PM-A4 very low distortion class A amplifier

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

Measured with 8 ohm load at 1kHz. Measurement bandwidth 22kHz.

THD at 1kHz

THD measured with notch filter at 1kHz, with 8 ohm load at 8.1Vrms (8.2W / 8 ohm). We can see only 3rd harmonic component with -111dBV level. This is -129dB below 1kHz fundamental (8.1V, +18dBV) which makes 0.000035% harmonic distortion. Measurement bandwidth >500kHz.

THD+N vs. frequency at 8.2W / 8 ohm

Load 8 ohm, output voltage 8.1Vrms, bandwidth 22kHz.

THD vs. frequency with 4 ohm load

Same measurement but with 4 ohm load at 7.1Vrms, output power 12.6W / 4 ohm.


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


Pavel Macura, 06/2016