OK, so I have a prototype for the Pro-Ject DAC Unity Gain Tube Analog Stage and linear 5V PS 'Mod Box' for lack of a better name yet!
First of all, the Pro-Ject DAC itself is actually not altered in any way! Its stock supply is a wall wart with micro USB connector, 5VDC.
The 'Mod Box' contains a tightly regulated, completely linear 5V DC supply, rated for 2A current.
The 'Mod Box' contains a 6922 based Unity Gain Tube Analog stage and RCA inputs and outputs.
The 'Mod Box' can be used between ANY digital source and preamp/amp. It has no volume control and is Unity Gain, i.e. no gain added. If offers a WIDE frequency response from 20Hz - 100Khz flat. It also measures less than .02% THD. So, it is not adding distortion, nor is it limiting frequency response. It does however offer an EXTREMELY low output impedance, allowing for excellent matching with any preamp/amp of even the lowest input impedance and will drive any cable easily.
I have always felt that tubes add 'Magic' to the music, for lack of a better word. Many attribute it to even order distortion, which is also true. Tubes impart their own sonic character to the signal that IS additive and in my mind makes things sound more natural and musical.
Many reading this will say that we are simply adding 'TONS' of tube distortion (note the < .02" THD of the device itself) and that we are adding a 'band-pass' box, that is lobbing off the highs and lows to accentuate the mid-range (note the BW of 20Hz - 100Kz flat!).
Today's listening tests were done by myself, Kristin and Julia. Kristin and Julia work for ModWright and while not Audiophiles, both LOVE music! Julia attends a lot of live music and both ladies are young and have excellent hearing. I would often ask my wife to listen in the early years, because women, who are not audiophiles in particular, simply have great hearing and will give their impressions, free of 'audiophile' adjectives and expectations.
The digital filter settings were more subtle and both Julia and Kristin found a couple that they preferred, the differences were not huge. I personally found that some of the digital filters make the music more aggressive while others are more musical. Your tastes will vary here.
I played a track for both Julia and Kristin on the Oppo and then on the Pro-Ject with the Mod Box in line. They both agreed that it was very close. Julia said that if she had to pick between the two, she preferred the 205 with tube mods. We all pretty much felt that in many ways, the Oppo had a slight edge, but we also all agreed that the difference was SMALL! Considering the cost difference: $~$3800 for 205 with mods and ~$1K for Pro-Ject DAC and Mod Box, this speaks volumes.
When I took the Mod Box out of the loop and just played the Pro-Ject DAC by itself compared to the 205, both Kristin and Julia said that the DAC sounded thin and 2-dimensional compared to the modiifed 205. Kristin said it sounded more like her car radio and Julia said that it just sounded very thin. Keep in mind, there are no mods to the Pro-Ject DAC, were are simply adding the Unity Gain Tube analog stage to the stock outputs of the DAC.
Lastly, keep in mind that both Pro-Ject S2 DAC and Oppo 205 are both feeding a SWL 9.0 Anniversary Edition tube preamp, into a KWA 150SE SS amp. I mention this because many people ask, "why not just add a tube preamp?".
So in short, based on my listening and that of my employees today, I am very pleased with the level of performance that we are able to achieve with the Pro-Ject DAC and 'Mod Box' with Unity Gain Tube Analog stage.
The strengths of the Pro-Ject DAC are:
Extreme flexibility with regards to formats (DSD, MQA, all hi-rez inputs) and multiple selectable digital filters.
It also acts as a preamp allowing for direct connection to the amp.
It has a built in headphone amp that drove even my HE1000's well! They would have liked a bit more power, but the S2 DAC will drive most modern headphones well!
It is inexpensive! It can be had for $350-$400 new!
The 'Mod Box' offers both upgraded power supply - 5VDC - and Unity Gain Tube Analog stage, which elevate its performance to an entirely different level, making it a VERY high value solution for DAC, HP amp and preamp!
I am going to do further experimentation with the 'Mod Box' - and find a better name! - including tube-rectification and pass through RCA to allow for comparison of the stock source with and without the Unity Gain Tube Analog stage. I may make allowances for different types of tubes also. I like the 6dj8 family very much however for its low plate impedance, high transconductance and ultimately its sound! I will test with 6CG7 at least, as this tube has EXACTLY the same tube curves and sound of the 6SN7.
Please let me know if you are interested in this and if so, what options you would like to see. I plan for this to be a product that can be used with any number of different digital sources, in-line with their RCA outs. Additionally, I may make the power supply option modular such that it could accommodate different power supply voltages and connections. Certainly we could provide 5V, 9V, 12V DC and could offer different connectors to suit a group of different products. We could also simply have a USBA connector on the back and make umbilicals that were terminated with the connector suited to different products.
Thanks and I look forward to your feedback!