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:

Description

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.

Specifications

• Output power 2 x 20W/4 ohm, 2 x 12W/8 ohm (2016 version)
• Output power 2 x 35W/4 ohm, 2 x 20W/8 ohm (2022 version)
• Input impedance 47kohm
• Frequency range 2Hz - 270kHz (-3dB)
• Amplitude response drop at 20Hz: -0.02dB/4ohm
• Amplitude response drop at 20kHz: -0.01dB/4ohm
• Output noise voltage for bandwidth 22kHz: 32uV (2022 version)
• S/N ratio for bandwidth 22kHz: 99dB/2.83Vef, 112dB full output voltage (2022 version)
• THD+N at 1kHz for output power10W/8ohm, bandwidth 22kHz: 0.00036% (2022 version)
• Square wave response 10kHz
• Gain 27.01dB (22.41 x)
• Dimensions 450 x 440 x 140mm
• Weight 20kg approx.

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

Literature

[1] Sassen, S.: ExtremA, a reference class A-DIY Amplifier. http://www.hardwareanalysis.com/content/article/1842.6/extrema-reference-class-a-diy-amplifier/

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