**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:

**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
- Input impedance 47kohm
- Frequency range 4Hz - 150kHz (-3dB)
- Amplitude response drop at 20Hz: -0.15dB/8ohm
- Amplitude response drop at 20kHz: -0.06dB/8ohm
- S/N ratio for bandwidth 22kHz: -94dB/2.83Vef
- S/N ratio for bandwidth >500kHz: -85dB/2.83Vef
- THD+N at 1kHz for output power10W/8ohm, bandwidth 22kHz: 0.0008%
- Square wave response 10kHz
- Gain 26.13dB (20.25 x)
- Dimensions 450 x 440 x 140mm
- Weight 20kg approx.

**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.

**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*