Jax' place

Power emplifiers with tubes

It's difficult to remember but I think I built my first stereo amplifier back in the year 1968. It used one ECC83 and one 6AQ5 per channel in single ended configuration with an output power of 2W.

Fast forward to the year 1989 and my next design where I tried to replicate a design by Jean Hiraga who had designed an amplifier using EL34 in push-pull. I could not make it work properly and Hiraga never published the complete schematic.

I scrapped that, or rather scrapped the input stage and redesigned the outputs to class A ultra-linear push-pull. A low-μ triode as input and a long-tailed pair as a phase splitter. Low gain and low feedback and low power output but sounded nice. Schematics can be found here. This one served me with music until 2004 when I decided to design a new based on what I have learned so far. The basic idea was to make all stages in the amplifier balanced, even the input stage. This way I could use the input as a combined amplifier and phase splitter and the former LTP second stage became a balanced amplifier driving the output tubes. The input pair is biased with a constant current sink. This is a configuration I have kept ever since then.

Balanced amplifier stages have an intrinsic power supply rejection which stops both hum and what is called motorboating. This is also the first design I could simulate in SPICE. Schematics can be found here.

Later I experimented with low frequency phase correction in an attempt to better the phase margin at the low end of the frequency spectrum. This is the version I'm currently using. Design is the same plus a correction network between input and driver. Schematics can be found here.