Using a transformer as a filter

[Zodiac]

Hi all,

The dilemma is this. I am trying to design a filter stage for a non oversampling DAC that outputs a lot of ultrasonic noise. I have evaluated a few options so far (passive filtering, problem: needs big inductors; active filtering, problem: opamp in signal path). Another potential option is using a bandwidth limited 1:1 signal transformer. The idea is that the natural behaviour of the transformer would filter off ultrasonic frequencies above the audio band. Any comments? Also how would one set about evaluating transformers for this purpose?

[EC8010]

The behaviour of transformers outside their pass-band is not something generally discussed in polite society as ultrasonic resonances abound. The inductors needed for passive filtering need not be large if the impedance is sufficiently low. Even at 75R terminating resistances, audio reconstruction filters are perfectly possible with air-cored inductors. If you were to drop to 10R, or less, you could terminate your (current output) DAC with the filter, follow it with an MC transformer, add a bit of low-noise gain with a 6C45, and buffer the output with a 6C45 cathode follower.

[Zodiac]

Thanks for your reply. The issue is that I was going to put the filter right at the output of the dac, after I/V conversion (using a transistor circuit to boost current first) and directly before the amp. The filter would therefore be driving a 75k rather than 75r load. I think I will have to look into active filters, as using 600mh inductors is not sensible IMHO. Cheers!

[fdegrove]

Hi,

The issue is that I was going to put the filter right at the output of the dac, after I/V conversion (using a transistor circuit to boost current first) and directly before the amp.

That's a rather unusual approach...

Why not use a step-up xformer and do the I/V and de-emphasis at its input?

Cheers,