PARASOUND A 51 DESIGN OVERVIEW continued
16
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 dis-
tortion is less offensive because it is consonant,
rather than dissonant. Our MOSFET driver stage pre-
vents 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 reliabil-
ity, 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 51.
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 51 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 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 ter-
minals. 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 51 is direct-coupled from its input jacks to its
speaker terminals. This advanced circuitry never
needs adjustment or maintenance. It operates out-
side the audio signal path to keep the DC offset at
the output of the A 51 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 51 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 51 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 51 to constantly moni-
tor the current flow through the output transistors.
If the current drawn by this stage exceeds a predeter-
mined 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.
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