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Old 6th May 2010, 12:30 AM   #1
diyAudio Member
Join Date: Jan 2002
Location: Canberra, Australia
Default output transformer secondary configuration

My scratchbuild amp has donated output trannies from an old unit.

Input connections are your regular 5 leads for centre taps, ends of primaries and UL terminals, no rocket science there. I'm just trying to get sorted how I configure the output side...

On the secondary side there are six leads in three twisted pairs, two lots of Brown/Black each of which are reading about 4.9mH and one Yellow/Black reading about 9.8mH

My understanding is that I'd have three ways to connect the output:

Series all three sets of windings: Speaker + to yellow to black, to brown to black, to brown to black to speaker - (highest voltage gain for the tranny but highest output impedance and suitable for high impedance load speakers only)

Parallel the black/brown windings and series that with the yellow black winding: Speaker + to yellow to black to 2xbrown to 2xblack to speaker - (compromise with lower voltage gain, but lower output impedance and suitable for medium impedance load speakers)

Series black/brown windings in parallel with the yellow black winding: lowest output impedance, best load drive but least voltage gain.

Have I got this right?

TIA Drew
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Old 6th May 2010, 04:54 AM   #2
diyAudio Member
Join Date: Feb 2002
Location: Western Sydney
Drew, I'm struggling with a similar problem in my 'Using Old Hammond Iron' thread. What I did was connect 15v ac to one side of the trannie and measure the voltage on the other windings. This will give the voltage ratio of the transformer, from there you can work out the impedance ratio by squaring the voltage ratio.

You multiply the speaker impedance by the impedance ratio to get the primary impedance your output tubes need.

Different secondary connections will give different impedance ratios, so it’s a matter of working out which combination of windings will give the required primary impedance.
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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