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High-Voltage Entertainment in the Home
Pre-Amplification  The
Line Stage
The big silver box with three knobs contains
a line stage and a headphone amp. The active component of the line
stage has gone through many iterations. It currently uses a 12B4 as a
single voltage amplifier stage, with C4S active loads and the usual
RC combo for bias. Each 12B4 plate feeds two halves of a 6BX7,
configured as
cathode followers, which act as line drivers. One 6BX7 section, coupled
to the
output
by a 0.33uF copper foil-in-oil capacitor,
is
full-range to any reasonable load, and drives the subwoofer amps (or,
could drive the primary
amplifiers if the subwoofers were to suddenly vanish, as, say, the
result of a horrible domestic dispute). The
other 6BX7 section is
coupled to the primary amplifiers by a 0.01uF copper-foil-in-oil cap,
which, with the grid resistors of the amplifier, form a first-order
high-pass filter
with f1=100Hz.
This allows the primary amplifier to bypass the bass management of the
subwoofers, which was not to my liking. The plate loads on the VA stage and cathode loads on the driver stages are handled by constant current sources and sinks. B+, at 300V for the VA stage and 350V for the driver stages, is supplied by the massively filtered preamp power supply, described below. The volume control is a 45-position ladder L-pad constructed from 1/4W Holco resistors, as supplied by Michael Percy. This was not too difficult to put together, and has been absolutely reliable and noise-free. The only drawback is that it is a dual-mono volume control, which means one has to count clicks to set the volume identically in both channels.
All
of the schematics on this page were produced using the open source
package gEDA.
Here is a printable
copy of the schematic.
The Headphone Amp The headphone amplifier also went through many iterations too tedious to repeat; the current version is a parallel feed cathode follower using a constant current source for DC, and a Sowter 8665 transformer with a 5.0uF coupling cap for AC (see immediately below). I use it to drive Grado SP-2 headphones, which, at 30 ohms, represent a difficult load for tube amplifiers without transformer coupling. I tried all sorts of combinations of capacitor coupling (without transformers) and cathode followers and could never get good low frequency and high frequency response simultaneously. The Sowter unit is tonally well-balanced, and looks good on square waves between 60 Hz and 30 Khz. Power is also supplied by the massively filtered preamp power supply, which is a handy thing to have around the house.
The
Preamp Power Supply
The
phono preamp, line stage and headphone driver are all powered by a
dual-mono power supply. The single, potted transformer on the
left feeds a HexFred full-wave bridge, followed by a small cap (about
2uF) to keep the voltage up, a 6CJ3 for slow turn-on, and then
parallel LCLC* filters, where C* is a capacitance
multiplier; the resulting 400V
has less than 0.2VAC ripple.
Each stage then feeds , in parallel, 350VDC and
300VDC series
regulators (0B2+6BM8+6080; Steve
Bench's web page presents an
excellent tutorial, although Morgan Jones, in his excellent book
Valve Amplifiers
(Elseveier), vouches for a solid state design
I have yet to implement, but am eager to try). The voltage
regulators
are actually not for filtering (although they do that, anyway), but
for regulation, so that whether or not I have the phono preamp and/or
the headphone amp plugged in, the line stage gets exactly what it's
planned for. The transformers and capacitors on the right are for
heater supplies for both this unit and the preamp, headphone amp and
phono stage.
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