My adventure with the HT SMPS modules continued. Did further testing with higher power modules which worked ok.
The headphone stage (HP) has a 1MHz bandwidth, impressive. Here is the circuit breadboarded after the modification of the 2P29L HP amp:
The cap multiplier isn’t mandatory as the gyrator PCB has a high PSR so a good well-filtered HT supply will work. I added the cap-multiplier for 2 reasons. I have the PCBs made and also I was looking to improve the supply noise of the SMPS modules. My intension was to power this up with an LT supply module I have based out of 2 variable SMPSs.
The Chinese 40W HT SMPS modules that can do 450V work fine. They need a good HF filter as well as a proper LC stage to decouple and filter the ripple to make it absolutely dead quiet.
Here is the variable HT supply I build some time ago which I use for multiple circuits:
With this stage I have the choice of swapping the output caps. Either a massive (aka expensive) Mundorf EVO Oil 100uF/350V for direct coupling of the 300R headphones, or via the Sowter HP output transformer with a smaller cap.
I personally prefer the OT coupling in this circuit, but both sound extremely well.
More work and measurements to follow.