McIntosh MAC 1900 Manuel d'utilisateur télécharger pdf (Page 20)

the rear of the MAC 1900 connect the filter amplifier

output (PREAMP out) and power amplifier input

(AMP input).

At the input of the power amplifier, two transistors

are connected as a differential amplifier. The two in-

put signals to the differential amplifier are the input

signal (output of the filter amplifier), and the negative

feedback signal from the output of the power am-

plifier. The use of a differential amplifier permits the

best use of larger amounts negative feedback to

maintain low noise and distortion. The combined out-

put of the differential amplifier feeds a linear voltage

amplifier which in turn drives two medium power

driver transistors. The driver transistors drive the out-

put stages.

The output section is arranged as a direct coupled

series push-pull amplifier. The power transistors used

in the output circuit are selected for their high power

dissipation capability, wide frequency response and

large "safe operating area."

The power transistors are mounted on large black

anodized heat sinks to assure that under normal op-

eration the transistors will operate at a low tempera-

ture. If operating temperatures should increase due

to a shorted speaker, or restricted ventilation, an

automatic sensing device turns the AC power off to

the MAC 1900. The MAC 1900 will turn on again when

the temperature has returned to normal limits. This

additional protection assures you of the complete

reliability of the MAC 1900 even under the most ex-

treme operating conditions.

To further insure reliability a special power out-

put SENTRY MONITORING CIRCUIT prevents failure

of the power output transistors due to excessive mis-

match or shorting of the output. When the MAC 1900

operates normally the SENTRY MONITORING CIR-

CUIT has no effect on signals passing through the

power amplifier. If the power dissipation in the out-

put transistors should rise above normal design lim-

its, the SENTRY MONITORING CIRCUIT restricts the

drive to the output stage which reduces the dissipa-

tion in the output transistors. The SENTRY MON-

ITORING CIRCUIT acts instantaneously for any input

signal or load combination. This arrangement as-

sures circuit reliability. Only Mclntosh gives you this

degree of protection.

The direct coupled series push-pull amplifier cir-

cuit places the output at ground potential when there

is no signal input which eliminates the use of a

coupling capacitor in the output circuit. The available

low frequency power output is restricted in other cir-

cuits that require a coupling capacitor in the output.

The MAC 1900 Power Amplifier is all direct coupled

to insure maximum low frequency performance. In

direct coupled circuits failure of any transistor in the

power amplifier could cause a DC potential to ap-

pear in the output. To assure that no damaging or

interfering DC appears across the output terminals,

a protector circuit (Patented) has been designed into

the MAC 1900. This circuit constantly monitors the

output circuit and if at any time a constant DC level

appears, the circuit "crowbars" a short across the

output to ground. The protective circuit is very fast (it

reacts in milliseconds} and the short will remain

across the output until the cause has been fixed. This

extra protection prevents damage to your speakers.

Under normal operating conditions the protective

circuit has no effect on the operation of the output

circuit. This is another example of the continuous

protection Mclntosh provides.

POWER SUPPLIES

Two high current power supplies, a positive 40

volt and negative 40 volt DC, are used to drive the

output power amplifier. Very large capacitors (9300

microfarads) are used to store a large amount -of

energy to provide good filtering, and excellent volt-

age regulation. Good low frequency response and

negligible low frequency distortion in the power am-

plifier stages depends on the regulation of the power

supply.

A second power supply provides the power for

the power amplifier drivers. It is a high voltage full

wave rectified and filtered supply.

Two additional electronically regulated power

supplies are used in your MAC 1900. A regulated 16

volt power supply furnishes voltage to the AM and

FM circuits. A regulated 75 volt power supply with

an accessory filter chain provides 75VDC and 20VDC

to the phono and tape amplifier circuits. Both elec-

tronically regulated power supplies are low imped-

ance series regulated transistor circuits using tight

tolerance zener diodes as references. Hum and ripple

reduction has been a major design consideration on

the MAC 1900 power supplies.

FM SECTION

The Radio Frequency (RF) Section houses the

complete FM-RF front end and part of the AM-RF

circuitry. A seven section variable capacitor is the

heart of the RF section. Four sections of the vari-

able capacitor are used for FM and the remaining

three are in the AM section. By interleaving the sec-

tions (AM-FM-AM-FM, etc.), spurious responses have

been significantly reduced. The use of the latest

"state of the art" in field effect transistors and a well

designed variable tuning capacitor has provided

a high degree of RF selectivity and excellent spuri-

ous response rejection. Image rejection and spurious

responses has been brought to a minimum in the

RF section of the MAC 1900.

A dual insulated gate metal oxide silicon field ef-

fect transistor (MOS-FET) is used as the first RF am-

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McIntosh MAC 1900 Manuel d'utilisateur Page 20

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