McIntosh MA6400 Owner's Manual page 23

Each switch consists of a sealed glass tube filled with an inert oxygen free atmosphere. Tiny
connecting leads protrude 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. The switching control signals come from the MA6400 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
optimized 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
performance 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 100 watts at the SPEAKERS Outputs. 10
millivolts input results in 1 volt at the Tape Outputs. The Tape Output source impedance of the
phono preamplifier section is less than 100 ohms, and will drive a load impedance of 1000 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 MA6400 uses an active loudness
control circuit design. An integrated circuit operational amplifier is used with two separate
feedback circuits. One feedback loop has flat frequency response, and the other loop has
loudness compensation. A potentiometer placed between these two feedback loops makes
TECHNICAL
DESCRIPTION
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