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
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
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
VMOS bias at c. 200 mA per device with 3R3 resistor to
Rterm is 30K or 850K
Construction time = 2 days (less than a 1/4 of which was
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).
1 kHz THD at 2 Vac RMS into 600 ohms = 0.0011%
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.
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.
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.)
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.
The VMOS gain is less than unity, so localised
semiconductor noise is suppressed.
Source impedance for test = 50 ohms.
Load impedance = 600 ohms (distortion drops
significantly when driving higher load impedances).
All FFT measurments are 24 bit, 192 kHz sampling, 1
million point FFT transforms.
-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
(-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.
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.
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
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.