Zeus VMOS Line Driver

This line driver is intended for use with a separate preamp and a pair of Zeus power amps.

Basic gain is nominally 4 times, although in practise with the under unity follower the actual gain is about 3.5 times.

The circuit represents the most basic form of push-pull amplification, and is auto biased without the necessity of a separate bias power supply.

VMOS Open Chassis Line Driver

Construction is based on an 8 mm thick piece of aluminium with a couple of angle pieces for the front and rear bezels.

Front view. Small toggle switches are to select Rterm giving low or high impedance input for use with solid state or tube preamps outputs.

Rear view. As this is still a development unit I have not cut down the output transformer leads.

Top view. Overall size is c. 320 x 300mm.

  • XLR main cutouts made with a Q-Max punch.

  • Centre connector is external 24 volt power supply.

  • Local reservoir caps are 2200 uF each, on separate 1A fuses.

  • VMOS bias at c. 200 mA per device with 3R3 resistor to ground.

  • Rterm is 30K or 850K

  • Construction time = 2 days (less than a 1/4 of which was electronics).

600 Ohm Test Setup

This is the first set of tests made with basic setup on my bench.

Test setup left to right: Sowter 9063e input transformer, VMOS FETs on heatsink, SP Output transformer, 600 ohm load (switchable from 150R to 1K5 in 150 ohm steps).

General Line Driver Schematic (Note: R-term across output of X1 not shown).

Test Configuration

1 kHz THD at 2 Vac RMS into 600 ohms = 0.0011%

FFT Tests - 600 ohm load


  1. Levels shown in FFTs plots for -6dB levels will drive Zeus power amp with 1:10 step up input transformer beyond standard 34v supply power rail.

  2. FFTs below -12dB become confusing as the levels are masked by the soundcard's own distortion, see loop-back tests on second half of page.

  3. dB levels shown in graphs are relative to the measurement system e.g. 4.12 volts = -6dB, 2.91 volts = -9dB, etc. (Not ideal, I know.)

  4. The line driver is operating without any form of global negative feedback. The signal goes in and comes out. The only feedback is within the VMOS FETs with the follower configuration.

  5. The VMOS gain is less than unity, so localised semiconductor noise is suppressed.

  6. Source impedance for test = 50 ohms.

  7. Load impedance = 600 ohms (distortion drops significantly when driving higher load impedances).

  8. All FFT measurments are 24 bit, 192 kHz sampling, 1 million point FFT transforms.

  9. -3dB bandwidth is <10Hz to 200kHz.

Line Driver output = 4.12 volts into 600 ohms - 1 kHz FFT (-6dB) - 0.0019% THD

Line Driver output = 2.91 volts into 600 ohms - 1 kHz FFT (-9dB) - 0.0012% THD

Line Driver output = 2.06 volts into 600 ohms - 1 kHz FFT (-12dB) - 0.0011% THD

Line Driver output = 2.05 volts into 600 ohms - 19+20 kHz FFT (-12dB) - 0.0210% IMD

EMu 1212M Sound Card Loop-back Tests - 10K input impedance.

Sound Card Loop-back - 1 kHz FFT (-6dB) - 0.0007% THD

Sound Card Loop-back - 1 kHz FFT (-12dB) - 0.0008% THD

Sound Card Loop-back - mute - residual noise and pickup.


X1 - Input Transformer: Sowter type 9063e

Sowter 9063 Transformer

MuMetal can is very important in reducing extraneous emc/rfi pickup.

N.B. Although Sowter specify a typical  series output -3dB bandwidth of 90 kHz in my configuration I am getting 200 kHz.

X2 - Output transformer: Sowter type 9940

 EI120x2", 4 x 0.71mm, 0.08mm gap; 1:1, 2:1 or 4:1 step down.

Dual chamber, 4 x 120 turns quad-filar per side, on a 2in EI120 former.

600 ohm Line Driver Output Transformer.

Although this size transformer may seem overkill the transformer has a large gap to reduce distortion so many turns are needed for the required inductance.

This output stage will drive headphones down to 32 ohms (use 4 x parallel output windings).

Design by: Susan Parker, MIEE.

The information contained here may be used to construct one system specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies.

Email: susan@audiophonics.com

All design and other information, drawings and images on this website are
Copyright 1992-2010 Susan Parker MIET (unless otherwise credited).

These designs and other information may be used to construct systems specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies - see T&C.