Its level is independent of preamp volume. I hear it running RCA from the preamp secondary outputs to the subs. I tried an RCA to XLR level converter with volume which affects the hum level. I hear it regardless of running RCA or XLR to the subs. If I bypass the Rythmik subs and plug the XLR straight into a DIY sub I also hear it.

My AtmaSphere M60 amps are dead quiet running XLR inputs. The amps are plugged into the same circuit as the subs, which is close but different than the circuit to the front end.

I lifted the ground on the subwoofer with no effect on the hum (I pulled the ground out of the IEC cord and shrink wrapped it). I then used the same ungrounded power cable on the preamp with no effect. I think I need a galvanic isolator to lift the signal ground, but before I buy another box I thought I'd ask for suggestions.

Here are a few I found that look interesting.

The first uses Jensen transformers to isolate the RCAs. I have a good opinion of Jenson, but if I don't have to spend $250 I'd rather not.
https://www.parts-express.com/Jense...pzKx0UnPjeN2V4-ahpzXzuppI2yj8A9IaAp2ZEALw_wcB

or

https://www.bhphotovideo.com/c/prod...800_1024_Twin_Iso_Balanced_Line_Isolator.html

There are other highly rated products for under $100...
https://www.amazon.com/Transformer-...ocphy=9032030&hvtargid=pla-761120909432&psc=1

https://www.bhphotovideo.com/c/product/380761-REG/Rolls_HE18_BUZZ_OFF_HE18_Buzz_Off.html?ap=y&smp=y

What recommendations do you have?

Thanks,
Evan

 

[cal3713]

Tracing those down can be a slog, sorry to hear you've got the issue. Are you using multiple wall outlets? Is it possible to use only one and see if that gets rid of the problem?

 

[ACHiPo]

The front end is on a dedicated circuit. The amps and subs are on a different dedicated circuit. Both circuits are within 25' of the subpanel. The amp circuit is 20A with 10awg Romex, the front end is a 15A circuit with 12awg Romex, so I would think there would not be a ground loop issue. The fact that I can lift the ground on the preamp and subs suggest it's not a ground loop with the AC ground, but rather something else on the signal ground. Also, there is no hum on the amps. I'm pretty close to pulling the trigger on a Jensen isolation transformer. I'm planning to sum the RCA outs from the preamp into the isolation transformer, then into the RCA to XLR level converter, then XLR to the subs.

 

[rbbert]

If there is no hum on the main amps you should try hooking up just one sub's high-level input from the speaker terminals of one amp and ground the two amps (sub and main) together. If the hum is still there I doubt there will be anything you can do to get rid of it. If there is no hum in this hook-up, there is at least a chance you can find a way to eliminate it, but it may not be simple.

 

[cal3713]

The front end is on a dedicated circuit. The amps and subs are on a different dedicated circuit. Both circuits are within 25' of the subpanel. The amp circuit is 20A with 10awg Romex, the front end is a 15A circuit with 12awg Romex, so I would think there would not be a ground loop issue. The fact that I can lift the ground on the preamp and subs suggest it's not a ground loop with the AC ground, but rather something else on the signal ground. Also, there is no hum on the amps. I'm pretty close to pulling the trigger on a Jensen isolation transformer. I'm planning to sum the RCA outs from the preamp into the isolation transformer, then into the RCA to XLR level converter, then XLR to the subs.

Not suggesting the single outlet as a long term solution (I also use dual independent lines for the front end and amps) just something to try during troubleshooting. Easier than buying additional equipment while trying to track down the source of the problem.

 

[cal3713]

I have a miniDSP in my sub chain and it was the source of a terrible ground loop that drove me crazy. The usb cable from it to the computer used for crossover adjustment was my culprit. Took forever to hunt that down. Computer was on a 3rd line with my TV and non-audio gear. Never suspected it because it wasn't in the audio chain at all.

 

[ACHiPo]

The fact that lifting the ground of the preamp and sub did not solve the problem makes me think it's something related to signal ground, likely in the preamp since the hum is independent of input and is eliminated when I mute the preamp. I don't understand why it is only on the sub, however? It seems like the balanced outs would be susceptible as well as the RCAs. I ordered an RCA isolation transformer from Jensen. Hopefully that will fix the problem.

 

[cal3713]

Good luck. I struggled with the logic of the problem when I had it. Hum only came through one sub channel. I was ready to send that amp in for repair because the other sub amp was fine (and plugged into all the same stuff). As soon as I pulled the offending amp from the system and got ready to box it up, the hum jumped to the other sub amp. Just amazingly frustrating.

Like you, I could manipulate it at the pre, although the pre wasn't the culprit in the end (and was plugged into a isolated balanced power conditioner).

 

[ACHiPo]

Good luck. I struggled with the logic of the problem when I had it. Hum only came through one sub channel. I was ready to send that amp in for repair because the other sub amp was fine (and plugged into all the same stuff). As soon as I pulled the offending amp from the system and got ready to box it up, the hum jumped to the other sub amp. Just amazingly frustrating.

Like you, I could manipulate it at the pre, although the pre wasn't the culprit in the end (and was plugged into a isolated balanced power conditioner).

This hum is on all 3 subs (2 Rythmik 12SE, one DIY dual 10") and doesn't seem to care whether I use a cheap Monoprice RCA or Mogami Gold XLR. The good news is that it is low enough level that I can ignore it/get used to it, although it bugs me. It's also pretty interesting how I can hear the room nodes--when I walk around the hum goes from barely to definitely noticeable.

 

[cal3713]

So frustrating. Looking forward to reading about the solution when you find it.

 

[MTB Vince]

Have you tried temporarily using a power strip and extension cord to plug all components including the subs into the same AC outlets as the front end gear @ACHiPo? This would potentially identify or eliminate the 2nd dedicated feed as the source of the ground loop. Best practice for multiple dedicated AC feeds is to use identical lengths of identical gauge in-wall wire so as to avoid differences in the resistance of the individual circuits. Your description of your two dedicated feeds suggests your electrician did not follow this advice.

 

[ACHiPo]

Have you tried temporarily using a power strip and extension cord to plug all components including the subs into the same AC outlets as the front end gear @ACHiPo? This would potentially identify or eliminate the 2nd dedicated feed as the source of the ground loop. Best practice for multiple dedicated AC feeds is to use identical lengths of identical gauge in-wall wire so as to avoid differences in the resistance of the individual circuits. Your description of your two dedicated feeds suggests your electrician did not follow this advice.

I have not tried plugging everything into the same circuit. My electrician is me, so no I evidently didn't follow best practice. The circuits are approximately the same distance, and both circuits are quite close to the sub-panel.

One interesting observation--if I put my thumb over the RCA cable shorting the shield to the center conductor, the hum starts. I'm guessing that means either the issue is in the subwoofer itself or the RCA cable has a flaky shield. The weird thing is that using the single-ended to balanced level converter I also get the hum using XLR, which should eliminate the cable as the source.

 

[cjfrbw]

If you subwoofers have line level preamp in AND preamp out, and you are using line level inputs to the subwoofers, try plugging a long cable into the preamp out on the subwoofers back to a preamp or receiver input (extra unused and unselected input) at the signal origin for all the subwoofers. If you have your subs daisy chained through the preamp outs on several subwoofers that have both R and L, try using one output to extend the daisy chain signal and then one for the line back to the unused input of preamp/receiver. I cleared up most my swarms' grounding problems this way.

Also, if your plate amp on subwoofer has a choice between an internal passive crossover and a line level without a crossover, and you are sending a signal already frequency limited, try the line level without the internal passive option and see if that helps.

 

[cal3713]

If you subwoofers have line level preamp in AND preamp out, and you are using line level inputs to the subwoofers, try plugging a long cable into the preamp out on the subwoofers back to a preamp or receiver input (extra unused and unselected input) at the signal origin for all the subwoofers. If you have your subs daisy chained through the preamp outs on several subwoofers that have both R and L, try using one output to extend the daisy chain signal and then one for the line back to the unused input of preamp/receiver. I cleared up most my swarms' grounding problems this way.

Also, if your plate amp on subwoofer has a choice between an internal passive crossover and a line level without a crossover, and you are sending a signal already frequency limited, try the line level without the internal passive option and see if that helps.

I wish I'd thought of this when I was playing my version of this game.

 

[ACHiPo]

If you subwoofers have line level preamp in AND preamp out, and you are using line level inputs to the subwoofers, try plugging a long cable into the preamp out on the subwoofers back to a preamp or receiver input (extra unused and unselected input) at the signal origin for all the subwoofers. If you have your subs daisy chained through the preamp outs on several subwoofers that have both R and L, try using one output to extend the daisy chain signal and then one for the line back to the unused input of preamp/receiver. I cleared up most my swarms' grounding problems this way.

Also, if your plate amp on subwoofer has a choice between an internal passive crossover and a line level without a crossover, and you are sending a signal already frequency limited, try the line level without the internal passive option and see if that helps.

Looks like a great idea to try. Thanks.

 

[ACHiPo]

My sub does not have an RCA out, just XLR out. Unfortunately I don't have any unused balanced inputs on my preamp. Plugging an RCA cable into the sub input (line or LFE) and an unused input on the preamp does nothing for the hum. I should get the Jensen isolation transformer Friday or Saturday. Hopefully that fixes it.

 

[ACHiPo]

The Jensen isolation transformer did nothing. I plugged the sub into the same circuit as the preamp and still I have hum. The hum goes away only when the preamp is muted or off. I’m starting to think it’s a preamp issue.

 

[cal3713]

Sorry to hear that.

What happens if you disconnect everything except for the preamp and the connected subwoofer (i.e., no sources & no other amps)?

And I'll note that my preamp was similarly "in control" of my groundloop issue, even though it was not the final culprit.

 

[MTB Vince]

Have you acted on the suggestion I previously made in this thread @ACHiPo? If not, I would do so prior to pointing the finger at your preamp...

 

[DaveC]

Have you tried temporarily using a power strip and extension cord to plug all components including the subs into the same AC outlets as the front end gear @ACHiPo? This would potentially identify or eliminate the 2nd dedicated feed as the source of the ground loop. Best practice for multiple dedicated AC feeds is to use identical lengths of identical gauge in-wall wire so as to avoid differences in the resistance of the individual circuits. Your description of your two dedicated feeds suggests your electrician did not follow this advice.

I'd try this... We often make life difficult for ourselves when running multiple lines. If the lines are not run together (both lines physically touching one-another until they get to the receptacles) you can end up with a large loop area, and if there is an EMF present it will cause current to flow.

The reason for this is the grounds from both lines will be combined in the system, and this forms a big loop. In some instances this may not be an issue, but it certainly can be. It helps that the subpanel is nearby but still, if the lines are not run together you'll have a big loop.

With multiple lines you can also end up with different ground potential because of several factors, voltage drop, leakage currents... and if this is the case it'll cause problems. You can measure this and see if it's a problem. The difference in ground potential is ideally zero, but I'm guessing you might find some. I forget off the top of my head what's an acceptable difference.