Mcintosh C2300 Manuel d'utilisateur télécharger pdf (Page 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

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Mcintosh C2300 Manuel d'utilisateur Page 20

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