Tweetldee said:
Don, now that I understand what the objective is, let me recommend a
different approach. Chuck the transformer and use a switching power
supply..
This is not only an excellent idea, but I wonder how the current
(or power) into each string is controlled now. You need something like
a linear device (power transistor or FET) per string, and it would
dissipate up to half a watt, and you'd also lose substantial power
through a bridge rectifier that would power each circuit.
I might use an off-the-shelf off-line switching supply that gives
10 to 25 volts (dependent on the max input voltage of the other
regulators I use) at the needed 250 watts, then use a small
2-amp-output switcher for each string. There seem to be lots of small
switching regulators that will give the required output (0 to 2V at up
to 2A) from Linear Technology, TI and such. I'd sync up the
oscillators of all the regulators so there's no chance of audible
difference frequencies being generated.
To your original question, are you sure the buzz is from the
transformer, and there's absolutely no buzz or hum from the wiring or
the strings themselves? Running (filtered and regulated, not pulsed)
DC through the strings would fix that.
They operate at frequencies that are out of the audible range, and
will run cooler than a transformer. If you rectify the transformer outputs,
and the end result is a DC source, then the switcher would serve your
purpose well.
Of course, if you need the 2V/2A at 60Hz, then you may be stuck with the
transformer approach. Since it's obvoiusly a custom build, then you should
be able to specify a maximum noise level from the transformer, and let the
manufacturer worry about the design & construction details.>
Good luck on your project... sounds like it may actually work.
I also have to admit it's a pretty neat idea.