McIntosh MAC 1900 Manuel d'utilisateur télécharger pdf (Page 22)

ceived, the automatic FM stereo switching circuit

activates the multiplex decoding circuit. This lights

the stereo indicator. The circuit switching is all done

electronically with no clicks. The automatic stereo

switching can be defeated by depressing the mono

pushbutton. (In this position the stereo indicator will

still light to indicate the presence of a stereo signal).

On monophonic transmissions the stereo switching

is inactive at all times, assuring a greater signal to

noise ratio. The stereo switching circuit has been

designed so that noise will not activate it. De-empha-

sis of the signal out of the multiplex decoding circuit

restores the frequency amplitude characteristics to

the same level they were before transmission.

AM SECTION

The AM-RF amplifier circuit includes a three sec-

tion variable tuning capacitor in the metal enclosed

shielded RF module which also houses the FM-RF

front end. A three section variable capacitor is used

for greater spurious rejection. The RF amplifier is

unique. The circuit has constant sensitivity constant

selectivity, high image rejection across the complete

AM band. Ordinary AM-RF circuits cannot do all of

these simultaneously. This circuit design achieves

equal sensitivity even down to the low end of the

band. Spurious, image, and intermediate frequency

rejection are all superior. The same circuit delivers

equal selectivity across the entire band. The Mclntosh

circuit is unique in a superhetrodyne AM receiver.

In addition, there is no loss of audio frequency re-

sponse at the low end of the band commonly known

to AM receivers. Another advantage of the Mclntosh

circuit is freedom from cross modulation and over-

loading by strong local stations.

A high-quality loopstick antenna is provided. It

can be rotated for maximum performance, optimum

signal rejection or minimum interference. In each

MAC 1900 the loopsticks are individually tuned for

optimum performance. After tuning the loopstick is

then sealed. This custom matching of the loopstick

to the AM-RF front end maximizes the performance

of the loopstick antenna. The antenna is rotatable

through nearly 180 degrees in all directions. With

this mobility you will not suffer loss of sensitivity

regardless of the angle at which the instrument is

mounted. A back panel antenna connector is pro-

vided for connecting an external antenna if desired.

To maintain the excellent image rejection and lack

of spurious cross modulation of the AM-RF amplifier

an autodyne converter circuit was used for the AM-IF.

AM-IF uses two double tuned IF transformers

designed to obtain a high degree of selectivity yet

allowing good audio fidelity.

A 10,000 Hz active filter eliminates the 10,000 Hz

whistle and irritating "Monkey Chatter" caused by

an adjacent station. The frequency response of all

stations is nearly flat from 20 Hz to around 3,500 Hz

then roll off begins. Because an active filter is used,

the output level at 10,000 Hz, or the whistle fre-

quency, is down over 30 dB or one thousandth of

what it would be without filtering. With the select

button IN, the active filter cutoff frequency is low-

ered. The filter then effectively suppresses the 5,000

Hz whistle from nearby television receivers.

The AVC (automatic volume control) system was

designed to prevent bursting or thumps when the

AM is tuned through a strong signal. Distortion at low

audio frequencies is minimized by using two AVC

filter sections instead of the conventional one.

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McIntosh MAC 1900 Manuel d'utilisateur Page 22

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