| This line stage
preamplifier is our
top-of-the-line design. Four
tubes (eight triodes) in
both channels are only
active elements in sound
path. Signal goes through
few components, beside
triodes, two resistors and
one output capacitor per
channel. Rest of the
circuitry provide stabile,
quiet surrounding and take
care that active elements
deliver most. Input selector
and volume pot must be
there, but balance pot is
always excluded from our
work. If you have channel
imbalance, it is better to
locate source of problem,
then to degrade sound using
such imperfect thing as
potentiometer. To be honest,
wipe out potentiometers
totally from audio
equipment,and use 24 contact
switches wherever you can.
If you use a pot, it MUST be
a good one, minimum blue or
black ALPS. |
| OK, cut general,
now about topology.
First stage is pretty much
standard triode amplifier,
but two triodes are
paralleled to get higher
transconductance, to work
with higher current and
lower stage output
impedance. But, this stage
use local feedback, (it is
better thing then global
feedback, because with local
feedback you can control few
very needed things as gain
and output impedance) which
directly controls majority
of cuircuit parameters. This
stage is DIRECTLY coupled to
low output impedance stage,
and drive easily long cables
and different amplifiers.
Output stage is triode
cathode follower, but
untraditionaly cathode
resistor is replaced with
current source cascoded with
tube. This stage is fast and
much more symmetrical then
cathode follower. |
| About parts:
Tubes are
easy-to-get, and you can
substitute them with
different tubes inside this
biggest family. BUT, second
stage tube must be a
ECC82/12AU7 family with BIG
plate (this is biggest thing
you can see without smashing
them), and for this tube
choose U.S military, Tesla,
EI or simply compare few
ECC82/12AU7 tubes upon dark
grey area-plate. The bigger,
the better. Why? This tube
run with very high current,
and plate dissipation is
about 0.88W per triode
(1.76W for whole tube), and
this is very close to the
limits (but safely enough if
tubes are good). |
| Other parts are
(or could be) standard
quality. Resistors are 0.5W,
unless in shematic is
mentioned different. 1 uF
and 4uF capacitors in signal
and local feedback path MUST
be polypropylene, best if
they are 630V (safety
first), but manufacturer is
up your choice and budget.
HOLCO resistors and WIMA
MKP-10 are good things, for
start. Buy new tube sockets!
|
| Power supply
must SUPPLY active circuit.
Transformers could be
separated, and connected
with preamp case with five
pole XLR and good cable
(nothing fancy, 4x1.5qmm
shielded) if you are hum
sensitive. I have not use
one transformer per channel,
because regulation is
separated, B+ and filament.
you can use standard diodes
or replace them with fast,
soft recovery types.
Standard diodes could be
bypassed with 10nF/1000V
capacitors, if you put
capacitors on upper side of
PCB, and then solder diodes
from bottom. Mosfet
regulator is simple, but
give a good regulation and
provide soft-start. Apply
generous heatsink for
regulator Mosfet and
filament IC (78S10), with
arr. 4C/W. |
| Timer circuit
is simple, but needed part.
After several years of
puristic attempt to
eliminate such things as
relay contacts from signal
path, it is clear thing that
more harmful is repair of
woofers or daily replacement
of every single fuse (best
fuses, and very, very fast
are transistors, they always
blow). Timer-delay gives the
rest of the circuitry 30-50
seconds to calm things down,
and to securely disconnect
preamplifier output from
amplifier in event of power
failure in 50
miliseconds...Ouput relay
must be small, in-line type,
SPDT or SPST-this one could
be mounted on PCB, but
without connecting output in
open state to ground. |
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Linestage
schematic| Linestage PCB|
Linestage
PCB layout
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