Hi Rollo and others who have posted, and thank you.
First of all, the analog stage of a DAC/CD/DVD player does a few things:
1) If a current output DAC (common in better DACs) there needs to be a current to voltage stage (I-V) after the DAC.
2) Analog filtering needs to be present to filter out ultra high frequencies.
3) Analog stage gain.
A preamp serves several functions:
1) Input select.
3) Volume control.
4)Tone controls in some, but not common these days.
5) Home theater bypass to allow integration of HT product with 2CH Stereo.
6) Possibly phase switching.
7) Possibly balance L/R control.
Today's DACs typically put out 2Vrms analog level in stock form. Most amps reach full power at 1V-1.5Vrms, unless a low gain tube amp or other unique designs. Pass's First Watt designs are necessarily low gain and there are some others that really do want additional gain.
So, a good DAC or player with a good (lossless) volume control and 2Vrms+ of output voltage, can drive an amp directly.
Why add a tube buffer then?
Well, first of all, how good is the digital component's analog stage?
We have been very successful with our tube modifications for the Oppo players and others, because we replace the stock analog stage with our own Class A, all tube analog stage, with zero feedback. Our designs also have low output impedance and can drive long cable runs no problem.
Most digital sources however (there are some exceptions at the much higher end), use standard op-amp based circuits, either single ended or balanced. Op-amps are inherently feedback based and they are cost-effective to design with. This is not necessarily a bad thing and many sound quite good.
To my ears however, no tubes, no magic!
Using a tube preamp with a SS based digital and even SS amps, works and we have done this for years. Our amps have mostly been SS and our preamps all tube.
However, for a simple system in which your main if not only source is digital, and has a built in volume control, then you don't need a preamp. This is where a tube buffer is ideal IMHO!
Our tube buffer design IS low distortion and does not limit frequency response.
My first intention with this, was to offer another option for lesser cost digital gear, alternative to a full tube modification at higher cost. I started with the Pro-Ject DAC because it offered DSD, MQA and SABRE DACs as well as HP amp and built in lossless volume control, for $350?! In stock form it is good, but a bit lean sounding. I found that when our tube buffer was added to the circuit, performance was tremendously improved, adding body and weight as well as a big 3-D sound stage! Now for this product, we also included a GOOD 5V DC regulated supply. The stock DAC operates from a 5V wall wart supply.
Now obviously, if it makes an inexpensive DAC sound this good, of course it will make a higher end DACs sound much better too. Brands such as Exogal, Mytech and other well respected DAC designs in a similar price range with built in volume control, would all benefit from tube circuitry.
And lastly, while you may change DACs with changes in technology, the tube buffer will never be obsolete. Also, being as how it is a separate unit, it can be used with any digital or analog source for the life of your system.
Ways this could be done:
1) Clean and simple tube buffer, one input (RCA) and two pairs of RCA outs, unity gain, no volume control.
2) Same as above, but add a well regulated linear supply for the source.
3) For balanced systems, a tube and transformer coupled solution would be ideal. At this point, it may be a two box design with external supply.
4) If more gain is needed, transformer coupling could offer +6dB and +12dB gain, without switching or volume controls.
Ultimately the ideal would be a compact, clean and elegant solution with minimal if any controls. It simply goes between your source and amp.
The buffer has the following benefits:
1) Impedance matching between any source and amp, offering a VERY low output impedance, allowing matching to any SS or tube amp.
2) Provides tube tonality and 3-D soundstage to a system without the need for a tube preamp or more costly modification to your source.
3) Optionally also includes an upgraded external supply for your digital source (could also simply be straight buffer).
4) Ultimately offering a big sonic upgrade whilst reducing system complexity (no preamp) and at reasonable cost.