An Original Tube-Opamp Hybrid
The SOHA came from a collaboration among Mark Lovell, Bill Pasculle and me. Mark, Bill and I have collaborated on a variety of projects, but the SOHA was definitely unique. The full history of the SOHA is described in our article on Headwize.
Our design goal was to improve on most of the tube/opamp amplifiers by arranging for the tube to operate at higher voltages. We expected that the tube would perform better even if the voltage was increased by only 10 or 20V. At the same time, we wanted to keep the amplifier simple, using a minimum of easily available components. We also wanted the amplifier to be inexpensive and for the amp's components to be easily available worldwide. To support this we designed three constant current sources so that builders would have enough selection to find components wherever they live.
We started a thread on Headwize that encouraged a few individuals to prototype the amp (after Bill and Mark had built theirs). These individuals got the same good results as they did. The design and prototyping process was proceding along to create a really nice final product, but the amplifier design was hijacked by an individual whose primary motive was to sell PCBs. Many of the participants in the design thread and in the Headwize community at large flocked to the PCB offering without considering its effect on the evolution of the design. Hence, the evolution of the SOHA was completely stunted by a handful of thoughtless individuals.
Lately there has been some effort to provide "air-wired" output buffers to attach to the SOHA PCB. This well-intended effort has only led to the continuation of the unethical behavior of the individual who hijacked the original SOHA.
Nevertheless, the SOHA now has a life of its own and is making quite a few headphone diyers happy that they've built it.
Most tube/opamp hybrids have used the same B+ for both the tube and the opamp. Using a common B+ limits the tube to the maximum rail voltage for the opamp and puts the opamp into a single ended mode that requires an output capacitor. The SOHA uses a different scheme, running the opamp from a split supply and generating the B+ from a voltage multiplier utilizing the same transformer used for the low voltage supply. Thus, the SOHA doesn't have a large output capacitor, but only a small interstage coupling capacitor.
This schematic shows the amplifier section and the three original constant current sources.
Power Supply Design
The key feature of the SOHA is the power supply. The schematic shows the standard split supply with the B+ voltage mulitiplier. The multiplier generates about 80V unloaded which then pulls down to 55-60V to provide the tube with 40V on its plate. The heater runs from a full-wave rectifier attached the same transformer. Thus, the design reaches its goal of true simplicity by using a single, easily obtainable transformer to power all the necessary parts of the amp.