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Macro-Tech 3600VZ Power Amplifier

Page 25

POWER

TRANSISTOR

POWER

TRANSISTOR

SPEAKER

LOAD

POWER

SUPPLY

–

Fig. 5.1 A Typical

Power Supply

The power transis-

tors stay cooler

and are not forced

to needlessly dissi-

pate heat. This is

the normal operat-

ing mode of the

power supply.

When the voltage

requirements are

high,

supplies

switch to a

series

mode

which pro-

duces higher volt-

age and less

current. The amplified output signal never misses a

beat and gets full voltage when it needs it—not when it

doesn’t need it.

Sensing circuitry watches the voltage of the signal to

determine when to switch

modes. The switching

circuitry is designed to prevent audible switching dis-

tortion to yield the highest possible dynamic transfer

function—you hear only the music and not the ampli-

fier. You get not only the maximum power with the maxi-

mum safety, but you also get the best power matching

to your load.

5.3 Circuit Theory

Each channel is powered by its own power transformer,

T100 or T200. Both channels share TF-1, a low

voltage

transformer. The secondary outputs of each trans-

former are full-wave rectified by heavy duty bridge rec-

tifiers and are filtered by large computer grade

capacitors. A thermal switch embedded in each trans-

former protects them from overheating.

The low

voltage transformer TF-1 uses a separate fan

motor winding. The TF-1 output is rectified by diodes

D1-4 delivering an unregulated 24 volts. Monolithic

regulators U1-2 provide a regulated ±15 volts.

5.3.1 Stereo Operation

For simplicity, the discussion of stereo operation will

refer to one channel only. Mono operation will be dis-

cussed later.

See the block diagram in Figure 5.5 and the schemat-

ics at the back of this manual.

The input signal at the phone jack passes directly into

the balanced gain stage (U104-C,D). Use of a

P.I.P.

module for input signal causes the input signal to pass

through the

P.I.P.

and then to the balanced gain stage.

Also, the lower the re-

sistance of the power

transistors, the more

voltage you can deliver

to the load. But when

you lower the resis-

tance of the transistors,

you increase the cur-

rent passing through

them, and again in-

crease the amount of

heat they must dissi-

pate.

5.2.2 The VZ Supply

An articulated power supply like

avoids much of

this problem by reducing the voltage applied to the

transistors when less voltage is needed. Reducing the

voltage reduces the heat, so the amplifier runs cooler

and more power can be packed in safely.

The

supply is divided into segments to better match

the voltage and current requirements of the power tran-

sistors. Remember that audio signals like music are

complex waveforms.

Fig. 5.2 Music Waveforms Are Complex

For music, the average level is always much less than

the peak level. This means a power supply does not

need to produce full voltage all the time.

POWER

TRANSISTOR

POWER

TRANSISTOR

SPEAKER

LOAD

STAGE

–

STAGE

–

VZ POWER SUPPLY

Fig. 5.4 VZ Supply in

Series Mode

Fig. 5.3 VZ Supply in Parallel Mode

The

supply is

divided into two

parts. When the

voltage require-

ments are not

high, it operates

in a

parallel

mode

to pro-

duce less volt-

age and more

current.

POWER

TRANSISTOR

POWER

TRANSISTOR

SPEAKER

LOAD

STAGE

–

STAGE

–

VZ POWER SUPPLY