View Full Version : Balanced power amps - a warning

2005-06-09, 05:57
If anyone is thinking of trying to drive their power amps directly from the analogue outputs of their SB2, and their power amps have balanced inputs, I have a short word of advice.

If you make up the interconnects in the standard way (source signal and ground go to the two signal inputs; screen connected only at the unbalanced input), then you'll get a massive hum. This is due to the fact that the SB2 is completely floating (which makes sense in a typical unbalanced installation, to avoid ground loops), and any mains noise on the (nominally 0V side of the) power supply goes straight through and is then amplified.

In my case no damage was caused, but it strikes me that a very high powered amp that happens to be connected to a small set of speakers might conceivably blow them up.

The answer in some cases is to connect the screen to the unbalanced ground. In my case this didn't work because there is a 47ohm resistor between screen and ground in the power amps, so I had to run a separate ground lead. I have no reason to believe my power amps are peculiar in any way (they are the triamp packs built in to my ATC SCM100A speakers), so this small resistance between screen and ground might be fairly common.