On 2014-06-22 1:37 PM, Lou Judson wrote:
>
> It turns out that since we did sound at this outdoor house concert
> last year, they added motion sensor lighting that is on the same
> circuit as the power that was giving the dirty bass RFI. Can motion
> sensor lighting systems do that?
>
> Sound likely?

Yes, it sounds possible if the motion sensors were using solid state
dimming type controls to turn the lights on and off. To save lamp life
and power, I have seen motion sensor lights that had built-in dimmers,
so if the sensors were turning on to anything other than 100% and
thereby providing lots of harmonic energy, yes then it would be possible.

I'm sure you've heard how a dimmer changes the sound of the buzz as you
dim, so if the motion sensors had dimmers (or "soft start" switches)
then I believe this as a possibility. The inductance in the cable going
back to the panel and the lower impedance of the source at the panel
would attenuate the high frequency components somewhat. This is the
"common impedance" that Neil always was talking about (I miss him and
miss talking at length with him about these things). The HF energy was
coupling directly into the amp's power though the common impedance of
the power conductors.

Once again, while it is a "problem" with the dimmers/motion sensor
system generating the RFI, it is also a system "problem" (common
impedance), and also an equipment "problem" on the amp end (poor
filtering/regulation of the power allowing the changing primary voltage
to leak through into the audio).

So the show went on without the buzz because of your resourcefulness to
try a different circuit. If there is one lesson we can learn here, it is
best if your power can be home run to the main panel. My studio has a
small sub panel feeding the transformer (240/120) but because of the
transformer, the studio has its own local neutral (single point
connected to ground) and no neutral or ground currents can circulate.
Further, all the studio power is in Type AC (aka "BX") wiring and comes
from the panel in the rear up both sides of the studio and the armored
cable is bonded together where it passes and crosses at the worksurface,
providing a ring ground for all the power outlets.

Cheers,

Richard

--
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.