Macintosh C36 Owner's Manual - page 16
LOGIC DRIVEN CONTROL
All inputs, outputs and mode switching on the C36 are controlled by logic circuits that drive
the electromagnetic switches. The front panel Record and Listen switches feed a pulse to
a binary up-down counter, then to a binary decimal decoder, and finally to a transistor array
that controls signals to the electromagnetic switches. The C36 pushbutton switches feed a
momentary pulse to control the electromagnetic switches for their specific functions.
ELECTRO-MAGNETIC SWITCHING
All signal switching in the C36 is done by ELECTRO-MAGNETIC methods. ELECTRO-
MAGNETIC switching is an old and proven technology that has been blessed with modern
materials and manufacturing methods.
Each switch consists of a sealed glass tube that is filled with an inert oxygen free at-
mosphere, and sealed with tiny leads protruding from either end- These leads extend into
the tube and overlap one another with a separation of a few thousandths of an inch. The
leads are made from a ferrous material that is influenced by a magnetic field. They are first
plated with gold as a base material, then with rhodium and ruthenium. Ruthenium is the best
contact material known. The glass assembly is then placed in the center of a multi-layer coil
of copper wire. The entire assembly is molded together in a tough shock absorbing plastic.
The switch and coil connections extend from the bottom in the form of printed circuit board
terminals.
When a DC voltage is applied to the coil, current flows and creates a magnetic field. The
force of the field causes the leads to bend and contact one another inside the sealed glass
tube. The inert oxygen free atmosphere eliminates corrosion of the contacts, insuring a low
resistance, distortion free switch.
PHONO PREAMPLIFIER
The phono preamplifier uses a high technology integrated circuit operational amplifier that
has an extremely wide frequency range capability. Its differential input stage has been op-
timized for low noise and low distortion. The open loop gain for this circuit is 100,000. With
high open loop gain, a large amount of negative feedback can be used around this preamplifier
section to reduce noise and distortion to an extremely low value. The feedback network also
provides precision RIAA frequency equalization which follows the required response curve
very accurately throughout the entire audio range. The network uses 1% tolerance metal film
resistors and 5% tolerance polypropylene capacitors.
To achieve low-noise performance, it is essential that the feedback network have very low
impedance. A circuit design of this type acts as a small power amplifier. This preamplifier
section will actually produce more than 100 milliwatts output power. This extra margin of per-
formance results in a phono preamplifier with extremely low distortion and noise.
This preamplifier circuit has a very wide dynamic range. It will accept up to 90 millivolts
of input signal without overload. This is far greater than the maximum output voltage capability
of any current model magnatic phono cartridge.
The sensitivity of this circuit is 2.5 millivolts for 2.5 volts at the main output. The gain is
40dB at 1000Hz. A signal input of 10 millivolts results in 1 volt at the Tape Outputs. The Tape
Output source impedance of the phono preamplifier is 100 ohms, and will drive a load im-
pedance of 10,000 ohms or higher.
LOUDNESS AMPLIFIER
High level signals feed into the preamplifier past the input and mode switching, through
the volume control and then into the loudness amplifier. The C36 uses an active loudness
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TECHNICAL
DESCRIPTION