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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 31.
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 31 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 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 31 is direct (DC)
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 31 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 31 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 31 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.