I'm not going to comment on the price, thats not my department! The design uses 1704 DAC chips which have a current output. The DAC chip drives a stepup transformer with an IV resistor after the transformer (PJ is looking into different arrangements, maybe splitting the resistance, some in front of the transformer and some behind, or just all behind, I personally like the all behind). This gives low impedance to the DAC (10 ohms) and still gives some voltage gain before the tube stage. The tube stage then gives some extra gain and buffering to the output.
This scheme is not very common, many designs put a IV resistor after the DAC chip, but you then have to choose, a high value resistor which gives enough voltage to drive a line output BUT causes distortion because of the high impedance the DAC sees, OR a small value resistor which gives appropriate impedance to the DAC chip, BUT needs a high gain amplification stage (similar to a phono stage). The technique we are using lets the DAC chip see the low impedance, but provides enough voltage gain that the tube stage only has to supply a moderate amount of gain.
The transformer also provides the high frequency filtering after the DAC chip so you don't need to add extra components to implement that function.
Right now I'm working on the digitally controlled super low jitter oscillator that can track any S/PDIF input. The original design philosophy was to send a S/PDIF stream from the DAC back to the soundcard which would sync its output to the DAC, this gives extremely low jitter IF you have a soundcard that supports this function. Unfortunately if you fed it from a source without the sync from the DAC you would occasionally drop bits. This turned out to actually sound pretty good, a lot better than I thought it would.
It turns out the company that makes the oscillators I'm using also makes a digitally controlled version which allows you to change the clock frequency in very tiny steps on the fly. I'm trying out one of these right now and have the frequency changing part working (I got it working about an hour ago!) With this I should be able to track the frequency of any input. It won't be quite as good as the source synced to the DAC, so that is still the preferred connection, but it should allow you guys to use this DAC with pretty much anything.
On the AES/EBU front, I'm not planning on putting it in the DAC itself, but it does use BNC connectors and you can buy BNC to AES/EBU transformers which should work. My take is that S/PDIF over BNC is technically a better interface than AES/EBU so I'm not going to compromise the DAC by building in what I consider to be an inferior interface. For those that have DACs with JUST AES/EBU you can use the transformers.
John S.
