Let me just say, I'm listening to "Ripple" through my new-used 550 amp and darn it, it finally sounds as good as it did when I first heard it through my stax headphones.
Jerry, we miss ya.
Hmm, so this avowedly "modest" DAC is driving your amp via the extra capacitive load of the passive switch box in between. I would think that with a passive preamp you would need a really high quality source with plenty of current drive ability (among other things) to get the best results.
Well, the dacmagic specs (
) well in that sense, with < 50ohm output impedance, 2.1v rca / 4.2v xlr.
The new speakers were already putting such a bulge on the budget that I had to cut back on all the other components, so as long as performance was acceptable, the $400 price tag (new) was hard to pass up.
I'm sure it has some issue somewhere, but I'd be surprised if it was that kind of stuff. I think it probably gets the first 90-95% of things right, but of course that remaining bit of excellence is worth searching for, and Van Alstine's DAC is probably getting there. That's why it's my next stop on this audio journey.
Passive preamps are 'lossless' in theory, but what you gain with them is additional capacitance which can do certain things to the signal. It requires the right pairing of source and amp. Switching to an active pre could well improve matters provided the pre didn't add appreciable distortion; or, switching to a better DAC could do the trick just as well. I think you're certainly right to try that route first.
My conclusion is that if you line up the source and the amp properly, then a tvc can work well; if you don't, it might be a little bit sucky. A good active preamp is a safer, more flexible choice. Based on my 550 experience If I ever decide that I have to activate at that stage, a Van Alstine will be the first one I audition.
I just don't think I'll ever make that decision... btw I think your comment about the capacitance of the passive would better apply to resistor-based passives such as the Placette. TVCs work differently from them. They have no capacitors and no resistors, other than what is contained in the wires--but you know that we're not supposed to talk about wiring too much on this forum.
btw I have really short rca cables coming out of the TVC into the 550 (50 cm), precisely for that reason. But when I tried longer cables (1.5 m) it was OK too.
Theoretically, the TVC's biggest problem is that it likes to attenuate (once again, the exact opposite of the vast majority of volume knobs out there, including the Van Alstine, I suppose). Attenuation leads to
better sound. So if the gain from the amp is not high, you end up losing some of the best that the TVC has to offer as you raise that volume knob past two o'clock... I don't know if that necessarily leads to something bad, or simply to the tearful loss of something excellent, but at least you can count on the TVC being pretty quiet.
The reason it likes to attenuate is that it behaves as an impedance step-down device: the more it attenuates, the more that it lowers its output impedance. However, even at zero attenuation (i.e. volume knob at maximum), its output impedance matches the DAC's output impedance. But if you attenuate by, say, 20 db, then you lower the output impedance by 99%. Yep, an output impedance of 100 ohms at the dac becomes 1 ohm at the TVC.
Much of the above concerns might be moot in my situation, because the amp has such a high input impedance (courtesy of the little tubes) that the DAC is just not driving much of a load. So even though I'm only attenuating by 10 db (say), that leads to a step down of output impedance by a mere 90%... into a one-megohm tube in the amp. That
should be OK.
I suspect that the biggest problem with a TVC is some kind of behind-the-scenes industrial production problem. It probably requires some careful hands-on attention that is antithetical with mass production. There are issues with tonal balance that require some pretty careful selection of the cores, for example. Having emailed back and forth with Nicholas at Promitheus, I felt like he'd done his physics homework enough to give him a shot, and I think he did alright.
In case you're wondering, this is how I tested the TVC. First I listened to my Stax with the dacmagic plugged straight into the stax amp (which comes with a volume knob). Then I put the TVC between the dac and the amp, maxed out the amp's volume knob and used the TVC for volume control. I couldn't tell the difference between the two scenarios (using the usual Beethoven's 7th test), and if a pair of electrostatic headphones cannot resolve the difference to my (tin) ears, I figure that the preamp is OK.
