Audio hum on mini hi fi

  tristar 19:00 10 Sep 2007

Hi, I recently bought a Hitachi AX-M137 mini hi fi from Curry's.
While the cd player is great, there is a background hum at low volume with either DAB or FM selected. I changed the original set from Curry's, but the replacement unit has exactly the same problem.
Can anyone suggest a possible cure for this problem?
Hitachi don't want to know as these sets were imported by Curry's not Hitachi. Sort of a 'grey import' I guess.
I feel if I could reduce aerial sensitivity it might make things better - but is this possible?

Regards, tristar

  holme 20:34 10 Sep 2007

"Can anyone suggest a possible cure for this problem?"

Apart from keep on rejecting as "Unfit for purpose" and trying replacement(s) or getting a full refund, there's nothing you can do as the consumer.

Sadly, the symptom isn't unusual. As the price of these sorts of consumer-electronics keeps on falling through the floor, the quality drops in sympathy.

From your report, and if it's a constant-pitch, fairly low-frequency hum (to be specific, 100 Hertz, i.e. twice the mains frequency), it's most probably inadequate smoothing in the circuit which converts from mains AC to a few volts DC. These are standard, off-the-shelf chips used by many manufacturers (grey or otherwise!), so can affect all makes and models. (BTW, nothing to do with aerial sensitivity).

The manufacturers/retailers rely on most people running the volume so high that the hum is masked... Take it back (again) I'm afraid. :-(

  tristar 11:19 11 Sep 2007

Thank you very much for your prompt and helpful response to my query.
Sadly, it confirms my suspicions that it is a fault 'built in' to the unit.
I guess you get what you pay for!!

I've printed out the body of your reply, and will return the unit as 'unfit for purpose.

I took special care in unpacking this unit - just in case I had to repack it, glad I did.

Thanks again,


  Arnie 17:49 11 Sep 2007

I've had the same hum problems with quality individual hi-fi units.
Sometimes hum loops can be caused by a poor choice of earthing points between the units.

However in your case tristar, as holme has stated, on cheaper equipment it's generally due to a poorly designed power supply.

Another possibility is, thin copper 0V return point tracks on the wiring board (PCB).
Because the thin tracks can produce appreciably higher resistance than thicker beefy tracks, a small ripple voltage can be produced which will be amplified along with the required audio output.

Not much you can do here of course.

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