The long cable itself can pick up hum, but even more likely, the necessity of plugging the distant subwoofer into a different AC wall socket can create a ground loop, producing a significant hum that is especially loud in a subwoofer.
We recently reviewed a hum eliminator, which introduces variable inverted 60 Hz hum to cancel the ground loop hum. It works well, but you still need that long RCA cable.
The ButtKicker RF Link
ButtKicker has introduced the RF Link, which is a wireless system for sending low frequency signals from your processor or receiver to your subwoofer (or ButtKicker couch shaker, which is how I tested it).
The system consists of a Send unit, its DC wall wart power supply, a Receive unit, and its wall wart power supply.
You connect a short RCA cable from the subwoofer output on your processor or receiver to the Send unit, and a short RCA cable from the Receive unit to the line-level input on your subwoofer or couch shaker amplifier.
With everything powered on, you play a movie with lots of bass action. Using the potentiometer control on the send unit, you turn the control until the red LED starts blinking, which indicates over-modulation of the RF signal. Then, you turn the control back just a bit until the red LED is no longer blinking. That's all there is to it.
I tested the RF Link with a ButtKicker LFE Kit (review coming shortly). With the shaker amplifier turned on, and the receive unit on as well, but with the main audio system not on, there was some low frequency noise, due to the receiver "looking" for a signal. As soon as I powered the main audio system on, the noise disappeared, since the receiver now had a signal to lock on to.
The RF Link completely eliminated the ground loop hum that I was experiencing when I used an RCA cable to connect the processor to the shaker.
To tweak the system, I adjusted the modulation on the send unit for each movie, as different movies have varying amounts of low frequency content. That way, I maximized the signal-to-noise ratio for each movie. If you want to just set it and forget it, then set it for the most intense bass output movie you have in your library.
The product operates at 900 MHz, which might interfere with older cordless phones. New phones operate at 2.4 GHz and 5.8 GHz, so they should not be affected.
The ButtKicker RF Link is a very clever and useful product that I have been wanting to see for a long time. The only suggestion I can make is to have the ability to select one of two RF channels so that stereo subwoofers or shakers can be used. It would also be nice if the receive unit could be purchased by itself so that one send unit could operate two receive units, one for a subwoofer and the other for a shaker. I suspect ButtKicker is going to sell a boatload of these things, because they really work beautifully.