For the most accurate information, I would strongly suggest you call the manufacturer. In the past, I have found them to be very forthcoming on such issues.
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Bmdduck - My DAC1 revision is H. Early Benchmark had few problems. First was thin sound of Signetics NE5532 opamps until factory burned down in 2000 or 2001 and Texas Instr bought license from them. Texas Instr redesigned die (larger) and sound got fuller. NE5532 is very cheap because of horrible DC spects (not needed) but sounds good. Award winner LM4562 from National Semi is used in DAC1 USB in the output stage to lower output impedance (stronger amp) and in more places (or all) on DAC1 Pre. LM4562 has great DC spects but I don't know if it sound better. Second problem was high output impedance on unbalanced outputs - it was so high that people used headphone outputs instead. All these problems are fixed now except a little high output impedance on XLR output at -10dB jumper position. 0dB and -20dB positions are much better but I need -10dB (covers all range of CDs + HDTV stations). It is not bad in my system (only 0.5m XLR) but it was reported in one review that 0dB position is slightly more vivid. If you buy used check warranty (and revision) - it might be for first user only (5 years). Volume pot developed static that disappears after "exercising" pot a little. Pot is small and looks cheap but tracks very well. According to Benchmark signal path is the same with volume pot or without - no gain of using preamp other than possibly better pot.
I have connected DVD, HDTV, and Airport Express (Mac mini server). To make all the connections I needed Toslink to coax converter and coax to XLR (Canare on ebay) converter.
Benchmark doesn't care about the source. The only difference between good and bad transport is jitter and Benchmark has huge jitter suppression. Same for digital cable - it doesn't matter. Normally Toslink has roughly twice jitter of coax but Benchmark ignores it. Cheap DVD player is sufficient, provides great tracking and plays (inherent) MP3s. Next best after Benchmark is Bel Canto DAC3 (also upsampling = jitter rejecting) but it is $2.5k. Weiss Minerva is even better but price is "better" as well $5k. Weiss Medea is great but for $15k etc. I think Benchmark is incredible but be careful since according to Benchmark's technical director John Siau it wasn't designed for warm sound since warm sound is not so great for some instruments like piano or percussion (great for guitar or voice). So Benchmark is very reveling and neutral. I suffered for a while with a little bright Paradigm Studio/60 v2 and not forgiving neutral Rowland 102 power amp (class D) until I bought relaxed and detailed Hyperion 938 speakers - heaven. I use 0.5m XLR interconnects. My Benchmark is dated 10/05 revision H (this revision was made 11/04)
John Siau warned about replacing amps. Their study and measurements showed that some expensive amps like Burr-Brown OPA628 (number might be wrong - my memory is not getting better) were actually worse performing (produced more harmonics). He even gave details which harmonics got higher. Benchmark had loaner program - free 30 days evaluation. I bought directly from Benchmark
Thanks for the detailed explanation. I was thinking about a used Dac1 so I will probably contact Benchmark with a serial number if I find a good candidate.
I may spend a little more for a Dac1 USB for future resale value and flexibility. Sounds like from you post the USB versions are all using current output stage parts.
Benchmark claims that there have been no revisions in the USB models. I agree with Kijanki on the neutral sound. If I were to take a step up, it would be the Weiss Dac 2 which is a Minerva without the dressing for 3k. If you go with a Benchmark play close attention to your power and power cable. I disagree with Kijanki about not hearing a difference with USB/toslink/spdif. I have heard differences in USB cables and flat out think that toslink sucks compared to the other two, although I have not tried real glass cables.
Bmdduck - USB is safe bet but USB sound is not necessarily better than Spdif. Your plan with serial number is the best think you can do. Ask them what else changed afterward.
One more thing - If you use it with volume control (like I do) disable third position of the switch behind (for instance metal plate allowing only two positions) so that it would not put full amplitude straight to power amp if you switch by accident (cable press against it etc).
One other point for you to accept or discard: my Benchmark DAC1 USB opened up a bit when I tried a Furutech power cord. I'm personally a big believer in Furutech products, so I may be hearing what I want to hear. But for me, the improvement was in the 15-20% range...not dramatic, but not miniscule, either.
4est - I'm surprised that you could hear any difference between toslink and coax. Benchmark has few Hz jitter bandwidth and at the frequencies of interest - kHz has in order of -100dB supression of the jitter. Typical jitter artifacts (sidebands) are at about -80dB (but still very audible since not harmonically related to root frequency). Combine two and you'll get -180dB jitter. I cannot hear any difference between Aiport Express with toslink and DVD player with coax but I will try it again. Sound with toslink is as detailed and clean as it gets. People often compare toslink to coax using two devices where one of them is not "bit transparent" (digital volume control, DSP processing etc). I don't dispute what you're hearing - I'm just surpised.