McIntosh C38 User Manual

LOGIC DRIVEN CONTROL
All Inputs, Outputs, CD, Tuner, SCR and Video selector lines are controlled by logic circuits
in the C38. This logic is changed by front panel switches or by two microprocessor IR decoders,
one for LISTEN/MAIN (Area A), and one for RECORD (Area B). These microprocessors are
programmed with exclusive Mcintosh software. They receive data from the C38 front panel
IR sensor, or from external IR sensors and provide the command signals for all switching
functions and volume control. A third microprocessor decodes the CD1, CD2 and TUNER
Control commands.
The logic circuits activate Electro-Magnetic switches for the final step in all signal switching.
ELECTRO-MAGNETIC SWITCHING
All signal switching in the C38 is done by ELECTRO-MAGNETIC methods. ELECTRO-
MAGNETIC switching is an old and proven technology that has been upgraded with modern
materials and manufacturing techniques.
Each switch consists of a sealed glass tube filled with an inert oxygen free atmosphere,
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 ex-
tend 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. The switching control signals come from the C38 logic
circuits.
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 magnetic 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 less than 100 ohms, and will drive
a load impedance of 1000 ohms or higher.
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TECHNICAL
DESCRIPTION

McIntosh C38 Specifications

General

Brand	McIntosh
Model	C38
Category	Control Unit
Language	English