Handbook for Sound Engineers

Amplifier Design 725

resistance when cold and a decreasing resistance as the
device heats up from the passage of current. This
behavior limits the peak current drawn from the line
when the circuit is first energized from a cold start.
After the capacitor bank is fully charged the thermistor
is shorted out by a relay-controlled switch as part of the
start-up protocol of the dc-to-dc converter. With the link
connected as shown, the storage capacitors act as
voltage doublers. When the Hi side of the line is posi-
tive, diode A charges the upper capacitor with the indi-
cated polarity. When the Hi side of the line goes
negative one-half cycle later, diode B charges the lower
capacitor with the indicated polarity. Voltage doubling
occurs because the two capacitors are permanently
connected in series. For a 240 V supply line, the link is
moved to the 240 V position and the circuit is then that
of a normal full-wave bridge rectifier having an effec-
tive capacitance of 0.5C. The nominal total dc output
voltage is the same in either case.

The core of any switch-mode power supply is the
dc-to-dc converter. Switch-mode supplies that are called
on to deliver significant power usually employ either a
half-bridge or full-bridge converter with the full-bridge
converter being favored for employment in the supplies
of the most powerful audio amplifiers that make use of
full switching technology in their design. A greatly
simplified diagram for such a full-bridge converter is
exhibited as Fig. 20-34 from which the basic operation
may readily be understood.

In Fig. 20-34 the switches S1 through S4 represent

either insulated gate bipolar power transistors or N

channel power MOSFETs. On the first half of the

switching cycle, switches S1 and S3 are closed in

concert and connect the primary of the transformer

across the bulk dc supply so that current flows in the

primary from left to right for some variable period of

time. On the second half of the switching cycle with S 1

and S3 now open, switches S2 and S4 are closed in

concert and connect the primary of the transformer

across the dc bulk supply so that current flows in the

primary from right to left again for some variable period

of time. The switches are activated by the control

circuitry and by varying the duty cycle of the switches it

is possible to maintain the level of the rail voltages in

spite of varying rail loads and variations in bulk dc

supply values. The secondary of the transformer is

center tapped and feeds a full-wave bridge rectifier and

a capacitor filter for each rail. The capacitance values

required here are modest as compared with those in the

bulk supply as the ripple frequency is twice the

switching frequency and can range from about 60 kHZ

to 200 kHz. The RC network in parallel with the trans-

former primary is a snubber network that in conjunction

with the diodes in parallel with the switches allows

switching transients to be damped while returning

energy to the bulk supply.

20.3.5 Technological Innovation

The most recent high-power analog amplifier design is

based on a patented design that introduces a new class

of operation. This design is based on an innovation that

Figure 20-32. Typical EMI filter.

Figure 20-33. Line-operated rectifier and capacitor filter.

To

rectifier

G

Lo

Hi

Shield

120–240 V

C

C

A

NTC B

RS

Hi

Lo

(^240120)
Link
–Out
+Out

• 
  


• 
  

  ¾¾¾–
  ¾¾¾–
  Figure 20-34. Full bridge dc-to-dc converter.
  +Bulk supply
  –Bulk supply
  S 1
  S 2
  S 4
  S 3
  +Rail
  Gnd
  –Rail