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RE: Single stage DHT headphone amp

Posted by drsx (A) on August 24, 2014 at 11:01:00

In Reply to: Single stage DHT headphone amp posted by hvbias on August 24, 2014 at 10:26:40:

Your DAC chip basically has the voltage you need, so you need to find a tube and transformer that balance each other and act as a buffer. I built such a thing years ago with output autoformers. Ignore the stuff in there on damping factor, etc. as it ignores the copper losses, etc.

26 has a very high rp and very low current, so you really need a very high plate load, and a very high primary to secondary ratio. However, you'll lose your voltage in the process making it a poor choice.

71a, which is what I used, may work better, though it is still not quite ideal. rp is ~2K and mu is 3. The LL1930 in parafeed configuration has a voltage ratio of 5.8:1. Used that way, you get a ~10K plate impedance. With a stepup on the input (maybe a LL1674 doing 1:4), you'll be OK, probably even after copper losses.

LL1660 for single feed is 4:1 which gets you ~5K on the plate which is OK and reduces the gain necessary from an input stepup so the LL1676 may work OK. However, recordings that are not red-lined will still be low, so the 1:4 input is still probably prudent as the 71a's mu of 3 will almost entirely be eaten up by the copper.

Probably the right way to do this is with something like the Emission's Labs 20A or 30A.

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