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MichaelB 8th January 2006 01:57 PM

Rotel RB 993 humming

I've recently purchased a used Rotel RB-993 3 channel amp on eBay
to accompany my RB-1080 for surround speakers.

Performs well but there is always a gentle humming noise (yes,
humming, no hiss...) coming from all three speakers. Not very
loud but clearly audible from all surround speakers in silent dvd

I've checked cabling, transformer, everything, only speakers attached,
no change. Humming starts immediatly when the output relays turn on.
Power-supply seems to be ok, 74V DC with roughly 35mV AC on top
with no load.

Since the amp is a class-2 device (no earth at all) bad earth can't be
the reason. The transformer itself is dead silent, too.

Any ideas what could be the cause for the humming?
Maybe somebody experienced such effects with this amp before?

A pointer to the service manual might be helpfull, too.

pinkmouse 8th January 2006 02:05 PM

Sounds like noise/ripple coming through from the PSU. You may have a dried up cap or faulty bridge diode. It may be a faulty component on the amp PCB, but as it is affecting all channels, that is less likely.

inteificio 9th January 2006 09:43 PM


are you sure this is not grounded?

I own one of theese wonderful amps as well. I got a buzzing aswell whenever my PC was plugged in (whether on or off).

I cut the ground wire in the 993, no buzz.

I am considering fitting a MOV in the gap but that should return it to being safe.

Check again that your unit has no ground, I am pretty sure mine did. If not how the hell did breaking the ground do anything if it was not there to start with!

anatech 9th January 2006 10:06 PM

Hi MichaelB,
Does it hum if the inputs are not plugged in? Do you have cable hooked up?


mastertech 9th January 2006 10:12 PM

Anatech by the sound of it is not a ground loop, of course they are hooked up
whats your diagnosis


anatech 10th January 2006 01:00 AM

Hi mastertech,
One never can tell until the basic tests are done. If cable is involved, the cable ground can be the largest, most evil ground loop ever designed.

So it's time to gather information.


mastertech 10th January 2006 03:20 AM

what if isnt that (grd loop) and there is still some light hum as the
fellow has said on very low audio levels, whats your diagnosis

MichaelB 10th January 2006 06:40 AM


the mains plug has no earth contacts, so it can't be an earth loop.

No input cables attached, only speakers, there is humming from the
speakers. Having my pre connected or not makes no difference.

The source for the hum has to be somewhere in the amps circuitry,
most likely the PSU, as pinkmouse suggested, since all three channels
produce humming noise.

I will check the PSU more closely as soon a I have some spare time

Does anybody by chance have the service manual available?

Tube_Dude 10th January 2006 06:43 AM

Is the signal ground connected to the case?

inteificio 10th January 2006 07:14 AM

just one extra. After a quick chat fitting an MOV in line with earth does not make it safe, well safer than not at all.

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