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Old 5th August 2002, 11:07 AM   #1
diyAudio Member
Join Date: Feb 2002
Location: Germany
Default Common mode choke as audio frequency transformer?

Many posts here deal with fancy audio frequency coupling transformers on DAC analog outputs or amp inputs and tube amp outputs. Unfortunately, those fancy specialist transformers have all but disappeared from most supplier catalogues.

May aim is much more humble. I feed the signal of my computer and stereo VCR to my amp. Even when I listen to CD, the amp ground remains connected to the PC and VCR grounds which is not ideal. So galvanic insulation might be a good idea. Those sources not being of the highest quality, I don't really really need a close-to-perfect transformer.

I saw one catalogue that suggests to use common mode chokes as audio coupling transformers. Considering that those are little, unsaturated toroids, this sounds like a good idea.

However, the largest value I can get is 100 mH. Assuming I load the output coil only very lightly, at 20 Hz, the impedance of the input coil is only 12.6 Ohms. Assuming my source has 600 Ohms output impedance, I will attenuate a 20 Hz signal by 98% while a 2 kHz signal passes with only 33% attenuation.

Reasoning correct?
Anybody tried this nonetheless?



I just found some "real" couling transformers in a catalogue. Strangely, no inductances are given, only resistive impedances. Even the 1:1 unit is supposed to have 4R6 on the primary and 280R on the secondary.
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