1:1 Coupling Transformer 20Hz to 10KHz

Robert Stevens · 2016-03-23T01:54:14+00:00

I am looking for a transformer to couple a 10W audio signal to a resistive load. It needs to have a "flatish" frequency response from 20Hz to 10KHz, or else vary no more than about 25% over the above range. Turns ratio should be relatively low, preferably 1:1. Input impedance is the usual 4-8 Ohms. I have tried a variety of off-the-shelf mains and PA transformers. They either have too high a turns ratio or patchy response. Surprisingly, the best was an old ignition transformer. Flat from 5Hz to 10KHz. But the turn ratio is way too high, and I only have one. Can anyone please suggest something that is ready made and not too expensive? Robrt Stevens

Mar 23, 2016 26 Replies

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bill.sloman 10 years ago

But these aren't wide band transformers, but supply frequency transformers. There's a lot of interwinding capacitance, which you do have to drive, and that takes more current when you move above supply frequencies.

And the hysteresis losses happen every time the core is driven through it's magnetic field range. Do it more often, and you dump more heat in the core, which you have had to extract from the circuit driving the transformer.

What makes you think that?

You said something like it

"Toroidal types work as well or better than E-cores as wide band audio transformers, without the need for layered windings."

Bill Sloman, Sydney

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bill.sloman 10 years ago

It is a neat idea, and can work, but there are potential problems - which is why the world doesn't use re-purposed power transformers for every transformer job.

Bill Sloman, Sydney

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Phil Allison 10 years ago

** I am not going to play your silly game any longer.

Since the valve era, laminated steel core transformers have been used to cover the audio frequency band for a host of applications - so they work very well and to beyond 10kHz.

Toroidal core types are just another example with some advantages and some differences.

.... Phil

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Winfield Hill 10 years ago

Don't be too sure, power transformers often have rather high leakage inductance, which is a killer for high-frequency response. Audio transformers use special interleaved winding techniques to solve the problem. But a pair of split secondary windings could/might be wound in a way that'll do the trick.

Thanks, - Win

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Phil Allison 10 years ago

** Certainly true if the primary and secondary are wound on separate halve s of a bobbin. Ones that have the secondary wound directly over the primary are much better coupled and toroidals are the best with the secondary woun d all over the primary, all around the core.

** Almost guaranteed to be bi-filar wound in case of toroidals.

.... Phil

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