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PARASOUND A 23 DESIGN OVERVIEW
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To create the +/- 55 Vdc B+ and B- supply rails for each
channel, we use high-speed rectifier diodes and four
enormous 12,000 uF Elna electrolytic filter capacitors,
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.
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 23’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.
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.
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 23 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 23 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.