As I promised, I took the HP amp back to the workshop for a bit of abuse on the bench. Here are some interesting measurements which correlate with some of my listening impressions so far.
I added a set of 300Ω dummy loads at the output to simulate the HPs. Firstly I tested the optimal circuit which has the Sowter 8665 output transformer in Parafeed mode (4:1) to drive the load. You can get a very clean and flat response across the audio band from 3Hz up to +90kHz. There is a minor hump to be tuned due to the RLC circuit formed by the parafeed circuit. I used a 4.7μF Mundorf EVO oil cap. It’s pretty flat to me, so will leave it there.
The performance of the circuit is amazing. Distortion is very low, up to 0.07% at 100mW (which is VL=5.47Vrms):
