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Congratulations! But....

there are a couple of questions (I presume you've copied the spdif tube buffer circuit from the Lampizator site):


1. a tube cathode follower (CF) cannot drive super low impedance satisfactorily, (75 ohm in your case), although a Whites CF with suitable tubes may drive as low as 100 ohm. A good tube circuit design requires that loading of a tube to be 3 to 5 times the Anode load, the same applies either to Anode follower or Cathode follower circuits. This CF circuit has a loading of 150 ohm, and the tube (looks like a Chinese 6N1 or Russian 6H1) has a Anode resistance of a 2-3 Kohms. The load line of this CF circuit is very steep, it has a low voltage swing and high current characteristic, low but not super low impedance. The tube will distort more than a traditional design due to in appropriate cathode loading.

2. I presume the SPFIDF signal is taken from Tx Pin 16. Suppose the CF has a gain of 0.9, the 3.3V SPDIF signal will be reduced to 2.97V and than halfed by the resistor network=1.485V. This exceeds the SPDIF spec. requirement(0.5 to 0.6 V peak to peak voltage, i.e. 1.2V).

The tube buffer circuit is fun but far from ideal. You should conduct blind listening tests with and without the tube buffer, better ask someone to listen rather than yourself. Without tube buffer means taking signal from the Tx with the recommended resistor network on P.22 of the data sheet, with or without transformer. Let us know the result.


Edits: 06/16/10
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