Parasound A 21 - Manual (Page 20)

PARASOUND A 21 DESIGN OVERVIEW
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chosen for their low Equivalent Series Resistance
(ESR) and dielectric absorption. In addition, these filter
capacitors are bypassed with smaller polypropylene
capacitors to reduce AC ripple in the DC supply and
to further eliminate noise and interference that is
generated in AC power lines from computers and
other appliances in the home.
Relay-Bypassed Soft Start Circuit
When the A 21 is first turned on, there is a significant
amount of in-rush current required to charge the
enormous power supply capacitors. In order to
suppress this in-rush current and to prevent nuisance
tripping of circuit breakers, we employ NTC (negative
temperature coefficient) resistors. These resistors cut
the in-rush current by approximately 50%. Once they
heat up, they essentially become a jumper with zero
ohms resistance. However, the A 21 goes one step
further for this circuit. After the NTC resistors have
done their job of suppressing in-rush current a gold
contact relay automatically is activated to jump across
the NTC resistors to completely bypass them. This
extra step insures that the resistors do not restrict any
current whatsoever to the power supply once the A 21
is in full operation.
Audio Circuit Path Topology
Parasound’s circuit topology is a hybrid of carefully
chosen discrete transistors that result in superior
performance at each stage. We use JFETs (Junction
Field Effect Transistors) for the input stage; MOSFETs
(Metal Oxide Field Effect Transistors) for the driver
stage and bipolar transistors for the output stage.
Discrete transistors are more sonically accurate than
integrated circuits commonly used by other brands.
Complementary Configuration
Each stage of amplification has transistors fed by
the positive DC power supply and complementary
transistors fed by the negative DC power supply.
Thus, half of the devices amplify the positive half
of the musical waveform while the other half of the
devices amplify the negative half. This complementary
topology is inherently linear, which reduces distortion
and improves sonic accuracy.
The Input Stage
The A 21’s input stage uses matched pairs of discrete
JFETs arranged in a differential configuration. JFETs
are ideal for the input stage because their inherently
high impedance is unaffected by the impedance
of source components. Differential configuration
provides superior noise reduction. These precision
input JFETs are also cascoded to produce the current
necessary to drive the MOSFET drivers in the following
stage.
Circuit designed by John Curl
Parasound design consultant John Curl has been a
legend among audiophiles and electronic engineers
for decades. He pioneered measurements to correlate
musical accuracy with the materials used in parts,
worked with world-class touring companies, has
designed highly coveted audio classics, including
the original Mark Levinson JC-2, Denneson JC-80,
Vendetta Phono Preamplifier, and CTC Blowtorch
preamplifiers; master recorders for Wilson Audio and
Mobile Fidelity; and the mixing consoles used in live
concerts by The Grateful Dead and the Montreux Jazz
Festival in Switzerland.
As our featured amplifier designer since 1990, he has
created many products that have earned Parasound
worldwide acclaim. John is particularly proud of
what he and Parasound have accomplished together:
“The circuits I design for Parasound are extremely
sophisticated and are typical of products that are far
more expensive. I can’t think of any other audio
products that offer nearly as much bang for the buck.”
Parts Selection
Every part within the A 21 is carefully chosen for
its accuracy and reliability. Metal film resistors with
1% tolerance are selected for their precision and
because their values don’t drift as they heat up during
operation. Polypropylene and mica capacitors are
used extensively for their superior linearity and low
dielectric absorption. Semiconductors are selected
for superior performance in their specific roles in
the circuit. Gold has the best conductivity of any
metal, so we use high quality gold-plated input
connectors and speaker terminals. The double-sided
circuit boards are FR4 glass epoxy for long-term
durability. The chassis is made of heavy gauge steel
to safely house the internal circuitry. This attention to
detail when selecting parts makes the difference between
a very good amplifier and an outstanding amplifier.
The Power Supply
The heart of the power supply is a 1 kVA toroidal
power transformer, chosen for its efficiency, low
hum field, and high power rating. Encapsulating this
massive power transformer in an epoxy-filled steel
canister assures ultra-quiet performance.
The A 21 power transformer employs multiple
independent secondary windings so that each
amplifier channel has its own power supply, assuring
more than ample DC voltage at all times and under
all conditions. It also reduces inter-channel crosstalk
that can blur the sound and impair the correct sense
of where instruments are positioned.
To create the +/- 80 Vdc B+ and B- supply rails for each
channel, we use high-speed rectifier diodes and four
enormous 25,000 uF Elna electrolytic filter capacitors,