Bryston PRE-AMPLIFIER 12B User's Manual

 INSTRUCTION MANUAL FOR
11B/12B PRE-AMPLIFIER
Congratulations on your selection
of the very finest sound equipment.
Your Bryston preamplifier has been
engineered to provide many years
of trouble-free, highly musical lis-
tening enjoyment.
This instruction manual refers to
the Model 12B. With the exception
of the phono inputs, the Model 11B
preamplifier is identical, and the
instructions pertain to it in exactly
the same way. Where a difference
occurs, it will be noted in the text.
The Model 12B is a basic signal
preamplifier with switching facilities
for all the sources likely to be used
by even the most elaborate domestic
sound system, including dubbing
capability for two tape recorders,
and the convenience of recording
from any source while simultan-
eously listening to any other through
the speakers. The included diagram
will illustrate the switching functions
of the Bryston Model 128 preampii-
fier.
no inputs; Including Moving
oil
The Model 12B is supplied with
an internal moving-coil step-up
transformer. This extremely wide-
band transformer, with almost no
added noise or distortion, increases
the voltage gain through the phono
section by 22.5dB, and adjusts the
inputimpedance to suitthe vast ma-
jority of moving-coil cartridges. (itis
available on special order with 6dB
less gain, and a higher input im-
pedance, for those few high-output
MC cartridges which may perform
better with these settings).
The 12B will also accept moving-
magnet cartridges by switching the
MC transformer out via a switch on
the front panel. In this case, the
cartridge is plugged into the MM
inputs on the rear panel. The MM
inputs (only) are equipped with input
loading capacitors of about 275 pf
per channel. When added to the
normal turntable wiring capacitance,
this yields about 300-400 pf, which
is in the optimum range for most
cartridges. (This internal phono
capacitance may be special-ordered
to any value at no extra charge).
The Mode! 11B is identical to the
Model 12B except that it is not
equipped with the MC transformer.
Thus both phono inputs become
MM inputs, as above. They may be
selected between Phono 1 and
Phono 2 via the switch on the front
panel. (The MC tranformer may be
retrofitted at any time, converting
the preamp to a 12B). Since the
phono stage on the Model 12B in-
verts absolute phase, the MC trans-
former is connected to invert phase
also. Thus, MC cartridges play in
proper phase with no special con-
nection necessary. in case of MM
cartridges, reversing the hot and
ground leads on the cartridge pins
is a convenient way of obtaining the
same function.
Tape Monitor Function
Although the 12B's tape monitor
function is arranged differently from
most standard preamplifiers, it is not
complicated to use. The two selector
switches operate independently, and
affect only the outputs to which their
labels refer. Thus the “MAIN
OUTPUT” switch is used as the
tape monitor, also. Inciude your tape
deck in the preamp's tape loop in
the normal way (i.e., its record head
is connected to Tape 1 output, and
its playback head is plugged back
into Tape 1 input). To monitor the
recording, just move the MAIN
OUTPUT switch to “Tape 1”. A sec-
ond tape deck may be included in
the tape loop in a similar manner,
using Tape 2 input and output, and
“Tape 2" selector position for mon-
itoring.
Two Tape Recorders; Dubbing
Facility
Plug in the playback and record
heads of the two decks to their res-
pective tape inputs and outputs, as
above. Normally, both decks will
receive the same signal for record-
ing. When “Tape 1" is selected on
the TAPE OUTPUT switch, how-
ever, the playback head of deck 1
is connected to the record head of
deck 2, via Tape 2 output. This
obviously allows direct dubbing
from Tape Deck 1 to Tape Deck 2.
Selecting “Tape 2" on the TAPE
OUTPUT switch allows dubbing in
the opposite direction by reversing
these connections.
External Equalizer, Noise Re-
duction, or Other Audio Pro-
cessor
An equalizer, noise reducer, etc.,
may be included in the tape loop
(as shown in the diagram) by con-
necting its input to one of the tape
outputs, and its output back to (the
same numeral) tape input. Turn the
MAIN OUTPUT switch to (that
numeral) “Tape” position whenever
you wish to use the audio proces-
sor in the signal path. During that
time, the TAPE OUTPUT switch is
used as the selector for whichever
source is to be processed. Turning
the MAIN OUTPUT switch back to
this source position will bypass the
processor.
Headphone Jack
The Model 12B is equipped with
a headphone jack for convenient,
private listening. The signal is taken
directly from the output circuit which
normally feeds the power amplifier,
and is thus controlled by the volume
knob. This output will drive head-
phones of any type, but lower
impedance headphones may re-
quire somewhat more volume
contro! rotation for the desired
loudness. To facilitate headphone
listening without disturbing others,
the preamplifier automatically dis-
connects the direct feed to the power
amplifier. (This is indicated by the
green pilot light on the front panel
changing to red whenever the out-
put relay is opened). This circuit does
not disconnect the remote outputs.
(See under “Remote Output”). Since
some headphones may require
substantial volume control rotation,
be sure to reduce the volume
before removing the headphone
plug, or you may be startled by the
high sound level when the relay
re-connects the amplifier.
Remote Output
The 12B preamplifier is equipped
with a special remote output, which
is switched via the “Main Output”
selector, but which exits ahead of
the volume control. This line-level
output may then be used to-drive
amplifiers in remote locations, with
local volume controls. This allows
remote systems to function inde-
pendently of the volume level set in
the main system. The automatic
amplifier disconnect which is part
of the headphone circuit does not
affect these remote outputs. (As an
option, this function may be replaced
by a second set of main outputs,
connected in parallel with the first.
Thisallowstheconvenientconnec-
tion of two amplifiers which are
desired to be controlled together,
such as subwoofer installations, or
passively biamplified systems).
Low Filter
The inflection point of the rumble
filter has been chosen at 31.7Hz to
correspondto the lowestsignalfre-
quencies likely to be found in re-
corded music, and to a suggested
revision to the RIAA recording curve
for LP discs. For these reasons, and
for the more important reason of
avoiding low frequency transient
overshoot, ringing and phase mis-
alignment, the low filter is a
“minimum” type, with a 6dB/octave
siope below cutoff. This is quite
effective in reducing subsonic
woofer-wooble and its attendant
intermodulation effects, however,
since the most problematic turntable
and warp-induced rumble frequen-
Cies, in the area of 2-5Hz, are
reduced an average of about 20dB
or a factor of 100 in terms of power
demand. The low filter switch is
active in the ‘In’ position, and
affects only the phono inputs.
Mute Switch
The listener often is called
momentarily away from critical lis-
tening, to answer the phone for
example, and wishes to reduce the
gain for a short time without disturb-
ing the volume control setting. The
mute switch reduces preamp gain
by 20dB. Thus the music may still
be heard, butat a background level.
- This is also useful for changing
records, cueing the stylus manually
to a particular song, cleaning the
stylus with a brush, etc. The mute
switch is active in the ‘In’ position
and affects only the main outputs
without disturbing the tape outputs.
Mono Function
Occasionally a system will display
some offset in gain favouring one
channel over the other due to slight
differences in speaker efficiency,
differences in room loading, or a
somewhat off-center listening pos-
ition dictated by room decor. Thus it
IS convenient to “mano” the signal
to obtain the most unambiguous
center image, convenient to adjust-
Ing the balance control. For reasons
of simplicity, rather than having a
separate switch for this function,
the “Mute” switch also connects
the two channels in Mono.
Obviously, the listener may wish to
advance the volume control to pro-
vide sufficient level for establishing
center image. Do not forget to
reduce the volume control again
before deactivating the mute
switch in this case, or you may be
startled by the sudden increase in
sound level. The balance controlis
a tailored inflection type which has
very gradual action near the center
of rotation for fine adjustments to
the left-right balance of the stereo
image.
Turn On Delay
The Model 12B contains an output
relay with an approximate 3-second
delay at turnon, to allow any transient
thumps to settle. The relay opens
instantly when the preampis switch-
ed off to prevent any power-supply
decay artifacts from appearing at
the outputs. The relay is hermetically
sealed in an inert atmosphere to
completely eliminate contact con-
tamination and provide nearly infinite
lifetime (rated in tens of billions of
cycles). This same relay is employed
In the automatic amplifier disconnect
function which is part of the head-
phone circuit, and in the protection
mechanisms connected into the
power supply. Whenever the pre-
amplifier is under power, and the
relay is open for any reason, the
green pilot light on the front panel
changes to red. (Since the power-
supply filter capacitors are quite
large, the red LED indication will
fade slowly after the preamp is
switched off).
Accessory Outlets
The 12B includes three 3-prong
grounded, 120 volt receptacles on
the rear panel, two switched, one
unswitched. Please observe the
caution printed below the re-
ceptacles regarding maximum
power draw from these outlets.
Large power amplifiers, especially
including the Bryston Models 3B
and 4B should not be plugged
into these receptacles. To do so
may damage the preamp's power
switch. It is permissible to plug the
Bryston 2B-LP into these outlets,
however, or any class AB amplifier
up to 50, (or class A amplifier up to
15) watts per channel. In general, it
Is inadvisable to switch large power
amplifiers on and off by the switch
in a preamplifier, for the above rea-
son, and also because it is likely to
cause a fluctuating voltage-drop on
the preamplifier's line cord, which
in worst case conditions can
become an audible problem.
DC Power-Supply Output
On the rear panel of the Model
12B (and 11B) there is a DIN con-
nector which provides +24V DC of
regulated power. This is provided
for future Bryston products, such
as equalizers, electronic crossovers
and the like, which will be available
with or without power supply, for
reasons of economy. This output is
fully protected, and contains a circuit
which opens the output relay auto-
matically if excessive current is
drawn. |
The Twelve-B has been designed
from a rational, function-oriented
ideal of maximum musical accuracy,
with an absolute minimum of use-
less gadgetry or extraneous con-
trols. The signal-handling circuit is
novel and fundamentally correct,
both electrically and more important,
sonically. We believe you will find a
renewed sense of awareness of the
listening pleasure in your music
collection, played through your
Bryston preamplifier. All your signal
sources will sound cleaner and
more muscially transparent than
you may have experienced before.
Please do not hesitate to write us
If you have any questions or com-
ments about any of our products.
Your opinion is of paramount im-
portance to us.
®
: (CY)
LTD.
LTEE.
57 WESTMORE DRIVE, REXDALE, ONTARIO, CANADA M9V 3Y6 TELEPHONE: 416-746-1800, TELEX: 06-989548
Printed in Canada
IN THE U.S.A. TELEPHONE 802-223-6159
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11B/12B Preamps with Balanced Outputs
Assembly Notes
for
Hookup for Balanced Output board and XLR board
5 CONDUCTOR CABLE
Component placement for Balanced Output 11B/12B Preamps
Foil side view
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1> Assemble Balanced Output Board (BALOUT 2v2)
2> Five conductor cable is prepared ond soldered on to balanced output
board.
3> Install balonced output board on mother (main) board.
4> Feed five conductor cable through hole in power supply shield and
soldered onto 2XLR—M board.
5> Insert male XLR jocks through power supply shield (from outside) &
solder on to 2XLR-M board.
XLR jocks are NOT screwed down, they will be sandwiched between
the main board, the chassis rear panel and the top panel and thus
held sucurely without any need for screws.
6> Apply self—ahesive label to outside surface of rear panel over XLR jocks