Discrete DAC – Bartola® Valves

Custom transformer for Discrete DAC

Some 3 years ago I built the famous “Starlight” discrete DAC and CD transport, the DIY version from Sonic Illusions. This is, in my opinion, one of the best DACs I’ve every listened to.

One of the disadvantages of the DAC output is the high (10kΩ) resistance and AC coupling requirement (unless you are prepared to balance DC, etc.) The traditional output configuration is an output capacitor feeding a step-down transformer to provide lower output impedance. The DAC can do only 1Vrms maximum due to the TTL output logic. With a 4:1 output I had to date, I wasn’t able to drive properly my VT25/10Y preamp into the 4P1L PSE amplifier.

I decided to try a 1:1 output transformer but I had a clear challenge. The big primary inductance (Lp) needed demands a careful design and construction of the transformer to minimise the leakage capacitance and inductance which will impact the HF response. As I’m driving a long cable into the DHT preamp, this presents a demanding load of 47kΩ||400-600pF.

I contacted DvB transformers to order a custom transformer to fit my needs:

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Starlight Discrete DAC Festival – London 2014

Jon’s ESL 6C4C drivers

Oh, are you taking a picture?

Bipolar discrete DAC ready

Final adjustments to the bipolar DAC guinea pig!

It never goes as planned

One hand is never enough

Jonathan’s Ti48 + DAC

A lot was already said about this fantastic day at Jon’s. The Starlight CD + Discrete DAC festival didn’t go as planned but we had a wonderful time. I had the chance to bring my own Starlight and sounded nice in Jon’s system despite the ground loop issue. Not clear what it was, despite it played well at low AVC levels but not when volume was increased. Perhaps was the load? Anyway, lots of tests from differential DACs to cathode follower outputs and Tom’s bipolar version.

I have to admit that the transformer coupled DAC sounded very transparent and detailed. The valve output gave a warmer tone and even harmonic contribution. The 6N1P implemented as cathode follower was very well received.

Tom’s bipolar DAC implementation is very promising. Clearly the tiny DC bias saturated the AVC and bass wasn’t good. Once we added the coupling PIO it came back to life with a subtle sound I enjoyed very much. Some challenges aside with the ESL resonances which I hope Jon can sort out soon.

Food and hospitality were superb in another unusual warm September day.

Most of all, a great day with fantastic company.

Thank you all

Ale

Starlight CD transport and discrete DAC finished!

After an incredibly long journey I’m glad to say that is nearly over. The Starlight CD transport (I2S output and 4x oversampling) with its discrete DAC sounds absolutely amazing. Getting back to play my CDs is what I was cherishing for a long time. I will be bringing my +500 CD collection from my previous home soon so that would be perfect timing.

I promise to write up a proper blog entry with a full description of the learning experience. For now, I will just sit down and relax. Listen to many CDs and enjoy!

All the acknowledgements and thanks go to Tom Browne for this fantastic design and all the help provided throughout this journey. I think this is the Starlight number 20 or so built so far. Not sure if there are other ones being built anyway. Also my thanks go to Jon Finlayson for his help on the DAC boards, Richard for the UI stuff and to Tony for inspiring me to give it a go…

Starlight Discrete DAC: a learning experience

You may find this post interesting or not. I just learned a lot with these tests and evolution of my design of the Starlight CD discrete DAC. Mostly, I want to thank Tom Browne for his patience and guidance throughout this interesting journey.

The Starlight discrete DAC has a relatively high output resistance: 10KΩ. With TTL-like levels it can deliver 1Vrms maximum. A commonly implementation of this DAC is with an output transformer in step-down mode (4:1) to reduce the output impedance of this DAC. The typical circuit is straight forward with a coupling capacitor between the DAC output stage and the transformer to block the DC current. The value of this capacitor is 2uF or higher. Many have used the Russian PIO with great results. All incarnations of this DAC sounded fantastic in my opinion, so there is plenty room for experimentation around this DAC.

I had a nice pair of LL7903 transformers. They are very nice and perform really well. These can be wired in 8:1 or 4:1. So they looked to be a right fit for this DAC. However, the high output impedance is a warning sign for any transformer as you would look to have a lot of inductance for a good FR. Higher inductance brings with it a higher parasitic capacitance on the winding which impacts the HF response. Achieving a high inductance and low capacitance is a challenge in any transformer design.

To confirm how good this transformer could be for this design, I measured the transformer to obtain its key parameters. To my surprise, it wasn’t that great.

Despite having a nice primary inductance (Lp), the capacitances are big. Looking at the FR response I found this not to be great (about 20KHz). Considering the Starlight CD player has LP digital filter, this will cause a slight loss of treble according to Tom. At least 30KHz or more is needed to make this not an issue.

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Starlight Discrete DAC – Build Part 1

I don’t like doing the metalwork and perhaps this is the main reason why I ended up delaying the build. Gladly I finish all the drilling and milling required for the DAC. The output transformers were changed to Sowter 9062 (more to follow on this). The outputs have a differential as well as RCA outputs. All good now to do the wiring:

Starlight: discrete DAC boards

Added the set of decoupling SMD low ESR capacitors for each board

Starlight Discrete DAC: more progress

I used a aluminium bar structure to hold both LL7903 transformers and the Russian PIO output capacitors. The LL7903 will be wired in 4:1 and the capacitor will prevent the DC current from the DAC to shunt to ground. Also used some silicon rubber to protect the mu-metal can of the LL7903:

Here are the DAC boards mounted on a PTFE board with a set of copper strips that will be used to solder 10 x 100μF SMD low ESR capacitors to improve decoupling of DC rails:

I think all fits in the box I’m planning to use 🙂

Starlight discrete DAC boards finished

After testing the accuracy of the R and 2R network, thanks to Jon managed to complete my discrete DAC boards for the Starlight CD player. Not very keen to solder SMD ICs but couldn’t get away from the tedious task of soldering 10 ICs:

Now need to continue with the DAC (or the transport). Lots of work to do still…