What to expect from a new DAC?

"So, my transport is a Levinson 31 Reference DAC.... so it was very expensive (and sounds great)."

I've modded ML transports. Not even close, even with clock upgrades.

I have a fully modded ML 38 preamp (my mods). Also not even close to a decent transformer linestage.

"My preamp is an Audio Research LS15 active preamp (tubes)."

That is not bad.

"I tried the passive linestage attenuator 20 years ago and found it not to be the way to go...(with lesser gear then I now use) If memory serves, the music just seemed dead. Funny how things go round and round... What's different with a passive transformer linestage ..... (at the time... I was told... well... the analog impedance matches were just not quite right and you lost too much with the passive Adcom line stage attenuator."

Myths. What is really happening with resistor passive linestages is 1) you are limiting the current going to the amp and 2) you are putting a low-pass filter in the cable. I dont recommend them.

Transformer linestages have neither of these drawbacks. The only real requirement is that the DAC have decent current drive to drive amps directly. Op-amps dont cut it. The ML will have 5 ohms output impedance. That is good. One must purchase a high-quality one however, or you risk transformer saturation distortion.

If you read the 6-moons review of the Music First TVC, you will discover that they believe this is the best linestage ever. More transparent than the most expensive active preamps. I would not go that far, but it is quite good. I have an $8K pure silver wound version that is cryo-treated.

"What I want to do now is add a USB to digital Low jitter interface for a PC server. What would be a good match for the Sonic Frontiers DAC/LS 15 combination?"

The best USB converters on the market are:

1) Off-Ramp 5
2) Diverter HR
3) Synergistic Research

All of these have low jitter. There is really no "synergy" here. If they are good, they are good with all DACs.

That does not mean that you will not take a step backward, because this is also possible. If you have a sibilant component or cable in your system, reducing jitter may aggravate this. More high-frequency energy is usually a result, but it will be "clean". The object will be to identify this component or cable and eliminate it from the system.

Be aware that you must make some wise choices in order to get stellar results from computer audio, namely the computer, the playback software, the ripping software etc.. It can be a minefield. Here are some tips:

http://www.empiricalaudio.com/computer-audio/

Computer Audio Asylum is another good resource.

Steve N.
Empirical Audio

Tom6897 - I really doubt if the "system" jitter is 10psec. Most clock specs are in this range, and that is when they are driven with very expensive low-noise power supplies. In systems, this number goes up usually by a factor of ten. Also, even with the best chips, the internal PLL is affected by incoming jitter.

I have used the Sabre chip that is purported to eliminate jitter. It doesn,t. I had a Wired 4 Sound DAC2 here about 2 weeks ago, which uses the Sabre chip. Each of the inputs; USB, S/PDIF and I2S sound different. The best of these is the I2S driven by an Off-Ramp 5. World-class sound quality. The others are not interesting compared to the I2S IMO. If the Sabre reduced jitter as much as advertised, none of these differences would be audible.

Virtually all DACs benefit from a low-jitter source. Maybe the only exception is the PWD in NativeX mode.

Steve N.
Empirical Audio

Knghifi - No joke - read this:

http://www.6moons.com/audioreviews/musicfirst/passive.html

This is an old review. New units are even better I understand.

Steve N.
Empirical Audio

As time goes by the new DACs are definitely better.
Apart from radically different designs they are all pretty well using the same chips.
I started with an Apogee some years back and even at one point questioned what the beneficial difference was.
Then I got into the Bel Canto's DACs.
The difference was like turning the lights on.
We then went into tubes and compared Ayon's SC1 to the Bel Canto VB models.
Yes the Ayon is better but marginally.
The latest buzz was the Lampizator and yes we ordered a level 5.
The wait time was so absurd we eventually cancelled.
I was happy we did as their build quality continued to haunt me throughout the waiting.
Needless to say we did not listen to the Lampi.
However after attending CES and THE this year I was presented with the holy grail.
Out of the 100 some odd exhibitors I visited one that continued to stay with me was Bluelight Audio.
Both their Evolution line and Playback Designs were a revelation for me
I couldn't get over the clarity of the source and inquired as to what chip they were using. Playback's president explained it was a proprietary system and not the same old chips.(Burr Brown or Sabre)
The system automatically recognizes any source (such as a PC, IPod, Discman, etc.) and uses new technology they have developed called PDFAS (Playback Designs Frequency Arrival System) to completely eliminate all jitter.

A special note should be given to the book shelves they presented at 2500.00 retail I have never heard anything remotely close at that price point.

Its a matter of preference, i know some people who like their older gear better than newest models. This is sort of like tubes vs SS.