Zeus Single Ended MOSFET Preamp / Line Driver

600 / 150 / 38 Ohm

Test setup. Output to HP8903B for RMS measurements + loading tests.

General Preamp / Line Driver Schematic.

Test Configuration

Q1: STW34NB20 mosfet
V+ 34 volt supply
T1 Input Transformer: TX-102 +6dB
T2 Output Transformer - see bottom of page for details.
Rterm: none
R1: 470R
Z1: 12V
500 mA mosfet bias (at 7.58 volts).
Output impedance: TX 1:1, 10.5 ohms.
Output configuration to drive: 600 ohm series, 150 ohm series/parallel, 38 ohms parallel.

1 kHz THD+N at 2 Vac RMS into 600 ohms = 0.009%

FFT Tests - 600 ohm load

Notes:

Levels shown in FFTs plots for -3, -6 and -9dB levels will drive power amp with 1:10 step up input transformer beyond standard 34v supply power rail.
Raising peak to -120dB based at 1 kHz test signal is a bad ground condition (discovered after testing made). This demonstrates clearly the importance of good power supply hygiene! (I will retest when time permits.)

Line Driver output = 5.79 volts into 600 ohms - 1 kHz FFT (-3dB) - 0.026% THD

Line Driver output = 4.10 volts into 600 ohms - 1 kHz FFT (-6dB) - 0.018% THD

Line Driver output = 2.90 volts into 600 ohms - 1 kHz FFT (-9dB) - 0.013% THD

Line Driver output = 2.05 volts into 600 ohms - 1 kHz FFT (-12dB) - 0.0093% THD

Line Driver output = 1.45 volts into 600 ohms - 1 kHz FFT (-15dB) - 0.0063% THD

Line Driver output = 1.03 volts into 600 ohms - 1 kHz FFT (-18dB) - 0.0056% THD

Line Driver output = 0.73 volts into 600 ohms - 1 kHz FFT (-21dB) - 0.0038% THD

(All tests 24 bit, 192 kHz sampling, 1 million point transforms.)

Transformers

T1 - Input Transformer: S&B Type TX-102 TVC

See: http://www.stevens-billington.co.uk/page102.htm

I am using the TX-102 in +6dB step up configuration.

Note for the Mk III version there are 23 output steps:

0, -2, -4, -6, -8, -10, -12, -14, -16, -18, -20, -22, -24, -26, -28, -30, -32, -34, -37, -40, -43, -46 & -52dB.

24th position is to cold i.e. no signal.

T2 - Output transformer: Susan Parker Type EI120x2", 4x0.71mm, 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.

SE Line Driver Output Transformer.

Although this size transformer may seem overkill the mosfet is biased at 500 mA so there is some considerable current flowing even when "at rest" and the winding wire used must therefore be of sufficient gauge for this.

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

Back to Zeus Main Page

Design by: Susan Parker, MIEE.

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

N.B. Personal liability disclaimer applies.

This page last modified on: 16th November 2005