Parasound A 21 - Manual (Page 21)

PARASOUND A 21 DESIGN OVERVIEW continued 18
Total Protection - DC Servos
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.
Parasound power amplifiers incorporate ingenious and
fast-acting DC servo circuits, completely eliminating
the need for coupling and blocking capacitors. The
A 21 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 21 at a constant 0.00 Vdc. The results
are startling clarity, freedom from listening fatigue,
and formidable bass response.
Total Protection - Relays
Each channel of the A 21 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 21 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.
Total Protection - Current Overload
Specialized current-sensing transistors are connected
to the output stages of the A 21 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.
Total Protection - Fuses
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 Driver Stage
The driver stage provides critical amplification for
which we employ a complementary matched pair of
MOSFETs selected for their tube-like sonic qualities.
MOSFETs tend to generate less odd-order harmonic
distortion than bipolar transistors. This is important
because odd-order distortion sounds unnatural and
fatiguing to the human ear, whereas even-order
distortion is less offensive because it is consonant,
rather than dissonant. Our MOSFET driver stage
prevents the harshness and brittle sound so often
found in other amplifiers.
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 four 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 21.
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 21
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.