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previous: BicBogartPeas I once heard that you can use a Pow... -- 8/27/1999 12:55 AM

Re: Power Transformers used as Audio Output Transformers

8/27/1999 1:45 AM
R.G.	Re: Power Transformers used as Audio Output Transformers Yep. Been there. You have to have the right power transformer, and then it's not a very good one. In general, you'll be looking for a low voltage secondary transformer. To determine the "impedance ratio" of a power transformer, take the voltage ratio, like 240:6.3, and square it, giving about 1451; then multiply by the proposed speaker load, like say 8 ohms, to get about 11K ohms to 8 ohms. A dual primary 120/120/6.3V power transformer would then have roughly the right ratio to match a pair of 6V6's or EL84's. 7Vac or 8 VAC might be better. You have to have a dual primary transformer, and you have to use it in circumstances that let you keep both the lowest frequency into the primary above 60 Hz and the RMS voltage across a half-primary under the rated voltage of 120Vrms, or 170Vpk, so this is only really suited to low B+ power amps unless you can find dual primary 240/240/secondary transformers. If you violate this, the iron will saturate and things will begin to overheat. Even if it doesn't overheat, the transformer is not interleaved, so the leakage inductance is high, which means that you'll have poor treble response and "crossover" distortion when one tube turns off in AB biasing, so it's really best kept in class A. Also, the primary inductance will be very low by audio amplifier standards, so the low frequency distortion and response will be impaired compared to an equivalent audio transformer. But there are some circumstances where it will work, after a fashion. It's not good.

8/27/1999 1:45 AM

R.G.

Re: Power Transformers used as Audio Output Transformers

Yep. Been there.

You have to have the right power transformer, and then it's not a very good one. In general, you'll be looking for a low voltage secondary transformer. To determine the "impedance ratio" of a

power transformer, take the voltage ratio, like 240:6.3, and square it, giving about 1451; then multiply by the proposed speaker load, like say 8 ohms, to get about 11K ohms to 8 ohms. A dual primary 120/120/6.3V power transformer would then have roughly the right ratio to match a pair of 6V6's or EL84's. 7Vac or 8 VAC might be better.

You have to have a dual primary transformer, and you have to use it in circumstances that let you keep both the lowest frequency into the primary above 60 Hz and the RMS voltage across a half-primary under the rated voltage of 120Vrms, or 170Vpk, so this is only really suited to low B+ power amps unless you can find dual primary 240/240/secondary transformers. If you violate this, the iron will saturate and things will begin to overheat.

Even if it doesn't overheat, the transformer is not interleaved, so the leakage inductance is high, which means that you'll have poor treble response and "crossover" distortion when one tube turns off in AB biasing, so it's really best kept in class A.

Also, the primary inductance will be very low by audio amplifier standards, so the low frequency distortion and response will be impaired compared to an equivalent audio transformer.

But there are some circumstances where it will work, after a fashion.

It's not good.