Rotel RB 993 humming

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Hi,

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
scenes.

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.
 
Hi,

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!
 
Hi,

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
again.

Does anybody by chance have the service manual available?
 
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Hi MichaelB,
Check lead dress inside in case someone has worked on the unit. The other, most likely problem is the power supply as others have pointed out.

The easiest way to find this out is to hang an oscilloscope on the supplies and have a look.

-Chris
 
Hi, When I had a fault with my rb-993, Rotel service provided me with a service manual.

I have since deleted it, but if you call and ask them they are surprisingly helpful.

I phoned last week about a fault on my RC-995, the tech talked me through the basic checks on it. it took a while but he explained in great detail everything i needed to do. mostly just discharging caps as I don't have a oscilliscope but give them a call.
 
I am not sure if the 993 is the amp, I will check with a friend of mine.

There was a Rotel multi channel power amp where the first batch produced had a problem. They needed some re routing done with their earth paths (breaking a track or 2 and running a wire or 2 instead to different points.) Here my friend did this beforte the amps got to market, perhaps some slipped through...
The symptom of the design fault / problem was...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................wait for it................................HUMMMMMMMMMMMM!!!!
 
I know this is an old post but I have a similar problem with my rotel rb-993 power amp. I've had it for 15+ years and I think this hum has always been there i've put up with it and i've moved several houses since.

The hum / buzz occurs in both tweeters of my b&w 703s.

I've tried:
- Disconnecting HK receiver and rca cables
- Just having rotel power amp on and speakers connected
- Just having the rotel power amp connected to power board (filtered)
- Power amp connect straight to wall

Some of you have mentioned MOV, earthing issues, discharging caps with an oscilliscope... can someone help me with some basic diagnosis and if I need to do any of the following please walk me through it as i'm not an electrical engineer and don't want to get fried :)
 
>I'd be looking to replace C906-909 and check D005 and D006

That's clearly not happening (by me anyway).

I'd say it's more a buzz.
It drops in tone but is consistent.. zzzzzzzzzzzlllllldddddzzzzzzzzzzzzzzlllzzzzzlllllzzzzzzzzz

Only from the tweeters not the mid or bass drivers.
 
I took the p amp into my local hi-fi specialist and we did the following.

'hooked it up to reference speakers using original power cable'
- a hiss could be heard from both tweeters when ear is close to them

'plugged in a shielded power cable, apparently worth $600'
- same result

'plugged into power filter'
- same result

We then looked at another setup using a more expensive and new power amp around $5k and $9k speakers. They too had a hiss, I'd say probably 1/3 the volume of my rotel + speaker hiss. The company apparently prides themselves on creating the purest noise fee equipment, i didn't get the name of the company.

So if anyone stumbles across this thread my general consensus is you will experience a small hiss/ buzz/hum noise coming from your speakers when connected to a power amp and it's perfectly normal. Unless your dealing with more expensive power amps, $5k+ and beyond might be a different universe ....that's something I'll probably never know.
 
I took the p amp into my local hi-fi specialist and we did the following.

'hooked it up to reference speakers using original power cable'
- a hiss could be heard from both tweeters when ear is close to them

'plugged in a shielded power cable, apparently worth $600'
- same result

'plugged into power filter'
- same result

We then looked at another setup using a more expensive and new power amp around $5k and $9k speakers. They too had a hiss, I'd say probably 1/3 the volume of my rotel + speaker hiss. The company apparently prides themselves on creating the purest noise fee equipment, i didn't get the name of the company.

So if anyone stumbles across this thread my general consensus is you will experience a small hiss/ buzz/hum noise coming from your speakers when connected to a power amp and it's perfectly normal. Unless your dealing with more expensive power amps, $5k+ and beyond might be a different universe ....that's something I'll probably never know.
The trick is to change the nature of that hiss.

Dan.
 
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