Parasound JC 1 Amplifier - Manual (Page 19)

PARASOUND JC 1 DESIGN OVERVIEW continued 16
Class A/AB 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 far too much heat. Class AB 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 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 JC 1 input and driver stages are
always on for 100% pure Class A while its output
stage provides up to 25 watts of pure Class A - more
than amplifiers selling for three or four times its price.
The result is less fatiguing, more natural sound at a
refreshing price.
Total Protection - DC Servos
Direct Current (DC) burns out speakers. Therefore,
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, which completely
eliminate the need for coupling and blocking capacitors.
The JC 1 is direct-coupled from its input jack 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 JC 1 at a constant 0.00 Vdc. The results are startling
clarity, freedom from listening fatigue, and formidable
bass response.
Total Protection - Relays
The JC 1 has a high-quality protection relay with
gold-plated contacts for long-term reliability. This relay
functions to protect either the amplifier, the speaker,
or both. When the JC 1 is first powered on, the relay
remains 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 speaker damage
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 stage of the JC 1 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 the output transistors
or other parts from failing.
Total Protection - Fuses
The JC 1 has separate fuses 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.
Chassis
The JC 1 chassis is heavy gauge steel with an aluminum
rear panel where its critical input circuit board mounts.
Aluminum is non-magnetic and cannot be energized
by nearby high voltage and current flow, thereby
removing a potential cause of “smearing” or other
audible intrusions.