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Subwoofer hum

wadoo

Member
Hey Folks,

looking for your advice in how to address the following problem: I have added a subwoofer (KEF Kube10b) to my setup to complement my LS50s. It's a nice match, however there is ever so a slight but annoying 50Hz hum emanating from the subw. The connection into the subw is via the speaker level inputs, and setup as in the figure below (showing just a single channel):

subw -----(speaker cable)----- LS50 -----(speaker cable)----- Power amp (NAD272) -----(RCA Cable)----- (line level out) Pre-amp (Audiolab 6000a) -----(optical input)

There is no hum with the inputs disconnected from the subw.
Disconnecting the NAD, or speakers or the 6000a from power, or using another power socket makes no difference.
The hum only stops when I disconnect the RCA cable (of both channels).

I have tried using a groundloop isolator filter on the RCA cable, but at least the one I have (used previously on the line level of a guitar pre-amp) triggers the protection circuitry of the NAD (amp comes on, with a click in the speakers, and immediately enters protection mode). The hum in the subw is less though (though clearly not a solution as the amp needs to be off). There's no hum in the LS50s at any stage.

Any tips on how to solve this problem, also in terms of the characteristics of a groundloop isolator that I should perhaps look for? Is there any chance that the subw is at fault (it is still under warranty)?

Thanks
 

maxkolonko123

Active Member
Did you tried other RCA cable? Like some decent quality one or so? Start from there if the hum is gone when it's disconnected, or it might be the source but yeah deffo worth to give a try a decent RCA cable.

Quite funny though cause I notice subtle hum on my subwoofer which u can't hear from mlp position but it's there, so I don't know if there is something like hum free equipment especially when the gains are quite high
 

wadoo

Member
Did you tried other RCA cable? Like some decent quality one or so? Start from there if the hum is gone when it's disconnected, or it might be the source but yeah deffo worth to give a try a decent RCA cable.

Quite funny though cause I notice subtle hum on my subwoofer which u can't hear from mlp position but it's there, so I don't know if there is something like hum free equipment especially when the gains are quite high
Yes, I tried different RCA cables - no difference between the high quality and the common ones.
What's also puzzling is why the power amp goes into circuit protection with the filter connected on its input stage. Is there some specific kind of filter to be used?
 

maxkolonko123

Active Member
Yes, I tried different RCA cables - no difference between the high quality and the common ones.
What's also puzzling is why the power amp goes into circuit protection with the filter connected on its input stage. Is there some specific kind of filter to be used?
It's hard for me to tell cause I don't use any filters to be honest, only thing I tried was Ferrite rings....

Maybe you should try something like this Behringer MICROHD Hum Destroyer HD400: Amazon.co.uk: Musical Instruments

If no joy you can always return it
 

IWC Dopplel

Well-known Member
I would see if there is an earth lift on the amp. Earth loops can be very annoying
 

Ultrasonic

Member
subw -----(speaker cable)----- LS50 -----(speaker cable)----- Power amp (NAD272) -----(RCA Cable)----- (line level out) Pre-amp (Audiolab 6000a) -----(optical input)
Have you tried using an RCA connection from your pre-amp to the subwoofer instead of using speaker cable from the LS50s? I'd be doing that irrespective of the hum issue anyway personally.

Id use a power cable without the 3rd (earth) pin.
A quick Google suggests that the sub only has a two-pin input and so there is no scope for an earth loop via a mains earth connection. I would though be reluctant to advise anyone whose sub did have a mains earth connection to stop using one, since it is there for a safety reason.
 

wadoo

Member
Update, and a sort of review of ground loop filters :)

1. Av-link simple filter
As described in my post above, it did attenuate the hum (measured -2dB difference) but inexplicably made my power amp go into protection mode each time it was attempted to be turned on - useless thus. Suppose that it could be to a very low resistance on the output path.

2. Oehlbach 9052
Priciest of the lot, and actually the one that almost entirely killed the hum (-4dB difference). Side effect, which for now I'll find acceptable is that there now is a small buzz in my LS50s - audible only when not playing anything and at close range.

3. Behringer
Only product that did nothing at all, at least for me (0 dB difference in the hum).

I'll be trying next to Y-split my pre-amp out and hook up power amp and sub using RCAs instead of the speak cables.
 

Ultrasonic

Member
The other thing you could try is having speaker cable going directly from the power amp to the sub rather than between the LS50s and the sub. This is the configuration that is normally recommended if using a high level connection I believe. Move the sub just for a quick test if the cables you have aren't long enough otherwise.
 

DodgeTheViper

Moderator
What about a grounding wire from the sub amp plate to the chassis of the pre-amp ?

Some sub cables have this facility built into them.
 

Ultrasonic

Member
What about a grounding wire from the sub amp plate to the chassis of the pre-amp ?

Some sub cables have this facility built into them.
The sub is double-insulated, so this would be difficult to do and potentially unwise.
 

DodgeTheViper

Moderator
I've not tried it myself as I don't need to or looked into the specifics, but I have these cables which is what led me to the suggestion.

 

wadoo

Member
Update:
Just for completeness I got myself 30m (!) of decent RCA cables and connected them to the subwoofer via a Y splitter on the pre-amp output. The result was.... Absolutely no hum, and all the other buzzing noises have also disappeared! Better than any filter!

Conclusions thus:
- While the speaker level inputs on a sub are a nice convenience (eg 30m of cabling less), they're not the way to go. The speaker inputs, at least on the KEF Qube10 and the rest of my setup are prone to a ground loop hum
- Out of the filters tested at line level the Oehlbach 9052 was a league above the rest in eliminating the hum.
- Using RCA inputs on the sub totally eliminated the hum and and noises, which I guess is due to their high input resistance.

PS Out of curiosity I tried the Oehlbach on my phono input stage, where as expected it didn't do well (added to background noise noticeably). It's definitely a great line level filter though.
 

Ultrasonic

Member
Glad the RCA cable suggestion worked :) .

FWIW I don't think the reason this worked has anything to do with input resistance but all that matters is it worked!
 

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