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The Simple Transformer Explanation…

There are a lot of reports and discussions about using a transformer as I/V stage for Sabre32/Buffalo DAC, but is it such a good idea?

There is simple explanation why it is not.

According to the specifications, and the Sabre DAC whitepaper, THD will increase from -120 db to -108 db if you use the DAC in voltage mode. That is the same as saying that you need to hold the output voltage at a constant value to obtain maximum performance/minimum distortion.

A transformer basic voltage equation is Vp=RtVs, where Vp is the voltage across the primary, Vs is the voltage across the secondary and Rt is the turn ratio.

For a 1:1 transformer then, Vs=Vp.

Thus if you expect a voltage across the secondaries, there will be a voltage across the primaries and therefore you cannot hold the output voltage of the DAC constant.

  1. Russ White
    April 10, 2010 at 04:32

    Yes, you are quite right.šŸ™‚

    For some people the distortion is not a problem.

    In reality there is no such thing as using a transform for I/V conversion.

    That is because for a current source to look like a current source, the target has to see it as a very high ratio of output impedance compared to its own input impedance… Something like 100:1 would not be unreasonable. Even more would be better, and is attainable.

    With a DAC output impedance of just 195R that means the next stage has to have a very low impedance indeed!!!


    • Hifiduino
      April 10, 2010 at 15:37

      Yes. I suppose that is why an opamp works so well because it can be configured as a “transconductance” device. Also read that a transformer can be used to reduce common mode noise and as a low pass filter to remove noise above the audio band. What do you think about such applications?

  2. Russ White
    April 12, 2010 at 12:25

    Transformers certainly have desirable characteristics, like any other audio decision it is a compromise. You trade some linearity and distortion for simplicity and noise filtering. It is of course more complicated then that, but you get the idea. Some people really like the way it works, and they are not wrong. Some people prefer other approaches and they are no wrong either.šŸ™‚

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