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: TX102 +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%
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 TX102 TVC
See:
http://www.stevensbillington.co.uk/page102.htm
I am using the TX102 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 quadfilar 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).
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.
This page last modified on: 16th November 2005
All information, drawings and images Copyright © 1995 
2005 Susan Parker unless
otherwise credited.
