McIntosh C32 User Manual download pdf (Page 19)

gain and differences in input level.

The C 32 uses active circuitry, electrically independent

of the volume control. Close conformity to Fletcher Munson

Equal Loudness Curves is attained regardless of volume

control position.

Active Filter Circuitry

Loudness, equalizer, and band pass filters of the C 32

are all constructed with ion implanted junction field effect

operational amplifiers (popularly called "Op Amps").

These new devices amplify with a 6 dB better signal to

noise ratio than bipolar input op amps.

Program Equalizers

To adjust for program limitations and personal prefer-

ences the C 32 LISTEN program line has a five band boost/

cut program EQUALIZER based on resonant circuit opera-

tional amplifier. The preamplifier is flat when the EQUAL-

IZER controls are at the 12 o'clock position. EQUALIZER

center frequencies of 30, 150, 500, 1500 and 10,000 Hz

were chosen to cover the audio spectrum in approximately

equal two octave bandwidths. Maximum boost and cut is

12 dB at band centers.

HF and LF Filters

The HF and LF filters are selected by pushbutton switches

in a manner electrically similar to the program EQUALIZERS

and are pushbutton switch selected. The HF filter attenu-

ates all frequencies above 5 kHz, reaching a rate of 12 dB

per octave, at 7 kHz (at which point attenuation is 3 dB

below the 5 kHz level). The LF filter attenuates all fre-

quencies below 50 Hz, at a rate of 12 dB per octave.

All program EQUALIZERS and HF/LF filters are bypassed

when the EQ OUT pushbutton switch is depressed. The

EQ OUT switch also removes subsonic and supersonic fil-

ters below 20 Hz and above 20,000 Hz, extending total

frequency response to 10 Hz to 100,000 Hz.

The Expander

To overcome the compression imposed in radio broad-

casting and recording, and to reduce hum, scratch and hiss,

the C 32 has a logarithmic program expander in the

LISTEN program line. Controls are provided to adjust the

operating point, attack time and expansion ratio.

Signal from the equalizer circuit is applied to a voltage

controlled amplifier (VCA) which operates as a variable

gain block. Control voltages for this VCA are developed

from a sample taken from the output of the equalizer. The

electronic processing of this sample is detailed. It includes

band shaping, logarithmic amplification, full wave rectifi-

cation, level setting, expansion ratio regulation, attack

timing, level setting and DC amplification. The resulting

voltage controls the gain of the VCA to cause logarithmic

gain reduction below set level and logarithmic expansion

above set level.

A switched bypass link provides a program path around

the expander, to maintain a constant signal level with the

expander in or out of the circuit. The gain of the amplifier

block incorporated in the link approximates the gain of

the expander circuit.

The Record Program Line

The C 32 is designed to permit simultaneous recording

on up to three tape recorders of any program input selected

on the RECORD switch regardless of what is being heard

on the LISTEN program line. The RECORD switch selects

its input from the same inputs, but entirely independently

from the LISTEN switch. As this selection is by means of

high impedance FET Analog switches, switching to or over

an input in use on the LISTEN program line does not cause

any audio interference.

Output from the RECORD switch is fed through tape

interlock switches to the three pairs of tape recorder output

jacks. In the event that one of the tape recorders is chosen

as an input signal to the RECORD program line, its asso-

ciated tape interlock is opened to prevent feedback through

that tape recorder back into the C 32 input.

Monitor Amplifier/Headphone Circuits

The C 32 contains a stereo power amplifier which pro-

vides adequate power to drive electrostatic headphones.

The upper HEADPHONE output jack is connected directly

to this MONITOR AMPLIFIER output, making the full 12

watts per channel power available to drive those head-

phones which require it. The lower HEADPHONE jack is

fed through a network designed to match low impedance

dynamic headphones.

In addition, the MONITOR AMPLIFIER may be switched

to either program line, controlled and connected with

enough flexibility that several other uses for it are possible.

Some of these are:

1. As a preamplifier/power amplifier in a completely

separate stereo system.

2. To power rear channel speakers when used in a 4-

channel discreet system.

3. To operate speakers in another area.

4. To feed a 600 ohm line.

On the rear panel are two green AC power outlets and

a slide switch marked AUTO/MANUAL. Plug the AC power

cables from the turntable into the green TURNTABLE

POWER outlets on the rear panel. When the switch is in

the AUTO position, power to the black AC power outlets,

can be controlled by the front panel POWER switch or

by the current drawn by a turntable plugged into the

green TURNTABLE POWER outlets. The current sensing

green AC power outlets will control the AC power to the

entire system from the AC power on/off switch on the

turntable. When the turntable is turned off with its own

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McIntosh C32 User Manual Page 19

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