Yes, looking quite promising. I have detailed simulations including power supplies, to study things like turn-on transients and PSU intermodulation near clipping. No cheating in schematic at present. Getting about 8W just below clipping.
Some interesting nice properties that have popped out include:
- Short circuit safe
- Stable with capacitive loads (bit of overshoot). For belt and braces, could add output Zobel, but not really needed.
- Small turn on thump, only about +/- 50mV. Negative for a few ms, then +ve for 20ms, then falling back to 0. Could be connected safely to very delicate drivers like tweeters. 50mV into 8 ohms is 300uW.
- Distortion is frequency independent over wide range
- Low noise floor (something like -110dBW), which hardly changes until close to clipping
- PSU intermod sidebands are really low (>120dB down) even at 1W. Not studied these before, but takes some care to eliminate.
- Distortion is dominated by 2nd harmonic up to about 1W, although 3rd harmonic rises with increasing power (as expected) and dominates close to clipping.
- Gain about 14dB, so full power requires about 1.6V rms, very compatible with red book digital sources.
- No bias adjustment needed.
- Offset adjustment
- Easy to change upper and lower bandwidth limits, for integration into active systems.
Uses a dual JFET in the input LTP, specified LSK489, as this has low noise and good offset match, and the right transconductance, available from Mouser. All other parts are fairly cheap and available from multiple suppliers. Needs two transformers, a small one for front-end (15-0-15) and bigger one for output stage (12-0-12, 160VA)
I will put a schematic up when I have re-drawn it enough to be understandable - very messy at present with wiring wiggling all over the place as I have changed my mind and added and removed parts.