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Old 12th February 2003, 07:48 PM   #1
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Join Date: Jan 2002
Location: Wellington, New Zealand
Default Annoying subwoofer hum

I've finally completed my active subwoofer but I now get an annoying hum when connected to the rest of the system . I'm guessing its an AC / power related hum (sounds like 50 - 60Hz which is what our AC frequency is here in NZ).

What I've found:
1. The sub amp is earthed (eg. 3 prong mains cable)
2. The sub does not hum when not connected to anything else and volume turned to maximum
3. The sub does not hum when connected with a short (< 0.5 metre) line level lead from my portable CD player
4. The sub does not hum when just the high to line level converter by itself is plugged into the sub amp (I need this since my receiver only supplies a high level output and sub amp only takes line level input)
5. The sub hums when when plugged into my main sterero receiver (high level output) running into the sub amp via the high to line level converter using 8 metre length of 14g unshielded clear speaker cable
6. The sub hums more when my non-earthed DVD player is plugged into the mains and when it is also connected to the rest of the system (but without the dvd being plugged into any other component)
7. The sub hums even more than 6. above when the video or audio outputs from the dvd player are connected to my TV

All of the above tests (1 - 5) is without any audio signal being played - simply plugging compnents together / into the mains.

The high to low level converter was custom made by the sub amp manufacturer (not by my inept electronic hands :-).

I don't want to rush out and buy fancy cables until I know what is causing the hum (this project has cost me enough already :-)

My stereo receiver does not have low / line level output - therefore high level is the only option. My sub amp does not have a high level input - hence I need to use my high to line level converter "somewhere" in the chain

Is my problem due to:
- Poorly grounded A/V components?
- Using an 8 metre length of 14g unshielded speaker cable to link the sub amp to the receiver?

- Should I use an 8 metre length of shielded line level cable - reducing the length of unshielded high level cable that needs to be run. (I need the 8 metre length for corner placement of the sub and work around a ranch sliding door)
- Should I un-earth the sub amp? (eg. remove the ground wire from the power socket?) I saw this suggested somewhere but sounds a little dangerous!
- Should I try and earth the DVD player? (however I still get hum from my earthed receiver)

sorry for the length of this post - I'm trying to give as much info as possible to help you doctors diagnose my illness.

Please help me. Thanks,
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Old 12th February 2003, 08:00 PM   #2
audioPT is offline audioPT  Portugal
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Try this:

use a amplifier with a speaker in the phisical place of you sub and connect the audio cable right there.

If the humm continue, then:
1- check the cable
2- check the output of the receiver
3- try other cable
4- change the location of your sub if all 1 to 3 keep the humm
5- nothing more

Did I help?

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Old 12th February 2003, 10:45 PM   #3
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Location: San Jose, CA
You have a ground loop in you system. The way to remove it is with an isolation transformer, placed between the converter and the sub inputs. This would break the loop formed between the receiver, converter, and sub. Try Jensen transformers on the web, I use use one myself. Oh yeah, and don't try using any old scrounged-up transformer for this purpose. It will only cause you grief.

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Old 12th February 2003, 11:00 PM   #4
JBL is offline JBL  Canada
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IMO most hum problem should be cared of directly instead of using somthing else to remove the problem.

I am not a fan of transformer in the signal path and good one are very costly.

As audioPT said check the cable as this is most of the time the main cause for ground loop.

I just want to add one more advice plug a line level output of any kind from a main operated equipement(like the audio rca output of your video or something)
Jonathan Blanchard (J.Bl.)
---Nothing is impossible---
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Old 12th February 2003, 11:07 PM   #5
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One simple thing you could try is to run a mains extension cable from the socket that drives the rest of your kit around to the sub. This will ensure that the sub is on the same feed and may eliminate any earth loops.

You could also try the old 200ohm resistor in series in the signal ground line trick.
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
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Old 12th February 2003, 11:20 PM   #6
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Trying the series resistor may work, and tying the components to the same AC source would help some, but the fact is you still have a substantial ground loop.

Cables, any cables no matter how good they are, still form a loop, leaving you with the option of rethinking your hookup or inserting a very transparent component to remove the hum.

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Old 13th February 2003, 06:46 AM   #7
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Thanks all for the replies.

Pardon the dumb question - does the isolating transformer get put on the line or high level side? (I assume line level).

Would grounding both the receiver and DVD help? How do I do this - is a simple chassis ground enough?

How can you have a ground loop problem when the only component that is grounded is the sub???? My receiver is not earthed - and neither is the DVD player. (eg. both only have a 2 prong plug)

However the DVD player when plugged into the same mains outlet but having no other (eg line level) connections to anything else significantly increases the hum. Note - it isn't connected to the receiver or sub in any way (other than sharing the same AC mains outlet). Sure there is a slight hum with just the receiver, but the DVD increases it 10+ fold.

I suppose the only way is the isolating transformer you suggest (the 75-300-300-75). Isn't this one for video though? - or will it work adequately for audio?

Good idea about the line level from another component. I'll try the VCR as you suggest - using a similar cable length.

Tried running receiver, sub and DVD power from the sub AC outlet - no difference.

The 20Ohm resistor? - what power rating must the resistor be? and where do you put it? - in the -ve connection (return) between the sub and receiver? and also - do you put it on the high or line level side? - thanks!

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Old 13th February 2003, 10:00 PM   #8
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BTW..... my amp supplier suggested I read this site.... not bad for a good explanation of ground loop hums and links to other EMI information:

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Old 14th February 2003, 04:03 AM   #9
AR2 is offline AR2  United States
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Before investing in transformer try with "lifting" the ground ether from sub or receiver. If you lift the ground from the sub plug your signal input cable, since that is connecting your ground from the receiver. I have similar problem with my amps, and this solves it completely.
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Old 14th February 2003, 08:17 PM   #10
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The transformer is line level. Also, I think I should explain the concept of the ground loop a little further. The loop is created by having multiple ground paths for a signal to flow, such as when you hook up all of the line level outputs/inputs between the various pieces of equipment in addition to the AC power strip. Depending on where the equipment is in this loop, the signal will see two paths to ground, one through the AC strip, one through the line level connections. Each presents a different impedance. The result is that one, usually the line level signal path, is a few tenths of a volt or more above zero, which is amplified and what you hear as the hum.

You can experiment with where to put the transformer, because there is no magic solution for all cases of hum.

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