[regal]

This is an upgrade I am considering for a common DAC60. It has a PCM1704 which outputs +-1.2mA. Currently the I/V is a 220 ohm resistor which I think is too much and causing a congested sound with complex passages. This is an SRPP output stage with a 6DJ8

it would lower the I/V load to 50 ohms. Adding the a cathode bypass cap (C1) will boost the gain from 16 to 26. I estimate around 1.5 Vrms output. The filter is a copy of the HagDAC.

Does this seem to be a worthwhile upgrade?

 

[dsavitsk]

I personally think that SRPP, 6DJ8's, cheap inductors and caps as filters, and caps as cathode bypasses are all poor design choices. Instead, I'd find a better valve -- something with higher Gm like a D3a or 7788 so you can load it with a CCS and take the output from the plate. Even better, use a transformer on the output and you can do away with the lousy filter before the tube as the transformer will roll off the HF noise. And, if you use a Western Electric connection of the transformer, you can eliminate the cathode bypass. If you don't use a transformer, use an LED for bias. Last, think about your next stage. Does it amplify? The industry standard is 2V as line level output, and commercial offerings like Jim's need to reach this, but if you have more gain on the next stage than you need, you don't need to worry about this. All you are going to do is amplify, attenuate, then re-amplify, which is really silly to do if you don't need to.

 

[Roam]

Depending on the sensitivity of the amplifier or preamp, active components can be omitted altogether. A simple step-up transformer connected to the output of the 1704 will perform I/V conversion and HF filtering all in one go. However, this is not the best idea if lengthy or highly capacitive interconnects are used, or if the following stage requires a largish input voltage.