Unsolveable HUM problem??

[BABABONDOMAN]

Here is my problem and I'm sticking to it. :) HUM! INFERNAL HUM! I have an Anthem AVM50 audio video processer/preamp. I have 4 powered subwoofers in the room. Two are Definitive Technology subs with both in parallel. They are fed to Sub1 out on the Anthem. The other two are Velodynes. They are also hooked up in parallel. The Def Techs have NO hum. The Velodynes both have some hum at 50% volume. I've tried nearly everything to solve it. I cannot. I have the ENTIRE system running off of the same 20 amp house breaker. I have quality 12 cut Monster turbine RCA connectors and monster cable. Even if I remove the Def Tech speakers from the system and only use ONE Velodyne the hum is there. Naturally turning the gain down on the Velodyne takes the hum away. I've tried different cables to no avail. The hum is NOT horrible ut just enough to be "not right" and annoying. I just cant understand why the Def Techs
plugged into the same parallel plugs give NO hum and the Velodynes DO give some hum. I've tried reversing the AC polarization. No effect. My last hope is to procure some sort of XLR balanced out to RCA conversion box maybe with a 1:1 transformer inside to isolate the hum. The run is about 22 feet or so. Does anyone have any bright ideas OR can anyone suggest
a quality XLR to RCA converter box? Thanks in advance for any ssistance. BABA

[David Kulka]

No hum problem is unsolvable, but having lots of interconnected big amplifiers with unbalanced inputs does tend to invite ground noise trouble. There's a good chance that adding a decent input transformer to each of the offending subs will bust open the ground loop and resolve your hum problem.

If the gear on both sides is unbalanced, the transformer doesn't really need to have XLR's. Just use a good quality, well shielded, input transformer.

[tINY]

First thing to try is plug every thing into a isolated circuit (nothing else on it and everything plugged into a single outlet strip).

After that, if you don't have balanced inputs, you have to create them with transformers.

Good info here:
JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS




-tINY

[TMI]

You don't mention the Velodyne model. Those with external amps have a feedback cable to the accelerometer. There is a lot of gain here and it can be extra sensitive to coupled hum but this will hum with NOTHING connected to the input,

If that is not the problem, installing a transformer inside the amp with a TRS or XLR connector is a good way to break the ground loop from the preamp. Jensen is a very good source. They also make prefabricated boxes for the HT industry you can just plug in.

Try this test. Connect an AC milliamp meter (a multimeter in the MA setting with probes connected for 200ma) to the RCA jack outer ring on the Velodyne and the RCA plug outer ring coming from the preamp. With this RCA connected ONLY through the meter, turn the system on and measure the current between the two components.

Jensen has good instructions on how to terminate the shield through a .01 ceramic cap while using a twisted pair to the transformer primary on the amp side.