All of my OTL parts arrived today, will start working on the prototype Wednesday
In the mean time, I have been doing some deep dive reading on NOS R2R DAC design, specifically around I/V conversion stages. Many of these DACs are current output types and are intended to work into a minimal impedance, the lower the better. This current output needs to be converted into a workable voltage, hence the I/V conversion. That voltage can then be amplified to line level through some sort of gain stage, ideally with a low output impedance.
The "best" I/V conversion is a bit controversial I am finding, as usual there are pros can cons to each. The simplest, and probably most common, is a resistor - the current output from the DAC IC creates a voltage across said resistor and that voltage can then be amplified. The value of the resistor often is tuned by measurements and ear, finding a balance between distortion, output voltage, and noise.
Another less common passive approach is to use a DAC interfacing SUT. The transformer adds some gain to the I/V stage, allows the I/V resistor to remain low and helps match the impedance to the following gain stage, if needed. The transformer also has the advantage of a "built in" low pass filter. It also breaks the ground connection between the DAC and the output stage, keeping quantization noise out of the output ground. This is pretty appealing, although opinions are mixed as whether or not it is an improvement over a resistor.
The other option is an "active" I/V stage, using a transistor or op-amp typically. This is the approach taken by SW1X, for instance, and many sing its praises.
I think this is just going to take some experimentation! No rush whatsoever as I have to plan the power supply anyhow, which will be the bulk of the work. As cliché as it is, I am likely going to base this design on the famous TDA1541A.
