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Re: DAC I/V conversion

Posted by KevinC (M) on June 26, 2004 at 13:05:20

In Reply to: Re: DAC I/V conversion posted by Pooge on June 26, 2004 at 12:14:40:

"In my situation, your passive output has a too high output impedance for my TVC. The tube unit would not be something that I would spend that much for as an initial investment. That leaves me looking for an alternative right up front--which takes time--which would delay my purchase of the DAC, etc.... But then, most people don't have TVCs, so the passive stage might be fine for them."

I suspect that my words will not shake your opinion, but I will offer them anyway.

I am listening to the prototype of the passive transformer output stage (output impedance 2.8K ohms) driven by one of the RAKK DAC prototypes followed by a Stevens & Billington TX-102 Mk I TVC, which then feeds a fully push-pull transformer-coupled tube amplifier with an input impedance of 25K. This and 5 meters of moderate capacitance interconnect,too, made me worry a bit like you before I hooked it all up. BUT, I was simply speculating on technical grounds. Speculating ain't hearing. My ears tell me that the sound is really marvelous (now that the LL1674 transformers have broken in) regardless of my theoretical expectations.

The key technical issue with this passive output stage is the cable capacitance. At 3K source impedance, it only takes a couple of thousand pF of capacitance to produce a very noticeable high frequency roll-off in my system. My interconnects have just less than 1400pF and are marginal in this regard, but I still enjoy the sound immensely. The differential tube output stage has slightly more "sparkle", perhaps due to the much lower output impedance and resultant lower cable interaction. If you have a more moderate interconnect length, then I wouldn't think that the difference would be so significant.

My rule with audio is to think some at the beginning of a project, develop a reasonable idea that is appealing, build and listen carefully, then start to worry about the theoretical difficulties when and if I hear something that justifies the worries. Every approach carries technical risks; if we were to accept no uncertainties or compromises when we start out we would never start out. And the enjoyment of music would be much the worse for it!

Kevin Carter
K&K Audio
www.kandkaudio.com

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