McIntosh MA 5100, Owner'S Manual

Page: 5 / 20

TECHNICAL DESCRIPTION
PREAMPLIFIER SECTION
The phono and tape head preamplifier circuits in the MA5100 have
three transistors in each channel. The input selector switch connects
the input jacks to the first voltage gain stage of the preamplifier. The
input stage has high voltage gain and very low noise. The next stage,
an emitter follower, acts as an impedance converter that matches the
input stage to the second voltage amplifier. The emitter follower is
direct coupled to the second voltage amplifier.
Negative feedback is used around the low level section to reduce noise
and distortion to an absolute minimum. The negative feedback also
provides precise frequency compensation for magnetic phono cart-
ridges and tape heads. The feedback remains in effect throughout the
entire audio bandwidth, even at 20 Hz where gain is the highest. This
kind of careful Mclntosh engineering assures you of lowest distortion
performance.
The tape head input impedance is 500,000 ohms. High tape head
input impedance permits uniform high frequency performance from
typical tape transport playback heads.
The MA 5100 is ideal tor tape recording. With an input signal from a
phono cartridge of 10 millivolts, there is 1.4 volts available at the tape
output jacks.
Phono input signal overload is virtually impossible. At 1 KC the phono
input will accept greater than 125 millivolts without overloading. This
is more than 4 times the output from most phono cartridges when
playing a low distortion phonograph record.
The phono input impedance is 47,000 ohms. This matches the imped-
ance of magnetic phono cartridges.
The preamplifier output is connected by the input selector switch to
the tape output, the tape monitor switch, the balance control, and the
first section of the volume control. This arrangement permits recording
of the program without interruption and has the ability to monitor the
recorded tape.
The tone control stages are made up of a three stage amplifier. The
first two transistors are emitter followers. The first emitter follower is
driven from the output of the volume control. The second emitter fol-
lower is direct coupled to the third stage. The third stage is a high
gain voltage amplifier.
Signals pass through the input emitter follower and then couple to the
second and third tone control stages through the tone control network.
The tone control contours are obtained by controlling the large nega-
tive feedback around the second and third transistors. This negative
feedback is used to accurately shape the response. The large amount
of negative feedback also makes possible low distortion from the tone
control amplifier.
The output of the tone shaping amplifier drives the low frequency and
high frequency filters and the second section of the volume control.
The filters are designed to remove unwanted noises such as turntable
rumble and record scratch. The filters remove the maximum amount
of objectionable material and still have a minimum effect on the musi-
cal content of the program material.
The use of a two section volume control performs two important func-
tions. First, the input section of the volume control increases the signal
handling capability of the tone control amplifier. Use of this arrange-
ment makes overdriving the tone control amplifier almost impossible.
Second, the output volume control assures maximum signal to noise
ratio regardless of the volume control position.
The output of the volume control is fed to a two stage voltage amplifier
that has very low noise characteristics. Negative feedback is used to
improve the signal to noise ratio and assure an absolute minimum of
distortion.
The phase switch is part of the left channel circuitry. The switch
selects from two sources that are "out of phase" to each other.
In the normal position the phase is the same in both channels. When
the phase switch is in the 180° position the left channel is "out of
phase " when compared to the right.
The loudness compensation switch is also connected to the first sec-
tion of the volume control. Out of phase program source material is easily corrected with the use
of this switch.
3