RB-1090 humming with RCA ins Plugged into source, Balanced ins fine....

nacam

Hi all;

I have an issue with my Rotel RB-1090 amp. I recently picked up a Rotel preamp to run it and until now I have been using the balanced (XLR) in's running from a pro audio interface with balanced outs. It worked fine this way. Today when I got the preamp I disconnected the XLR's and connected the unbalanced (RCA) inputs to the preamp AND switched the input selector to on the amp to "Unbalanced". I got a hum out of the speakers. I tested the pre with another amp and it is working fine and clean. So in troubleshooting the amp does not hum with either nothing connected or RCA's connected but as soon as I connect the preamp or other source I get the hum (with the switch on "unbalanced" of course). I also tried different cables with the same result. Running the source component with the preamp with headphones is also clean.

Would it be possible the unbalanced in's are fried but not the balanced ins?

Any help would be appreciated!

tonyb

It would be more possible that you have a ground loop problem or a bad RCA. Try a 2 buck cheater plug on the rotel first. Thats a plug you can get at any hardware store that takes a 3 prong plug and converts it to 2 prong, in essence lifting the ground. What kind of RCA's are you using ?

nacam

Here's what I tried/found:

Had a cheater cord, hum was still there but not as loud. Tried different cheapo RCA's, hum got worse. The first set of RCA's are some monster thick cables. Also tried a single, shielded "video" cable on each channel (which I know works). Each channel hummed separately with no noise on the other. The preamp has no ground on the AC connection, the amp does (if this helps).

Msabot1

Ground problem!

Speedskater

The problem is near the "Unbalanced" switch.
So disconnect everything but the amp. (power & interconnect).
Get a battery powered music source, does it hum?

Bill Whitlock has some rather technical troubleshooting tips (pages 10 to 21)

http://www.jensen-transformers.com/an/generic%20seminar.pdf

tonyb

Try the cheater plug on the amp ? Trying a cheaper set of RCA's isn't going to do anything. Get some better RCA's and maybe a power conditioner. Ground loops can be a **** to nail down.

Back in the day, we used to just run a wire from a screw on the outside case to another piece, sometimes it worked, sometimes not.

blehmbo

Is your source a desktop PC? When you run the amp in unbalanced mode it's likely that the amp uses earth/ground as its signal ground which makes it vulnerable to ground loops with a desktop PC or any other device that uses earth as a signal ground.

nacam

Hey everyone, sorry for the silence. Out of town but back now!

My source is a Rotel RC-1070 preamp. It hums like crazy. I tried a battery powered music source (An android phone) and the hum was still there but not as loud (I would say about 1/4 of the volume coming from the 1070). The RC-1070 has a 2 prong connector on it and a ground screw. Should I possibly try connecting the RC-1070 ground screw to the chassis of the amp?

I am thinking about bringing the amp to a Rotel service center for repair. Anyone know of a good shop in the Toronto/Hamilton area?

blehmbo

I see that you have a DAC listed in your signature. Do you have a PC connected to your system? If you do, try disconnecting it and the DAC from your system.

Poee7R

Have you looked at the manual to see if the xlr connections need jumpers to run unbalanced?

Its been a while but I seem to remember needing jumpers to run the rca's on an older amp.


Dave

nacam

Well, it turned out to be the unbalanced inputs on the amp. There are two resistors on each channel that opened causing the ground loop. Fixed all in was a whopping $58.00. Now I am back up and running.

Thanks to all for the help! It definitely helped me narrow down the problem.