I recently bought one of these BEHRINGER: HA400 (http://www.behringer.com/EN/Products/HA400.aspx) to monitor live "broadcasts" to a large school building through a 100V "tannoy" type PA system's line out jacks.

It does the job fine, but exhibits a strange effect - the whole unit works as a crude microphone. When I tap the box or any of the pots , it outputs a sound. It has handling noise and actually transmits (very loud) vocal sounds made in its vicinity through the headphone outputs when the gain is turned up high. Not that it matters greatly, but what is going on here?

Just a wild guess, but maybe valid.

I suspect the 100 V PA systems Line Out may be floating and thus not referenced to ground.

I also suspect the 12 V power adapter is likewise floating and not referenced to ground, thus allowing the 100V PA system to pull the entire box up to it's own floating AC level.

If you have one available, I would use a high impedence multi meter to measure the voltage from a bare chassis access point (screw head or similar) to a grounded (earthed) receptacle.

It would not suprise me if you got anything from 50 to 100 V AC displayed.

Mix that voltage with high gain DC amplifiers in a steel box and all sorts of weird microphonic effects could be produced.

You might be able to cure it by grounding the case permanently, tho' quite where that leaves the PA Line Out signal is anyone's guess.

Hopefully the floating level is of such high impedance the grounding will have no effect.


CS

Interesting theory, and you might be correct in the matter of unreferenced "earth" connections, as we call them over here. I should say I first noticed this effect when testing the HA400 with an audio input from an iMac's audio out and outputs to headphones and a Roland PM-10 amp powered from the same AC socket - I'm not sure how that might affect it.