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Elektor78 8th October 2008 07:43 PM

Transformer Distorsion THD
Hi people :)

has someone ever measured the distorsion of an audio-transformer to AudioPrecision (THD vs Power) ?
and for the transformers interstage (THD vs Vout) ? :confused:

I have tried to look for in the web but I have found anything of nothing :bawling:

Thanks :D

lcsaszar 9th October 2008 09:16 AM

I am planning to do such measurement. I will use this arrangement:

low distortion audio generator - 100 Watt transistor power amplifier - transformer secondary : transformer primary - 100 Watt dummy load - voltage divider - distortion analyzer

The dummy load is a resistor with T^2 x 8 ohm resistance, where T is the transformer turns ratio.

richwalters 9th October 2008 07:05 PM

What are your aims in wanting to do such measurements ? Reasons ? Magnetics is as wide as the universe.


lcsaszar 10th October 2008 10:33 AM

As for myself, I am interested in the level/frequency dependency of the distortion magnitude and spectrum. Tubes and transformers are the two dominant nonlinear components in a tube amplifier. We know the characteristics of tubes very well, but we know very little about a custom-made transformer. Therefore it makes sense for me to measure a transformer off-circuit.
BTW I measure each resistor before soldering in the circuit. And for a good reason: I found a Mills MRA-12 wirewound nominal 33k, actual 66k.

Geek 10th October 2008 10:40 PM

I do believe there is an Edcor vs. Hammond THD shootout somewhere. Was it one of Tubelab's pages? I can't find the link right now :confused:


EC8010 11th October 2008 09:17 PM

There are plenty of transformer distortion measurements around. Sowter transformers plot distortion against frequency for a number of their transformers.

jon_010101 11th October 2008 09:41 PM


Originally posted by Geek
I do believe there is an Edcor vs. Hammond THD shootout somewhere. Was it one of Tubelab's pages? I can't find the link right now :confused:


Tubelab has some SE transformer tests, I recall...

Funwithtubes has some Push-Pull tests. I linked one below:

lcsaszar 16th October 2008 11:42 AM

2 Attachment(s)
This is the setup that I proposed. The generator and analyzer can be a PC soundcard with a suitable software (e.g. RMAA).

jrevillug 16th October 2008 04:19 PM

Since the 100w transistor amplifier isn't linear, I'd suggest measuring the 'THD' opf that and subtracting that from the transformer results.

Some parts may cancel, some may add, but with the proposed thest you won't just be measuring the transformer.


HDTVman 16th October 2008 05:26 PM

Don't forget that in most tube output circuits that use a transformer there is DC current flowing in the primary side of the transformer. This can have a large effect on the performance of the transformer. The DC current in a single ended output stage will mean that you need a transformer with a large core with a gap to prevent over heating and saturation of the core material.

In a PP circuit the DC current is less of a problem because power is generally supplied to the center tap and the plate connections to the ends of the coil winding causing the DC flux level in the core to be near 0.

If you don't put DC bias on the transformer during the measurement the distortion will not be the same in the circuit.

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