This is an introduction to what I hope to become an interesting topic.
There are quite a few DACs out there and even many DIY kits to assemble your own DAC. Many of those utilize tubes in their output stage. Naturally, all the reason more to contribute to the community!
The DAC you see here is based on a commercially available printed circuit board purchased on eBay. It cost me around 80 US dollars even though the prices might have gone up. Regardless of that, I think it is an exceptional value for money and there are several reasons for that:
- circuit board layout is excellent and workmanship is generally very good for a Chinese kit
- board is loaded with good quality components including Nichicon capacitors (unless they are fake even though they appear to be genuine)
- board is equipped with good digital integrated circuits like the Crystal Semiconductor CS4398 DAC chip and CS8416 digital receiver chip, both are 24 bit / 192 kHz capable
- both CS4398 and CS8416 are fitted on daughter boards that can be removed and therefore, it is easy to access all the necessary pins
There is an option to add upsampling by replacing the CS4398-equipped daughter board but considering the upsampling chip is the dreaded DIR9001, it is best to leave it in the stock form and not to install the upsampling board at all. While many will argue whether upsampling is necessary or not, logic dictates that if it cannot be done precisely in order for the benefits to outweigh the chortcomings, then it is best to avoid it. As for USB, external USB-SPDIF converter can always be added.
During the design process, several criteria were set:
- complete bypass of all post-D/A signal processing which basically means the output stage has to be fed directly off the feet on the DAC chip
- no unnecessary filtering in the analog stage, even the Bessel filter has to be avoided if necessary
- exclusively single-ended output from one triode operating in class A
- separate power supplies and transformers for both digital and analog processing
- use of highest quality passive and active components in the analog stage
I won't give too many details on what has been done but I will mention a few things.
Tube stage is based on a single 6H6. Nothing like a single triode output stage. No unnecessary component or matching that can only increase cost and still keep advantages in the domain for academic discussion. No filters in the high frequency band allow for more „garbage“ in the recording to be heard but it also allows for more music to be heard. Power supples are completely separated and the one for the tube stage has been built on a provisional prototype circuit board. All of the componentes are at the top of their class so Holco military grade 2 Watt resistors were used. In addition, two 100 kilo Ohm Audio Note tantalum resistors were used. Capacitors are Nichicon and Mundorf Silver/Oil. Everything is soldered point-to-point which is arguably the best technique when tubes are in question. Outputs are available on the back in the form of two new RCA connectors. There are two power switches, one for the digital, the other for the analog stage.
Here are several pictures.
What about the sound?
It is very much alive. I'd say it's the kind of sound that makes you tap your feet and clap your hands. It isn't the ultimate in precision or definition but that was not the intention. What it does and does well is play music dynamically and organic, with palpable contours of voices and instruments. Timbre in particular is excellent. The output levels are a lot higher than usual 2 Volt RMS for most DACs and consequently, signal-to-noise ratio is higher. It is but a mere specification but allows for higher resolution of low-level detail which in turn aids the already exceptional bottom end.
I am not sure where I want to take this project.