McIntosh C2300 - Manual (Page 20)

20
The McIntosh engineering staff has created a Tube Pre-
amplifier without compromise, using the most advanced
McIntosh circuit design concepts. Many months of design,
testing and measuring were required. Extensive controlled
listening tests, the ultimate form of measuring, were made
before the final design was accepted.
Silent Audio Switching
All signal switching in the C2300 is done by Electro-Mag-
netic devices. Electro-Magnetic Switching is a proven
technology that uses the latest in materials and manufac-
turing methods. Each switch consists of a glass tube that
is filled with an inert oxygen-free atmosphere and sealed
with tiny leads protruding from either end. These leads
extend into the tube and overlap one another with a separa-
tion 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 finally ruthenium. Ruthenium is
the best contact material known. Refer to figure 28.
The glass assembly is then placed
in the center of a multilayer coil
of copper wire. The entire assem-
bly is molded together in a tough
shock absorbing material. The
switch and coil connectors 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 gas eliminates corrosion of the con-
tacts and insures a low resistance, distortion free switch.
All inputs, outputs, and data ports are controlled by
logic circuits in the C2300. The logic is changed by Front
Panel Push-buttons or by a microprocessor IR decoder.
This microprocessor IR decoder is programmed with ex-
clusive McIntosh software. It receives data from the Front
Panel or Sensors and provides the command signals for
input switching, data switching, tone control and volume
control.
Phono Circuitry
McIntosh has long been known for its expertise in low
noise discrete amplifier circuits. The MC Amplifier con-
sists of two stages, which together have a gain of 60dB at
1000Hz. The first stage (pre-preamplifier) is a new design
Moving Coil Preamplifier using special ultra low noise cir-
cuitry with out the need for a step-up transformer. It offers
the ability to change the input impedance from 25 ohms to
1000 ohms in six steps to match a wide variety of Moving
Coil Cartridges. The C2300 utilizes a double filtered pow-
er supply design, just for the Moving Coil Preamplifier.
The second (compensation) amplifier stage uses
12AX7A tubes. Its input stage has been optimized for low
noise and low distortion performance. With high open loop
gain, negative feedback is used around the Phono Ampli-
fier to further reduce noise, distortion and has a very wide
dynamic range. The feedback network also provides preci-
sion RIAA frequency compensation. The output is buff-
ered by a unity gain amplifier for low output impedance.
The MM Phono Amplifier uses 12AX7A tubes. Its
input stage has been optimized for low noise and low
distortion performance. It offers the ability to change the
load capacitance from 50 pF to 750 pF in 50pF steps to
match a wide variety of Moving Magnet Cartridges. With
high open loop gain, negative feedback is used around the
Phono Amplifier to further reduce noise, distortion and
has a very wide dynamic range. The feedback network also
provides precision RIAA frequency compensation. The
output is buffered by a unity gain amplifier for low output
impedance.
Tone Control Circuitry
A control attenuator inserts a series tuned circuit into
either the feedback or input section of the tone amplifier.
The level is set by a digitally controlled electronic attenu-
ator. This provides a 12dB boost or cut in 1dB steps at the
frequency of the tuned circuit. The Bass turn-over frequen-
cy is 200Hz. The Treble turn-over frequency is 2kHz. The
overall gain of the stage is 0dB when the tone is adjusted to
flat. The TONE BYPASS switch removes all tone control
circuits from the signal path.
Volume Adjustments
The most frequently operated control on a Preamplifier
is the Volume Control. So it is no surprise that McIntosh
has devoted a substantial amount of time in pursuit of the
“Perfect Volume Control System”.
Conventional Mechanical Controls have poor tracking
especially when it comes to maintaining channel balance
and after a period of time they tend to introduce noise into
the audio signal, as they are rotated. The one desirable
thing they have is the rate of change in volume for degrees
of rotation, known as the audio taper. Using Electronic
Circuitry for controlling the volume can be designed to be
free from noise and maintain channel balance, however
the audio taper produced by the typical Electronic Volume
Technical Description
Figure 28