McIntosh C29 -, Руководство пользователя

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Technical Description
AUDIO SECTION
Each channel of the C 29 has four basic sections.
They are: phono amplifier, high level amplifier, filter
amplifier, and headphone amplifier.
PHONO AMPLIFIER
The phono amplifier uses a high technology in-
tegrated circuit operational amplifier. Its differential
input stage has been optimized for low noise and
low distortion performance. Open loop gain of this
integrated circuit is 100,000. With high open loop
gain a large amount of negative feedback can be us-
ed around the phono amplifier to further reduce
noise and distortion. The feedback network also pro-
vides precision RIAA frequency compensation. The
network uses 1% metal film resistors and 5% poly
film capacitors. To achieve low noise performance it
is essential that the feedback network be very low
impedance. As a consequence, the preamplifier
must be capable of operating as a power amplifier to
drive this impedance. The actual power output
capability of this preamplifier stage is more than 100
milliwatts, a great margin beyond that which is re-
quired.
Input sensitivity of the phono amplifier is 2
millivolts. The gain of the amplifier is 42 dB at 1000
Hz. The phono amplifier has a very wide dynamic
range. At 1000 Hz the phono input circuit will accept
100 millivolts without overload, a voltage far greater
than the output of any current magnetic phono car-
tridge. Phono input overload therefore is virtually im-
possible. A signal level of 10 millivolts at the phono
input at 1000 Hz will produce 1.2 volts at the tape
output. The tape output has a source impedance of
200 ohms, designed to operate into a load im-
pedance of 47,000 ohms or greater.
HIGH LEVEL AMPLIFIER
At the input to the high level amplifier the signal
passes through the mode switch, then through the
volume control and the loudness control. The
loudness control is independent of the volume con-
trol permitting any amount of loudness compensa-
tion to be introduced regardless of the position of
the volume control. The linear high level amplifier
has a gain of 20 dB. The same type integrated circuit
operational amplifier as used in the phono amplifier
is used to minimize noise and distortion in the high
level amplifier. The feedback loop around this
amplifier uses frequency selective bass and treble
networks with two 11 position switches which allow
up to 20 dB of boost or cut at 20 Hz and up to 18 dB
of boost or cut at 20,000 Hz.
FILTER AMPLIFIER
The filter amplfier serves three functions: a high
frequency filter, a low frequency filter and a 20,000
Hz low pass filter. Each channel of the filter
amplifier again utilizes the same type operational
amplifier as was used in the phono stage and high
level amplifier. The resistive and capacitive
elements form a 12 dB per octave active RC filter.
The high frequency filter is a 7,000 Hz active low
pass filter. The low frequency filter is a 50 Hz active
high pass filter. The filter amplifier also forms a
20,000 Hz active low pass filter to reduce noise out-
side the useful sound spectrum while leaving the
preamplifier response flat to 20,000 Hz. This low
pass filter is connected at all times to reduce wide
band noise yet maintaining flat frequency response
to 20,000 Hz. The non-linear distortion of the active
filter elements in the C 29 is very low even at cutoff
frequency. This is not always the case with passive
filters that present to the driving source a very low
impedance at cutoff frequency.
HEADPHONE AMPLIFIER
The integrated circuit headphone amplifier is
capable of driving two pairs of dynamic headphones
with less than . 1 % harmonic distortion. The max-
imum output of the headphone amplifier is 2.5 volts
into 50 ohms or greater. Internal impedance of the
headphone amplifier is 47 ohms. Long shielded
cables can be used without adversely affecting the
frequency response. The headphone level controls
adjust the output level and balance of the head-
phone amplifier.
TURN ON DELAY
The C 29 has transient free turn on and turn off
characteristics. A light dependent resistor, time con-
trolled, connects the output of the preamplifier to
the output jacks. The control to the resistor is deriv-
ed from a long time constant that turns on approx-
imately two seconds after the power switch. The
same circuit has a short turn off time constant
which turns off before the preamplifier's main power
supply has a chance to discharge.
AUTO TURN ON
On the rear panel are two green AC power outlets
and an AUTO/MANUAL slide switch. Plug the AC
power cables from the turntable into the green turn-
table power outlets on the rear panel. When the
switch is in the AUTO position current that is drawn
by the turntable will be sensed and will cause a relay
to close. This relay controls the AC power to the C 29
and the black AC power outlets. When the current
drain ceases the preamplifier and the AC power
outlets will turn off. The complete system can be
controlled from the turntable off and on switch.
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