Dreaded Subwoofer Hum

[ACHiPo]

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...

MTB Vince, yes I plugged the subwoofer into the preamp circuit. No change in the hum. One other interesting thing--the hum volume is independent of pre-amp volume, but does increase with sub volume and RCA-to-XLR level compensator volume. Turning off the preamp or muting the preamp, however, does eliminate the hum. One other interesting thing is that this is really a noise-floor issue--the level and frequency of the hum is equivalent to a large truck idling a few hundred yards from my house.

 

[ACHiPo]

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.

That's next on my list of things to try.

 

[ACHiPo]

I unplugged all source components from the front end circuit with no effect on the hum.

I made a mistake in an earlier statement. The hum does not go away when the pre-amp is off, but it does go silent when it self-mutes during power on and power off. It also goes silent when the preamp self-mutes when switching between sources.

At this point I'm inclined to live with it. It is barely audible with the Rythmik sub volume at 12:00, which is below the level I'd like the subs to be, but it's better than not having subs.

 

[cal3713]

Have you done the full bare minimum? A dedicated line with only one sub and your pre on it, no other wires headed in or out, and nothing else operating in the circuit?

If you've got the hum there, then you've almost certainly isolated it within the pre. Perhaps in that case building a copper (or mu metal) shield for its power supply could fix it?

 

[DaveC]

Ok, so with everything disconnected from the preamp except the sub, and the sub plugged into the same receptacle as the preamp, are you still getting hum, even with an isolation trafo in use?

If so there's probably an issue with preamp's internal grounding. Check the IEC ground and see where it goes. If it's connected directly to signal that's probably the issue and you can add in isolation, such as a Schurter DENO:

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[DaveC]

Have you done the full bare minimum? A dedicated line with only one sub and your pre on it, no other wires headed in or out, and nothing else operating in the circuit?

If you've got the hum there, then you've almost certainly isolated it within the pre. Perhaps in that case building a copper (or mu metal) shield for its power supply could fix it?

I think it's likely leakage current from the power trafo, it could be insulated from the chassis more but it's also likely isolating the chassis from the IEC ground pin with a DENO would work.

 

[cal3713]

I looked up your pre and see that my shielding solution is meaningless. Very surprised there's an issue inside there:

 

[ACHiPo]

Have you done the full bare minimum? A dedicated line with only one sub and your pre on it, no other wires headed in or out, and nothing else operating in the circuit?

If you've got the hum there, then you've almost certainly isolated it within the pre. Perhaps in that case building a copper (or mu metal) shield for its power supply could fix it?

Cal,
Not yet. I disconnected all the front end equipment from the circuit, but there are ancillary items plugged into that circuit as well. That will be next--I will pull the pre out of the rack with no inputs connected, connect it to the sub and see what happens. I know that if the amps are not connected with XLRs to the pre the secondary RCA output is much higher, but I haven't listened for hum with only the pre and sub.

Thanks,
Evan

 

[ACHiPo]

I looked up your pre and see that my shielding solution is meaningless. Very surprised there's an issue inside there:

View attachment 77430

I find it pretty unlikely that it's the pre as well, but it just seems really strange that muting the pre stops the hum. Will post what I find when I just hook up the pre to the sub.

 

[ACHiPo]

Ok, the preamp is quiet. I hooked up the pre with an RCA cable and it was dead quiet. I then hooked up the RCA to XLR level compensator and turned the level up--sure enough I got hum. So that POS is coming out of the circuit, which is really a bummer because I want a bit of a level boost.

I think I actually have two problems--1) the level compensator is junk and is injecting low frequency hum, and 2) one of the cheapo Monoprice XLRs I ran under the floor has a dodgy ground which is why I'm getting a hum with both the XLR and RCA.

Thanks for the help!

 

[ACHiPo]

Ok things are reasonably ok at this point. Evidently having the balanced ICs connected to my amps is enough of a load on the pre to significantly attenuate the secondary RCA outputs, so my sub levels, even will all of them maxed, is quite a bit below what I was looking to achieve. BUT there is no hum. Guess I'll live with it for now, but sure would like to find a GOOD buffer amp to boost the sub signal without adding hum!

 

[DaveC]

Ok things are reasonably ok at this point. Evidently having the balanced ICs connected to my amps is enough of a load on the pre to significantly attenuate the secondary RCA outputs, so my sub levels, even will all of them maxed, is quite a bit below what I was looking to achieve. BUT there is no hum. Guess I'll live with it for now, but sure would like to find a GOOD buffer amp to boost the sub signal without adding hum!

You could use speaker level inputs. If the sub doesn't have them I think Jensen may have a converter. It's also possible a SE>BAL conversion trafo would double the voltage, same as the compensator.... which btw you should have removed with the other front end inputs.... The whole point of removing inputs and plugging everything into one circuit is to make things as simple as possible, to remove variables as potential causes.

Good to hear the hum went away though.

 

[cal3713]

Excellent. Glad you've located the issue. Congrats!

If the volume reduction is just due to the current required by your sub + main loading, you could potentially build a B1 buffer (https://diyaudiostore.com/collections/pre-amplifier/products/mezmerize-b1-buffer) or Pass 6-24 crossover board (https://diyaudiostore.com/collections/misc-pcbs/products/diy-biamp-6-24-crossover) to put in front of your subwoofer circuit so that the preamp only sees that impedance. Those are both actual buffers though, so you won't get any additional gain in a default build.

I'm really surprised the built in sub amps don't have enough gain for your purposes.

 

[ACHiPo]

You could use speaker level inputs. If the sub doesn't have them I think Jensen may have a converter. It's also possible a SE>BAL conversion trafo would double the voltage, same as the compensator.... which btw you should have removed with the other front end inputs.... The whole point of removing inputs and plugging everything into one circuit is to make things as simple as possible, to remove variables as potential causes.

Good to hear the hum went away though.

I did remove the compensator. No hum. I added the compensator back in. Hum.

Options I’m considering include XLR splitters before the amps, a better line level amp, or the speaker level converters. I like the speaker level idea since I have no hum on the main speakers.

 

[cal3713]

Btw, your fix could also be something as simple as a cold solder joint in your compensator that just needs a quick touch up.

 

[ACHiPo]

Btw, your fix could also be something as simple as a cold solder joint in your compensator that just needs a quick touch up.

Thanks. Given that I bought it on Amazon it's pretty simple to return. I'm really liking the idea of running speaker-level to line-level, but Rythmik gave me another thing to try--if I split the mono cable and put it into both RCA inputs it will double the output by 6 dB, which may be enough if it doesn't also increase hum by 6 dB.

 

[ACHiPo]

Received two of these cheesy speaker-level to line-level converters and the various cables and adapters necessary to install them in my system. Installed one on each speaker (I use 3:1 Zero autoformers as speaker cables so with the adaptors wired to the speakers the amp sees ~30 Ohms rather than the ~10 Ohms of input impedance of the adaptor) with RCAs to each Rythmik sub and 1-into-2 splitters to drive both RCA inputs on the Rythmiks. There is no hum and I now have more than enough bass! Yeah, I am a bit embarrassed that two $8 adaptors did what $400 of cables and isolation boxes failed to do, but then again I don't really care--I have bass, no hum, with no noticeable degradation of either the main or sub sound quality!

Amazon.com

 

[cal3713]

Congrats!