This power amp is well documented on the Internet – Mark Alexander worked as a consultant at Analog Devices, and it is a transimpedance – current feedback amplifier – the circuit is quite different from a conventional amplifier. The design is protected – it belongs to Analog Devices, but is free for non-commercial, private use. High Fidelity Magazine, which I worked for during a long period, was, as maybe the only magazine in the world, given the option to buy a number of pcb:s for their employees – I bought a set with components, and everything has rested in my workshop for a number of years. In 2020 – I decided to build the amplifier as 2 mono blocks.
The amp was described in the High Fidelity magazine in a series of articles in 1994 – complete with measurements.
The Internet is full of different versions – generally speaking, there are 2 main features as a consequence of the well-executed current feedback design: the amplifier is fast – more than 100 volt per uSek, and has very low distortion. My version is close to the standard layout – see diagram – main difference is another opamp, right now OPA 134, and different power transistors – you can use Mosfet's or IGBT – I ended up (for the moment...) with Mosfet's.
Input stage is an opamp (fine with me!!) but here as a transimpedance device – very different from a conventional solution. Current feedback, and a DC servo.
My intened use: as amplifier for the midrange speaker in my largest loudspeaker, the TLL, and because of that, power supply is 500 V/A transformer, 28 – 0 - 28
Volt AC, dual rectifiers and 47.000 uF capacitors, making it close to a 100 W amplifier, ensuring fair headroom for a midrange loudspeaker amplifier. I only use one set of output mosfets – the driver can easily cope with more. Slew rate of my version is probably more than 150 Volt / usek, and distortion figures equal or better than the normal version, since my present opamp is better than the original, now obsolete SSM 2131. Present bias current is approx. 150 mA – the mosfet's have no tendency for thermal runaway. A midrange speaker (in a closed cabinet) is a much easier load than a woofer, so current capability is not a parameter of main importance, but I am still searching for a better solution for the opamp, and for the output devices.
Data in short:
THD: Less than 0.00 in the audio region, also with close to full power.
Slew rate better than 150 V / uSek.
I'm checking out output transistors – testing 2SK 405/2SJ115 and IRF 540 / IRF9540, but looking for options, drivers are fixed: 2SA 1142 and 2SC 2238.
02/2023: Input IC changed to AD711, output transistors changed to IRFP 240 / 9240 in TO3P housing with better thermal properties, output bias current is now a bit higher than before.
