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).

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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).

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Test Configuration

M1, M2: DN2535N5 VMOS FET (Supertex). V+ 24 volt supply X1 Input Transformer: Sowter 9063e, 1:4 CT step-up. X2 Output Transformer: Sowter 9940, 1:1 Quad Filar (gapped core). R-term: 20K0 (change to match driving impedance). R1, R2: 2K2 Z1, Z2: 12V - Note reverse polarity compared to conventional MOSFETS. 150 mA bias per VMOS FET. Input driven direct from EMu 1212M balanced output. Output impedance: TX 1:1, 14.5 ohms. Output Load: 600 ohm.

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

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FFT Tests - 600 ohm load

Notes:

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 = 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

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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.