McIntosh MAC 1900 Manuel d'utilisateur télécharger pdf (Page 19)

Technical Description

AUDIO SECTION

Each channel of the MAC 1900 has four basic

sections. They are: phono or low level amplifier, tape

or high level amplifier, filter amplifier, and power

amplifier.

PHONO AMPLIFIER

There are three transistors in each channel of the

phono amplifier. The input transistors are carefully

selected for low noise and high gain. High gain tran-

sistors are used in the other stages of the phono

amplifier to achieve high overall open-loop gain. With

high open-loop gain a larger amount of negative

feedback can be used around the phono amplifier

which reduces noise and distortion. The feedback

network also provides precision RIAA frequency com-

pensation required for magnetic phonograph cart-

ridges.

Feedback remains in effect even at 20 Hz where

higher gain is required for RIAA compensation. The

output of the phono amplifier provides a low im-

pedance output for the tape output jack and the in-

put of the high level preamplifier.

Input sensitivity of the phono amplifier is 2.0 milli-

volts. The gain of the amplifier is 42 dB at 1000 Hz.

The phono amplifier has very wide dynamic range.

At 1000 Hz the phono input circuit will accept 100

millivolts without overload, a voltage far greater than

the output of any current magnetic phono cartridge.

Phono input overload is virtually impossible.

A signal level of 10 millivolts at the phono input

at 1000 Hz will produce 1.2 volts at the tape output.

The tape output has a source impedance of 200

ohms. For most efficient signal transfer the tape out-

put should operate into a load impedance of 47k

ohms or greater.

HIGH LEVEL

At the input to the high level amplifier the signal

passes through the mode selector switch through the

balance control and volume control into the amplifier.

Loudness taps are provided on the volume control.

With the loudness switch in the circuit, the low fre-

quency output (below 400 Hz) is increased when the

volume control is at 50% of rotation or below.

A differential amplifier configuration is used at the

input of the tape amplifier. The tone controls are in a

negative feedback loop around the amplifier stages

to give excellent low noise and low distortion char-

acteristics. With the tone controls in the feedback

around the tape amplifier, the negative feedback re-

mains in effect even at the maximum boost setting of

either bass or treble controls.

The slide tone controls and their associated

components have been mounted on a double sided,

plated through, printed circuit board. This complete

tone control module is mounted to the front panel for

ease in operation and maintenance. A mechanical de-

tent has been provided at the electrical and mechan-

ical center of the sliding bass and treble controls.

The tone controls provide smooth gradual boost or

cut of output levels giving minimum change at mid

frequencies and maximum change at the audio spec-

trum extremes 20 Hz and 20,000 Hz. With the tone

controls in the center position the gain of the tape

amplifiers is 20 dB. The output of the tape amplifier

feeds directly to the filter amplifier.

FILTER AMPLIFIER

Each channel of the filter amplifier has two tran-

sistors connected in compound emitter follower con-

figuration. To maintain low distortion and noise and

present a uniform input impedance the resistive and

reactive elements which make up the filters are in a

feedback path. The Low Frequency (rumble) filter

starts to roll off at 50 Hz at a rate of 12 dB per octave,

reducing the output at 20 Hz by 18 dB.

The High Frequency (scratch) filter starts to roll

off at 7,000 Hz at a rate of 12 dB per octave, reducing

the output at 20,000 Hz by 18 dB. Filter components

have been mathematically selected in value to give

the desired frequency rolloff characteristics. Close

tolerance components used assure uniformity in filter

response. The filter amplifier output is an emitter fol-

lower with a source impedance of 200 ohms. The

output is fed to jacks on the rear at the MAC 1900. In

the normal operation, both filters out, the filter am-

plifier has a gain of 0 dB.

POWER AMPLIFIER

The input impedance of the power amplifier is

100,000 ohms and requires 2.5 V RMS to drive the

amplifier to rated output. The short jumper cables in

1 2 ... 14 15 16 17 18 19 20 21 22 23 24

Commentaires sur ces manuels

Pas de commentaire

McIntosh MAC 1900 Manuel d'utilisateur Page 19

Commentaires sur ces manuels

Produits connexes et manuels pour Équipement musical McIntosh MAC 1900