PM-A4 very low distortion class A amplifier (2016, modified 2022)

This is my 4th class A amplifier built since the year 2003, successor of PM-A1 and PM-A2. The goal is to investigate the class A output stage with the lowest nonlinear distortion and wide bandwidth.

Circuit design of the output stage is based on [1]. The output stage operates in the feedback of the operational amplifier with JFET input stage.

Simplified schematics is shown in this image:


2-channel class A amplifier with idle current 1.5A per channel. Class A operation is fulfilled up to 3A output current. Strong local feedback in the output stage (Q1 and Q2) minimizes output stage nonlinear distortion to less than 0.001% THD.


Measurements with Audio Precision 2700

THD+N at 1kHz vs. amplitude of input signal (2016)

Measured with 8 ohm load at 1kHz. Measurement bandwidth 22kHz. 06/02/2016

THD+N vs. frequency at 8W/8ohm


New measurements with 2022 version (increased supply voltage for opamp, updated PCB)

THD and THD+N vs. power at 1kHz/4ohm


Spectrum with 4 ohm load at 15W/1kHz


THD and THD+N vs. frequency at 15W / 4ohm

Load 4ohm, bandwidth 45kHz.

RightMark Audio Analyzer report


[1] Sassen, S.: ExtremA, a reference class A-DIY Amplifier.


Pavel Macura, 06/2016, last modified 02/2023