Dimmers and flourescent ballasts are also very good candidates for inducing hum. Compressors (like in your fridge) can even do it, for that matter.

I just swaped out my 7125 (two prong) for the 7700 (3-prong) and I have MASSIVE ammounts of hum. I am not talking about a little distracting but where that's all you could hear out of the speakers!!!

I was using two power conditioners to handle all of my equipment so I ran a wire between the ground lugs on both units and the hum was cut down by about 75%. I then found the source - it's the light kit on my celing fan! A few months ago I put a dimmer switch in and I am sure this is doing it as I have heard they are well know for that, do they make a special kind of a dimmer that does not introduce massive ammounts of noise?

One more thing - I am using balanced connectors.

Zofo wrote:

One more thing - I am using balanced connectors.

Without the external obvious causes of hum I had a ground loop hum (see above). Lifting the ground with a cheater as a test worked, but was not considered advisable as a solution by outlaw tech-support.

Using balanced cables, hum gone.

My hum problem is solved. I checked all the usual suspects including completly disconnecting my sat antenna but that didn't cure it so I checked the rest of the cabling and found one of the cables from the 990 to the 770 was not fully inserted in the back of my 990. Even with all the panel space on the back of the 990 those big Canare connectors and boots that Bluejeans uses make it a tight fit. When I seated it correctly there was complete silence.

I stayed up until 2:30AM last night listening to music, mostly SACDs, and thus far I am very pleased with what I've heard. Tonight a movie or three.

mike

Hi there. I've not troubleshooted it yet but wanted some suggestions. I don't have a loud hum as such, but I recently noticed that after turning off the 990/7700 combo with XLR cables that there is a high pitched whistle after turning everything off that fades out, kinda like something is 'draining' out so it goes from high to low and is gone in a second or so.

What could cause that?

It sounds to me like the amps' capacitors discharging, if I had to guess.

Hello,

I would like to add my "experience" with hum. I just received Model 990/7500 combo. And right of the bat, I was greeted by the hum. I pretty much disconnected everything and rearranged the power cables with no reduction of hum. I finally put everything back together and started all over again. During this exercise I noticed few things:
- I was able to "effect" the hum level while playing behind the rack (high, low, etc.)
- The hum was there when the RCA cable was connected between 990/7500 (attached on both ends).
- The hum was there when the RCA cable was only attached to 7500 but not to the 990.
- The hum was gone once I disconnected the RCA from 7500.
- I thought the cable was acting somewhat like an antenna.
- The hum was not there when I used my old Rotel Ampplifier (stereo) using the same cable.
- I changed the RCA cable with a Monster Cable ULT-I600, 4feet, the hum went away. Nada, zilch, zero.
- I will be bringing home some instrument tonight to see if I could measure any interference coming out of 7500 when compared to Rotel.
- Rotel is "power consumption" rated at 400 Watts vs. the mighty 1800 Watts of 7500. Though, I do not know if this should make any difference.

I will let you know if I have the right instrument (Tesla?, Gauss?, I am very poor in electromagnetics...) and the results of my measurements.

Cheers,

H2O

Thanks H2O for sharing your experiences (frustrating) with us about hum. When using 990/755 combo I had no hum even though the connections were RCAs-unbal. I did have the buzz referred to in this thread:

Other thread-buzz with voltage change due to wind turbine really cooking

I moved the 755 to a different location in the house and bought a 7700. Ran RCA's got hum. Used XLRs and no hum. Yes I went thru the drill of isolating each component out of the system as a potential source. Still haven;t figured out why, but I've solved the hum (ground loop) problem by using balanced connectors. Yes I tried a "cheater" but since Outlaw doesn't recommend it I won't "cheat".

The buzz with voltage increases from 122 to 125 volts still exists audibly, so that's a constant.
What is not a constant is that the XLR balanced cables eliminated the hum. Still dunno why.

"Input" (pun not-intended ) appreciated.
And I gotta tell you I love the 990/7700 combo.