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.
Continue reading “Starlight Discrete DAC: a learning experience”
