McIntosh C28 Owner's Manual page 19

HIGH LEVEL AMPLIFIER
The linear high level amplifier has a gain of 20 dB.
The same careful design considerations apply with
respect to noise and the reduction in non-linear dis-
tortion. Each channel of the high level amplifier con-
sists of 3 transistors selected for low noise character-
istics. In the feedback loop, two 11 position switches
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. At the input of the
high level amplifier are the bass trim control and the
switch which selects loudness or presence compen-
sation.
FILTER AMPLIFIER
The filter amplifier serves three functions: (1) a
high frequency filter, (2) a low freqency filter, and
(3) a 20.000 Hz low pass filter.
Each channel of the filter amplifier has two tran-
sistors connected in compound emitter follower con-
figuration. The resistive and capacative elements
form a 12 dB per octave active RC filter. The high
frequency filter is a 7000 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 outside the useful
sound spectrum while leaving the preamplifier re-
sponse flat to 20 000 Hz. A 20,000 Hz active, low pass,
filter is connected at all times to reduce wideband
noise yet maintaining flat frequency response up to
20,000 Hz.
The non-linear distortion of these active filters is
very low even at cut off frequency. This is not the
case in passive f i l t e r s where a very low impedance
at the cut off frequency is presented to the driving
source. This low impedance will cause high non-
linear distortion at and near the cut o f f frequency.
The design of the C 28 eliminates this problem.
HEADPHONE AMPLIFIER
The headphone amplifier is capable of producing
0.75 volts of output into 8 ohm headphones with less
than 0.1% harmonic distortion. This is adequate
power to drive any dynamic type headphone.
The maximum output of the headphone amplifier
is 2.5 volts into 50 ohms or greater at the LINE OUT-
PUT terminals on the rear panel. The internal im-
pedance of the headphone amplifier is less than 0.2
ohms. Long shielded cables can be used without ad-
versely affecting the frequency response. The head-
phone level controls adjust the output level and
balance of the headphone amplifier.
The power supply consists of a low impedance 75
volt power supply and a 14 volt power supply for the
headphone amplifier. The 75 volt supply is stabilized
by a series regulator transistor and a zener diode.
An accessory filter chain and voltage dividing net-
work provides 18 volts to the low level circuits.
The headphone amplifier is powered by a separate
14 volt supply which uses a transistor as an effective
ripple filter.
A principle design consideration was exceptionally
low hum level. First the power transformer of the C 28
uses a magnetic core with very low flux density.
Double shielding has been accomplished by sur-
rounding the transformer f i r s t with strips of copper,
and then with strips of treated soft iron. The shielded
transformer is potted in a heavy gauge soft steel can.
To prevent unbalance of the magnetic flux of the
transformer, the secondary voltage is rectified by
silicon diodes in a bridge configuration.
19
POWER SUPPLY