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ehous 30th April 2011 01:27 AM

Impedance of parallel transformers
I am building a custom preamp and am having trouble understanding ac impedance calculations.
Need to know what impedance to build my step attenuator.
I wil have a balanced step attenuator, a pair of 10K /10K 1:1 Sowter transformers with the inputs in parallel, one of the out puts will go balanced to a 68K per phase amp. The other out puts will go single ended to a 200K sub amp.
Am I right that the attenuator should be 5k or less or can It be 10K?

DF96 30th April 2011 12:26 PM

To a first approximation, a transformer does not have an impedance but transforms impedance. In your case it is 1:1, so the impedance presented to the attenuator will be 68K in parallel with 200K.

Running audio transformers into an impedance much higher or lower than they were designed for may affect frequency response.

ehous 30th April 2011 01:38 PM

Thanks for the "to the point answer". If i raise my amp input resistor from 68k to 200k, I can use the existing 50K step attenuator i have as all of them in parallel will be higher than 50k (200k/200k/200k = 68k i believe).

AndrewT 30th April 2011 07:35 PM

Where is Steve Eddy when he's needed?

In the meantime, a 10k audio transformer expects to be loaded with 10k, not 68k, nor 200k.

A 10k:10K (the same as a 1:1) will present an apparent load to it's source that equals the load you present to it's output.

The source sees a 10k load and each transformer needs to be loaded with 10k.

Two transformers in parallel each loaded with 10k loads will present an effective 5k to the source.

Bill Fuss 6th May 2011 11:23 AM

The Sowter people can give you the best answer, drop them an email.

A line output trafo usually does not need an appreciable load to operate correctly, it depends on the particular construction. In general, the impedance rating is the maximum load the trafo can handle and still meet specs.

Best, Bill

ehous 6th May 2011 09:41 PM

Thanks for the response,

I started with sowter and the manufacturer of the step attenuator. Sowters response was " some where under 5K input and somewhere over 20K output. Shich makes sense, I was hoping some would offer a better understanding of it. I have since built a 2.5K attenuator, though I have not tested it yet.

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