Specifications
•
Channels:
2 (stereo)
•
Valve line-up:
4 x 12AX7 twin triodes,
4
x 6L6 beam power tetrodes
•
HT supply:
-310V, actively filtered
Tested load impedances:
4n, 6Q, sn
•
Output power:
2 x 10W (80, 652), 2 x 9W (40) (see Fig.3)
•
Operating mode:
Class-A (80), Class-A/AB (60, 452)
•
Input sensitivity:
-1V RMS (80, with feedback enabled)
•
Signal-to-noise ratio:
77dB
•
Channel separation:
>60dB, 20Hz-20kHz (40, 652 & 852)
•
Harmonic distortion:
typically <0.1%, 60 & 80 (see Figs.3&4)
•
Frequency response:
±0.6dB, 30Hz-20kHz (see Fig.5)
•
Damping factor:
>20 (80), >10 (40)
•
Mains power draw:
typically 120-130W
•
Other features:
ultra-linear outputs, remote volume control option, delayed HT,
HT soft-start
•
Dimensions:
294 x 304 x 186nnm (W x D x H) including protrusions
ally liked to use a transformer with
much higher secondary voltages but a
specially-designed power transformer
would be much larger and more ex-
pensive, as already noted. Having said
that, there is future potential for this
amplifier to be upgraded with better
(more expensive) transformers to en-
able it to deliver substantially more
output power.
The valves can be replaced without
any disassembly. Their sockets are me-
chanically mounted to the thick (2mm)
PCB to prevent the solder joints from
breaking loose during valve removal
or insertion. The thick PCB also helps
to support the relatively high weight
of the output transformers, which
are mounted on the board for ease of
construction.
Temperature-sensitive components
such as electrolytic capacitors have
been kept away from the high-dis-
sipation components, primarily the
6L6 valves and associated 5W cathode
resistors. However, due to the compact
size we have not been 100% success-
ful; one of the large filter capacitors is
near the output valves. Checks of its
temperature during extended opera-
tion show that direct heat transfer is
minimal and should not be a problem.
Semiconductors
There are some semiconductor
components in this circuit but not in
the audio signal path. Mostly, these
perform power supply filtering, to
get rid of ripple and keep the ampli-
fier quiet. The HT delay and soft-start
circuit is also built using solid-state
components.
We should acknowledge considera-
ble input to the design of this amplifier
from Allan Linton-Smith, the designer
of the Majestic loudspeaker system fea-
tured in the June and September 2014
issues. Allan built the first hard-wired
prototype and the concept was then
considerably refined and transferred to
the final PCB featured in these pages.
Allan also suggested using the Al-
tronics line transformers, based on
a discovery by Grant Wills that they
could be used as cheap and effective
ultra-linear valve output transform-
ers - see http://harne.alplialink.
coin .au/-cambief6A N 8 a mp/G r a lit_
Will s_6 CM5 am 1),h t are.
Circuit description
Fig.1 only shows the circuit for the
left channel signal path. The right
channel is identical and the corre-
sponding component numbers are
provided in blue.
The line-level input signal from
RCA socket CON1 has a 1M52 DC bias
resistor to ground, in case the signal
source is floating. The signal then
passes through an RF-rejecting low-
pass filter comprising a 12052 series
resistor and 100pF ceramic capacitor.
The signal is then AC-coupled to
(nominally)
20kn
logarithmic volume-
control potentiometer VR1 by a 1.50
MKT capacitor. This gives a -3dB low-
end roll-off at 5Hz. Note that depend-
ing on part availability, a motorised
potentiometer with a value as low as
51d2 may be used, in which case the
-3dB point rises to 21Hz.
The wiper terminal of VR1 is con-
nected to ground via a 1MS2 resistor
so that if it briefly goes open circuit
during volume changes, the grid of V1a
does not float. The signal is fed to this
grid via a 221(52 RF stopper resistor.
V1a and V1b form the preamplifier,
which is very similar to Jim Rowe's
design from the February 2004 issue
of SILICON CHIP ("Using The Valve
Preamp In A HiFi System"). Essen-
tially, this consists of two common
cathode amplifier stages in series, with
negative feedback around both.
V1's plates are fed from a filtered
HT
rail
of around 224V DC, somewhat
less than the 308V DC main HT rail
due to voltage drops across the two
RC filter resistors (6.81d) and 471d2).
These filters reduce coupling between
channels, reduce coupling from the
output stage to the preamp stages and
minimise supply ripple reaching the
preamp. The preamp is the most noise-
sensitive section as the signal level is
lowest here.
In fact, because hum can be picked
up from AC-powered heater filaments,
we are running the 12AX7 filaments
from regulated 12V DC.
Self biasing
All valves in the circuit are self-
biased. V1a's anode runs at around
120V, ie, 224V minus the drop across
the 270k0 resistor. With zero bias, a
12AX7 will conduct around 3mA at
this voltage, dropping to near-zero
with a grid-cathode bias of around
-2.2V. With a 3.3kf2 cathode resistor,
V1a's operating point tends to settle at
about 0.3mA and thus the cathode is
1.2V above ground.
The output signal from V1a's anode
is coupled to V1b's grid by a 220nF
capacitor and this grid is DC biased
using a 1M52 resistor to ground. V1b
runs at a higher power than V1a,
with a 68052 cathode resistor giving
an operating current of around 1mA.
Therefore, its anode load resistance is
lower at
1001(0.
The output at V1b's plate is coupled
back to V1a's cathode via a pair of
parallel 470nF polyester capacitors
30
SILICON CHIP
siliconchip.com.au
