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Old 18th October 2012, 10:14 PM   #1
AJ34 is offline AJ34  United Kingdom
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Default Can I remove hum from active speakers?

Appreciate some advice on this one.

I use a pair of active (not powered passives), two-way speakers for listening to music. They're great in almost every respect but there is a slight hum I'd like to remove from each speaker.

The hum is just about noticeable at the listening position when environment is very quite with no music playing. Obviously, playing music masks the hum.

Hum is constant and independent both of speaker's volume settings and whether input connections have or have not been made. The hum doesn't change noticeably at different times of the day either.

The hum comes only from each bass/mid driver, not from the tweeters. This baffles me because, being an active design, the amps are directly connected to each drive unit. If the hum is mains related caused by the power supply, why can't I hear it through the tweeters too?

Hum sounds like a 100Hz type mains related hum but I'm no expert and could be wrong.

Could anyone advise on a generic fix for this sort of hum problem?
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Old 18th October 2012, 10:34 PM   #2
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thats why you can't hear it in the tweeters
is this a finnished product? or DIY?
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Old 18th October 2012, 10:37 PM   #3
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Could be a grounding loop.
Or could be the amp just wasnt designed quite right and they have some ripple.
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Old 18th October 2012, 10:47 PM   #4
AJ34 is offline AJ34  United Kingdom
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Thanks for your replies.

I purchased the speakers, they're a commercial product.

Quote:
thats why you can't hear it in the tweeters
I seem to remember connecting a 1.5 V battery to an old tweeter once, I heard that!
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Old 18th October 2012, 10:50 PM   #5
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Old 18th October 2012, 10:59 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

The hum level in each amplifier could easily be the same, simply the tweeter
can't reproduce it whilst the bass unit can easily. What are the speakers ? *

rgds, sreten.

* If its a real problem the internet is your friend.
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Old 19th October 2012, 12:51 AM   #7
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sounds like power supply hum. could be the amps are run from a single ended supply, and such amps usually have a little bit of power supply ripple in the output
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Old 19th October 2012, 02:11 AM   #8
cbdb is offline cbdb  Canada
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If you disconnect the input (or better still, short the input ) does the hum go away? If it does, then you have a ground loop. If it dosnt the problem is in the speakers.
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Old 19th October 2012, 02:14 AM   #9
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Quote:
Originally Posted by AJ34 View Post
.....The hum is just about noticeable at the listening position when environment is very quite with no music playing. Obviously, playing music masks the hum.....
If the hum is identical from both units with or without input leads shorted, this means typical budget system, low level hum to my understanding. One almost has to get used to this sort of performance from extreme cost-cutting measures in the electronics.

If you paid for something up-market or purporting to be hi-fi quality, you could expect better than that but a fix will depend on what's (not) inside. A schematic is needed (unlikely to get outside the service agent's control) or good pics of the works, including top and bottom side of the PCB. Then a fair bit of work and parts will be necessary if the problem is more than just poor location of the signal leads and boards. The problem could lie also in design issues like poor PCB layout, power supply, or insufficient filtering which can be major work to fix.

Generic fix?...Not without knowing specifically what you have there and whether modifications would be an economical proposition.

As other posts have suggested, the tweeter is filtered, by the definition of an active filtered loudspeaker, to exclude bass - why ever would you expect anything but a tiny amount of high residual harmonics to be heard through them? That has little to do with the tweeter's ability to reproduce bass, as you witness stabbing a bit of DC into one.
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Old 19th October 2012, 05:38 PM   #10
AJ34 is offline AJ34  United Kingdom
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Thanks to everyone for replying.

Quote:
If the hum is identical from both units with or without input leads shorted, this means typical budget system, low level hum to my understanding. One almost has to get used to this sort of performance from extreme cost-cutting measures in the electronics.
Hum is present and constant in volume with or without input cables connected. Hum is unaffected by speaker's volume control setting. Not tried shorting input because I wasn't sure if this would cause damage. Yes, the speakers are very cheap. These in fact:

http://www.studiospares.com/studio-monitors/studiospares-seiwin-powered-monitors-pair/invt/248030/

Trouble is, their audio quality is so good that Id like to improve them further by removing that small amount of hum. These speakers rival my main active system in some aspects of audio quality theyre a bit special and by far the best value purchase of hifi/audio reproduction equipment Ive ever made.

Quote:
As other posts have suggested, the tweeter is filtered, by the definition of an active filtered loudspeaker, to exclude bass - why ever would you expect anything but a tiny amount of high residual harmonics to be heard through them?


Maybe Im missing something here? This is the way I (currently) understand it if the hum is caused by an imperfect power supply then this feeds both amps that connect directly to their respective drive units. The audio signal passes through the active crossover before the amps so has no bearing on this.

Quote:
The hum level in each amplifier could easily be the same; simply the tweeter
can't reproduce it whilst the bass unit can easily.
That makes sense to me. I guess the tweeter's frequency response curve is somewhat depressed at 100Hz.

I take all points made about possible causes of the hum. Im willing to chance that its caused by ripple from the power supply. If we assume that is the case, what could cause this and is there any way of improving the supply by adding additional components or replacing something? I appreciate specific advice cannot be given but any general direction much appreciated.

Incidentally, there is none of the switch on & off thump that I've experienced with other single ended power supply amps but maybe some switch on delay circuitry is incorporated?
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