PARASOUND A 23 DESIGN OVERVIEW continued
18
Parasound power amplifiers incorporate ingenious
and fast-acting DC servo circuits, completely
eliminating the need for coupling and blocking
capacitors. The A 23 is direct-coupled from its input
jacks to its speaker terminals. This advanced circuitry
never needs adjustment or maintenance. It operates
outside the audio signal path to keep the DC offset at
the output of the A 23 at a constant 0.00 Vdc. The
results are startling clarity, freedom from listening
fatigue, and formidable bass response.
Relay Protection
Each channel of the A 23 has a high-quality protection
relay with gold-plated contacts for long-term
reliability. These relays function to protect either the
amplifier, the speakers, or both. When the A 23 is first
powered on, these relays remain open for three
seconds as the positive and negative power supplies
stabilize and reach equilibrium. This prevents
annoying popping or other transient noises. Relay
protection also prevents damage to your speakers in
case of a catastrophic amplifier failure. Any amplifier
that doesn’t use relay protection for its speaker
outputs compromises the safety of the amplifier and
your speakers.
Over-Current Protection
Specialized current-sensing transistors are connected
to the output stages of the A 23 to constantly monitor
the current flow through the output transistors. If the
current drawn by this stage exceeds a predetermined
safe level due to a load impedance below 1 ohm or a
short circuit at the speaker terminals, the output relay
will open immediately to prevent any of the output
transistors or other parts from failing.
Fuse Protection
Each channel of the amplifier has a separate fuse for
its positive and negative DC voltage rails. These fuses
provide backup protection in case the over-current
protection does not work in time, or if an internal part
fails. In the event of a part failure, these fuses halt
operation to minimize damage to additional parts.
The Output Stage
The amplifier’s sonic characteristics are established
by its input and driver stages. Now, the sole job of its
output stage is to deliver the enormous current and
voltage from its power supply to the speakers. Bipolar
output transistors are better than MOSFETS in the
output stage because of their higher safe operating
area (SOA) and inherent ruggedness. Each channel’s
output stage employs three pairs of high current (15-
ampere) bipolar transistors to insure long-term
reliability, even with continuous high power operation
and challenging speaker loads. Lightning-fast (60
MHz) transistors respond instantly to complex
demands in the musical signal, virtually eliminating
distortions that occur with slower transistors. Slew
rate limiting and Transient Intermodulation Distortion
(TIM) are simply not an issue in the A 23.
Class A-A/B Operation
Pure class A operation provides the purest sound.
However, an amplifier operating entirely in class A
operation would be enormous, highly inefficient, and
generate too much heat. Class A/B combines some of
the advantages of Class A with the efficiency of Class
B operation. It is a compromise that reduces the heat
generated in pure class A operation and the odd-order
harmonic distortion created in class B. In class AB, the
driver and output stages are always partially turned
on, which provides a nominal amount of pure class A
operation. At higher power levels, when the musical
waveform swings from positive to negative and vice
versa, each bank of transistors is allowed to rest
momentarily. This resting, or quiescent time, makes it
possible to deliver high amounts of power without
overheating. It also makes it possible to use passive
cooling and avoid fans, whose noise can be heard
over the music. The A 23 input and driver stages
employ pure Class A while its output stage operates
with higher pure Class A power than many amplifiers
selling for twice or three times its price. The result is
less fatiguing, more natural sound.
Total Protection
DC Servo
Direct Current (DC) burns out speakers. Every power
amplifier must have some way to insure that DC from
its power supply never reaches its + or - speaker
terminals. Most amplifiers simply use trim controls to
reduce their DC offset or capacitors to block DC.
Unfortunately, trim controls can allow DC offset to
increase over time, and even the most expensive
capacitors in the audio signal path will “veil” sonic
clarity and attenuate bass response.
