KISASS came about as a suggestion by Iain Churches that I design a SS
equivalent of Andre Jutes (so far mythical) KISS design, supposedly a
single-ended tube amp with 300B output tube, a double-triode
input/driver stage, zero loop feedback, and an output of less than ten
watts. Ignoring a bunch of tube-centric 'rules' spoiuted by Patrick
Turner, I had a think about how this should be done.

In terms of parts count, we cannot beat the Nelson Pass ZEN design,
which uses a single MOSFET, so we will use BJTs, the 'bete noir' of
the tube fan. In terms of excellent performance from a minimal parts
count, we have the classic 1969 Linsley Hood design, which ticks most
of the boxes but does use global NFB.

Where to go from here? OK, let's start from the simplest possible gain
stage, a common-emitter BJT with emitter and collector loads (Tr2 in
the schematic). This will set the transfer curve, and hence the sonic
signature, of the amplifier. Such a stage does of course have a quite
low input impedance, so we place a simple emitter follower buffer
(Tr1) ahead of it to allow a wider choice of source devices. The
output impedance (as with the 300B) is much too high to drive a
loudspeaker, so we need an impedance transformer. As one of the great
advances of SS was that it allowed the removal of the OPT, we will not
use iron. A simple pair of emitter followers (Tr3 and Tr4) provide
output impedance reduction without affecting voltage gain, and there
we have the design in a nutshell.

Further philosophical decisions were the avoidance of bootstrapping
from the output to the gain stage (that could be construed as loop
feedback), and the use of a heavily filtered power supply to keep the
noise floor clean. The latter was in the cause of maintaining the
'first watt is the most important' philosophy, without which this
design is utterly pointless. R15 and R16 reduce the rail voltage to
give a maximum output power of less than ten watts into loads between
8 and 4 ohms, meeting one of the principal design parameters, while
R12, R13 and R14 maintain the rail voltage of the gain stage at 5
volts higher than the output rail, allowing better driving of the
output while maintaining the natural transfer function of the gain
stage.

The bias current of the output stage is set to about 1 amp by R6,
while the quiescent output voltage point is set primarily by R1/R2.
This amplifier should be d.c. and thermally stable, but will reward
careful layout, particularly in the utilisation of the 'star earth'
philosophy, i.e. one central ground point to which all 'zero volt'
connections are made. Given this, I'd expect less than 0.1% THD plus
noise across the audio band at an output of 1 watt into 4-8 ohms,
rising to several per cent just below clipping, in the usual 'SET'
style. The output impedance will also be in the half-ohm region over
most of the audio band.

In terms of construction, it should be noted that R15 and R16
dissipate 5 watts each, so should be at least ten-watt rated, while R5
dissipates about 3 watts, and should be at least a 5-watt component.
R12, R13 and R14 should be 2-watt rated, the others should be
half-watt metal films. Note that for thermal stability, D1, D2, Tr3
and Tr4 must share the same heatsink.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

"Stewart Pinkerton" wrote in message
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:
: Stewart Pinkerton | Music is Art - Audio is Engineering

Thanks, Stewart :-)
Rudy