THD meter circuit needed

[Jim Patchell]

Martin Czech wrote:

> :::    Another posible way is to use a A/D converter on your PC.  Again, you
> :::will need to get a sound card that exceeds what you are attempting to
> :::measure, but I am pretty sure you can get one that will be better that 100
> :::dB S/N.  Once you A/D the signal, you can process it with an FFT.  This
> :::would probably give much more acurate results.
>
> I have now tryed a couple of pricey sound cards.
> In order to record some live event directly to hard disk.
> The result: the amount of useable bits is sometimes 13
> or 14, but never 15 and beyond. The reason is hf-noise
> and clock subharmonics as well as the unbalanced inputs.
>

    You need to try a Layla.  With the input shorted, I get a noise floor around
-100dB.  Trying to find an analog source that good, however, is a bit more
difficult.  The noise floor on my crappy mixer is about -60dB.

>
> We ended up with the good old trusty Sony dat...
>
> That's why I proposed the optical in card.
> Some of them seem to produce artefacts as well...
>
> m.c.

    -Jim