Annoying subwoofer hum

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I've finally completed my active subwoofer but I now get an annoying hum when connected to the rest of the system :bawling: . 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?

Solutions?
- 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,
Dave.
 
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?

:bigeyes:
 
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.

-Joe
 
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)
 
Ex-Moderator
Joined 2002
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.
 
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.

-Joe
 
Thanks all for the replies.


redgumjoe666:
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?


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


pinkmouse:
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!


Thanks,
David.
 
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.

-Joe
 
Thanks guys...

But... can anyone answer my point about the subwoofer having the only grounded connection?

The DVD and receiver do not have ground. Does that mean the hum I hear is transformer induced from the DVD player?

I'll do some more experimenting....

Here is the full story - my components are connected as follows:
DVD player has composite video and audio output (both channels) into the TV
The TV monitor out goes into the receiver
The receiver high level goes to the line level via 8 metres of clear 14awg speaker cable (then finally an adapter to get line level into the sub).

In the above:
DVD is not grounded
Receiver is not grounded
TV is grounded
sub is grounded.

What is bizarre - is you say "ok - the ground loop is being caused between tv and sub" - then how come it gets much worse when the DVD player is connected - it has no ground connection and is the last component in the chain?

I'm still confused!
Dave.
 
Whether or not they are all connected to "earth ground", which is the third prong on the plug, is not relevant. What is important is the common signal ground from which all the components are referenced. Signal ground is usually at the same level as earth ground, but not vice versa.
Even if all of the AC inputs are referenced to the same signal ground via a strip, the ground levels between various components will not be the same as signal ground. When you chain components together at line level, the more components you chain, the greater the risk of ground noise, because you have multiple voltages, with the hum being the AC component of the DC offset of each.
In your case, as is common with components like DVD players, they neglect the design of the audio ground signal path in favor of video, so that the DVD player presents a higher impedance, therefore generating more hum.
A good isolation transformer is one that I use (and I should mention that I'm not in any way connected with them) is from Jensen Transformers in the US, at www.jensen-transformers.com.

-Joe
 
Thanks leadbelly / redgumjoe666

As you can see my basic electrical understanding is well, non-existent!

I un-earthed the sub and the DVD did not add any other hum - but the hum was still there. Which redgumjoe, your explanation fits precisely.

I will re-earth the sub (wise idea!) and invest in a decent line leel audio isolator. I've noticed the jensen ones aren't cheap - but then again you must get what you pay for.

I'll look for a local supplier - because overseas shipping is pricey (I'd guess at least $30 US for me for an isolator)

Thanks,
David.
 
Found the problem - not enough paths to ground!

For the benefit of the rest.... I've found the problem and fixed it.

It was the DVD player not being grounded. It had (for want of a better word) "residual" current that was being fed through the line level connections. It wanted to find its way to earth and could only do so via the subwoofer (the only earthed / grounded component).

Solution?: I added an earth lead to the chassis of the DVD player plugged into the multi-board the same as the rest of the components.

For anyone else who has a similar problem like me (your sub hums and you only have the sub with a ground connection):
1. Remove all line level connections into your main receiver
2. Add each line level connection, until you introduce the component with the hum
3. Unplug the line level lead from the "humming" component
4. Touch the chassis of the component causing the hum - at the same time, brush your fingers across the line level lead you just disconnected from it
5. If you feel a spark / introduce the hum - proof that current is leaking from the component into the line level lead (ultimately, through your cable to the sub causing the hum). You should also feel a "buzzing" when you touch the chassis of the component
6. Wire in a ground lead to the chassis of the component - connecting to the earth pin of a standard plug and plugging that into your mains

Important - do not screw up the connection in 6. above - otherwise you could "liven" the chassis - leading to your own or a loved ones fatality!

Disclaimer:
If you do not understand the above instructions - do not follow them and instead seek expert advice. I do not take any responsibility for errors / omissions in this posting or your inability to diagnose or rectify any problem relating to an audio fault.

Dave.
 
I have a Sony STR-DE595 receiver and Cerwin-Vega AVS-5.1 6-piece speaker system and was having a problem with a subwoofer hum, too. I checked all the line-in components but couldn't find one that caused the hum. Then I tried the groundless wall plug to no avail. But then as I was trying to get the subwoofer to "come on" (from standby red to "on" green) by removing and reinserting the RCA mono audio cable into the receiver (in order to "wake" it up), I noticed the hum varied when I touched that cable. Seeing the cable (which CAME with the system so I didn't second guess it's shielding, initially) looked fairly thin and unshielded, I replaced it with a standard RCA stereo (red-black) cable (that comes with most audio components) and 95% of the hum is GONE!

Odd that Cerwin-Vega would provide such a shoddy mono cable (especially considering the sub has stereo inputs), but oh well...I'm happier now that that annoying hum is essentially gone!
 
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