On Wed, 14 Jul 2004 19:56:52 -0700, Roy Lewallen
wrote:

Welcome to the world of transformer design.

What you need to do is make the impedance of the winding considerably
higher than the impedance of the load. For example, the winding
connected to the 32 ohm load should have an impedance that's several
times 32 ohms. A factor of 5 or 10 is generally adequate to keep the
disturbance of the winding to a tolerable level, so shoot for about 160
- 320 ohms of impedance. You'll need to consult the data for the core
you're using to determine how many turns that will require. Do this
calculation for either winding -- the other will come out the same if
the impedances are matched.

When you're dealing with audio and ordinary cores, the winding impedance
will primarily be reactive, and the winding inductance will be fairly
constant with frequency for a given number of turns. Consequently, the
winding impedance (reatance) will be directly proportional to frequency.
That means you need to do the calculation at the lowest audio frequency
you intend to pass through the transformer. Capacitive coupling between
the windings and other effects tend to interfere with proper transformer
action at high frequencies, so high frequency performance degrades if
you use way more turns than needed. Truly high fidelity audio
transformer design is something of an art -- the audio output
transformer was often a major limiting factor in tube-type audio
amplifier performance.

Thanks, Roy. Looks like there's rather more to it than I imagined. :-(
I wonder if Reg has written a program to design impedance
transformers? :-)
I woke up this morning, having given it due thought overnight and
thought I'd got it cracked. But that was until you said the primary
inductive load Z should be many times the design load Z. Oh well,
back to the drawing board, I guess. Looks like it's gonna take a lot
more wire and a lot more turns than I'd thought. OTOH, I could just
stick an emitter follower stage on the end of the amp chain and have
done with it. :-/
--

"What is now proved was once only imagin'd." - William Blake, 1793.