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Mains hum from subwoofer

sleazymamba

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Hi,

I have recently purchased a Wharfedale SW-150 subwoofer which is being driven by my old Technics SU-A700 MK3 amp. Very happy with it.

However I am getting a pronounced and clearly noticeable mains hum when the Technics amp is powered off.
  • The subwoofer is connected from my amplifier “B” speaker output to the speaker level input connections on the subwoofer. I use 2.5sq mm pure copper speaker cables and banana plugs. Both left and right channels are connected.
    I do not have sub out on my amp.
  • The hum stops immediately when the amplifier is powered up, even if no source is playing. I guess the amp is grounding the negative cables but only when switched on.
  • The hum still occurs if I remove the speaker cables completely from the back of the amplifier.
  • The hum stops if I remove the speaker cables from the back of the subwoofer.

  • This is clearly an induced current in the connecting wires which, being speaker cables, are unshielded. I have tried moving them away from any mains cables and using a different (UK) mains socket for mains power but it persists.

I am considering getting some shielded speaker cables with a shield wrap that could be earthed to ground, in the hope that this might eliminate or reduce the induced current in the conductors. eg https://www.audiophonics.fr/en/cabl...e-ofc-2x25mm-frnc-shielded-o-83mm-p-9922.html

Does anyone have any experience of this approach, and it it likely to work ? Anyone got a better idea ?

The long term solution is to save up for a new amp with sub out, but that isn't on the cards for a while.

Need help.

Thanks.
 

DonH56

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Hi,

I have recently purchased a Wharfedale SW-150 subwoofer which is being driven by my old Technics SU-A700 MK3 amp. Very happy with it.

However I am getting a pronounced and clearly noticeable mains hum when the Technics amp is powered off.
  • The subwoofer is connected from my amplifier “B” speaker output to the speaker level input connections on the subwoofer. I use 2.5sq mm pure copper speaker cables and banana plugs. Both left and right channels are connected.
    I do not have sub out on my amp.
  • The hum stops immediately when the amplifier is powered up, even if no source is playing. I guess the amp is grounding the negative cables but only when switched on.
  • The hum still occurs if I remove the speaker cables completely from the back of the amplifier.
  • The hum stops if I remove the speaker cables from the back of the subwoofer.

  • This is clearly an induced current in the connecting wires which, being speaker cables, are unshielded. I have tried moving them away from any mains cables and using a different (UK) mains socket for mains power but it persists.

I am considering getting some shielded speaker cables with a shield wrap that could be earthed to ground, in the hope that this might eliminate or reduce the induced current in the conductors. eg https://www.audiophonics.fr/en/cabl...e-ofc-2x25mm-frnc-shielded-o-83mm-p-9922.html

Does anyone have any experience of this approach, and it it likely to work ? Anyone got a better idea ?

The long term solution is to save up for a new amp with sub out, but that isn't on the cards for a while.

Need help.

Thanks.
The (pre)amp's output impedance in the "off" state is often undefined, probably high, and routes to ground through a circuitous route resulting in hum. The obvious question is why are you listening to the sub with the amp driving it turned off? I doubt shielded speaker cables will do a thing. If you plan to listen with the amp off, then turn off the sub, or add an inline switch to disconnect it from the sub's inputs.
 

AnalogSteph

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I am considering getting some shielded speaker cables with a shield wrap that could be earthed to ground, in the hope that this might eliminate or reduce the induced current in the conductors. eg https://www.audiophonics.fr/en/cabl...e-ofc-2x25mm-frnc-shielded-o-83mm-p-9922.html
This is quite likely to help but may not eliminate the issue entirely. When the amplifier is turned off, its output is dosconnected and floating in thin air, so the output node suddenly goes from a fraction of an ohm to being dominated by the sub's input impedance which may be well in the kOhms, making the cable's poor shielding readily apparent as external E fields will now couple into it much more easily.

I would also advise examining the whole playback setup for a ground / protective earth connection. (Neither the SW-150 nor the SU-A700 provide one, so it would have to be at the source or whatever else is connected to the amp.) There should be exactly one - no more, no less. If the whole affair is floating, its ground potential would have a mains component on it when compared to the environment as a result of transformer parasitics and mains filtering, and capacitive coupling between the speaker cable and the environment would couple some of that into the sub input.

Both measures combined should eliminate the issue.
 
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sleazymamba

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This is quite likely to help but may not eliminate the issue entirely. When the amplifier is turned off, its output is dosconnected and floating in thin air, so the output node suddenly goes from a fraction of an ohm to being dominated by the sub's input impedance which may be well in the kOhms, making the cable's poor shielding readily apparent as external E fields will now couple into it much more easily.

I would also advise examining the whole playback setup for a ground / protective earth connection. (Neither the SW-150 nor the SU-A700 provide one, so it would have to be at the source or whatever else is connected to the amp.) There should be exactly one - no more, no less. If the whole affair is floating, its ground potential would have a mains component on it when compared to the environment as a result of transformer parasitics and mains filtering, and capacitive coupling between the speaker cable and the environment would couple some of that into the sub input.

Both measures combined should eliminate the issue.
Thanks for replying. The SU-A700 has a grounding terminal on the back which is currently unused. No other ground terminals that I can see, and the mains supply is non-earthed. Should I connect this to something, and if so what ? Mains earth ? Water pipe ? The only other component apart from the sub and speakers is a CD player.

The cables do look interesting, I have lodged a query with the seller to get their opinion too.
 
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sleazymamba

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The (pre)amp's output impedance in the "off" state is often undefined, probably high, and routes to ground through a circuitous route resulting in hum. The obvious question is why are you listening to the sub with the amp driving it turned off? I doubt shielded speaker cables will do a thing. If you plan to listen with the amp off, then turn off the sub, or add an inline switch to disconnect it from the sub's inputs.
Fair point. Because the power switch is round the back at the bottom, and the sub has an auto-off feature that doesn't work because it thinks the hum is a signal so it never shuts off. I was hoping to let it turn itself on and off as needed.

Interestingly the problem persists when the cables are disconnected completely from the amp and just hanging in air. That is what made me think this is not a classic ground loop problem, but I may be mistaken.
 

NiagaraPete

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LTig

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Get a power strip with a switch and use it to switch both amp and sub at once.
 
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sleazymamba

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Speaker cables won’t help. You have ground loop.
Forgive me but I don't understand. If it still produces hum when the cables are disconnected from the amp and hanging in air, where is the circuit to create the loop ?
 

NiagaraPete

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Forgive me but I don't understand. If it still produces hum when the cables are disconnected from the amp and hanging in air, where is the circuit to create the loop ?
I just reread your post. Weird, I can’t be any help with UK power. Others will have experience.
 

AnalogSteph

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The SU-A700 has a grounding terminal on the back which is currently unused. No other ground terminals that I can see, and the mains supply is non-earthed. Should I connect this to something, and if so what ? Mains earth ? Water pipe ? The only other component apart from the sub and speakers is a CD player.
Which, I presume, is just as ungrounded as the rest, as usually the case (at least for major Japanese brands, aside from some TEAC CD and SACD players from the 2000s). So that would explain your issue.

You can more or less ground to whatever seems convenient as long as it is known to actually be grounded (ask a multimeter if in doubt).

The earthing terminal on the amp is normally intended to ground a record player tonearm but will work for this all the same. You don't need any kind of massive wire gauge, all you're fighting is leakage capacitance in the low nF at best. Just about any random piece of wire without breaks or massive oxidation should work.

(BTW, isn't just having a CD player a bit inconvenient these days? Even a fancy early-'90s Sony pales in comparison to the possibilities of a computer-based playback setup.)
 
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sleazymamba

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Which, I presume, is just as ungrounded as the rest, as usually the case (at least for major Japanese brands, aside from some TEAC CD and SACD players from the 2000s). So that would explain your issue.

You can more or less ground to whatever seems convenient as long as it is known to actually be grounded (ask a multimeter if in doubt).

The earthing terminal on the amp is normally intended to ground a record player tonearm but will work for this all the same. You don't need any kind of massive wire gauge, all you're fighting is leakage capacitance in the low nF at best. Just about any random piece of wire without breaks or massive oxidation should work.

(BTW, isn't just having a CD player a bit inconvenient these days? Even a fancy early-'90s Sony pales in comparison to the possibilities of a computer-based playback setup.)
Ok thanks I'll give that a try. Probably connect a lead to an earth pin on a mains plug (leaving the other pins disconnected) and shove it in the wall.

If it doesn't work, or not completely, I'll try the cables. I guess I can ground the shield to the same point as it won't be part of the audio circuit

Also, I forgot to mention. I do have a Bluetooth receiver connected as well (Logitech) which allows me to stream from my NAS via my phone.

Advice much appreciated.
 
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sleazymamba

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OK so after much mucking about I seem to have a solution. Not an answer, but a solution.

First of all I grounded the amp to an earth pin in a mains plug (nothing else connected, no fuse, just earth). Made no difference. However I'll leave it connected.

Then I made up a set of test cables using some 5m long RCA phono cables (no power going through them, amp was off, only mains hum). I was able to fit the banana plugs over the centre terminals without touching the sides, and connected the shield to my new ground wire. Yes I know is this very rough and ready but I wanted to see of a grounded shield would make any difference at all. It actually made things worse. :(

I moved the mains power cables so they were all running off the same wall socket. Nada.

Then I connected some 10ft speaker wire to the back of the sub woofer and played around with running the wire along the floor in different orientations. Nothing connected, just two bits of wire. The hum was worse in some areas and less in others, but it was always there. This reinforces the theory of RF induction.

So after that I decided to run the wires to the sub from my existing speakers (this is listed as an option in the manual), rather than running them from the amp. This worked. Not only can the cables be much shorter, but connecting the black (negative) to the speaker terminal completely eliminated any remaining hum.

So I can run them like that until such time as I get a new amp.

I would like to say this was a great learning experience for me, except I suspect I haven't learned much. I would also say heartfelt thanks to all contributors for giving up your time to help a stranger.
 

AnalogSteph

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So after that I decided to run the wires to the sub from my existing speakers (this is listed as an option in the manual), rather than running them from the amp. This worked. Not only can the cables be much shorter, but connecting the black (negative) to the speaker terminal completely eliminated any remaining hum.
Which I think works because it puts the existing speakers in parallel, bringing down impedance levels into the low ohms even when the amplifier output is turned off. Clever. Plus with less wire length, there is less to couple into in general, so it's a win-win in your case. Good to know you found a solution anyway, even if it's not entirely like I envisioned it.

Still not sure why your shielded cable experiment did not go as planned... I would check whether sub RCA ground connects to high-level input negative directly (using a multimeter), which it well may not. That might explain it. I don't quite get your description of the setup either though. In any case I'm pretty sure this has nothing to do with RF, just plain ol' mains electric fields.
 
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sleazymamba

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Which I think works because it puts the existing speakers in parallel, bringing down impedance levels into the low ohms even when the amplifier output is turned off. Clever. Plus with less wire length, there is less to couple into in general, so it's a win-win in your case. Good to know you found a solution anyway, even if it's not entirely like I envisioned it.

Still not sure why your shielded cable experiment did not go as planned... I would check whether sub RCA ground connects to high-level input negative directly (using a multimeter), which it well may not. That might explain it. I don't quite get your description of the setup either though. In any case I'm pretty sure this has nothing to do with RF, just plain ol' mains electric fields.
I was using hashed together RCA cables with banana plugs screwed on to the centre conductors, grounded outer sleeves, plugged into the speaker level inputs (not the RCA inputs). Just to see if providing some sort of shielding would make a noticeable difference, like a poor man's shielded speaker cable. I wasn't expecting miracles, I just wanted to test the theory before splashing out on expensive cables. Glad I did.

I used the wrong terminology. I meant EM induced current, not RF.

Appreciate your help.
 

Zoomer

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Not sure if this classifies as a "better" idea, but I took the easy way out and went wireless with my subs.
 

kemmler3D

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This thread has been helpful, I got most of the hum in my own sub to stop by connecting it to mains with a 3- to-2-prong adaptor plug. (I'm guessing the visible sparks when I plug it in are normal and not at all dangerous.) :oops:

However, there is still an audible hum that persists even with RCA cables disconnected and doesn't vary with gain setting. I am guessing this is transformer hum? Any way to mitigate it? It's acceptable but loud enough that I'd prefer to get rid of it...
 
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