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PARASOUND A 51 DESIGN OVERVIEW
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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 corre-
late 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 pre-
amplifiers; 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 sophisti-
cated 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 51 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 opera-
tion. Polypropylene and mica capacitors are used
extensively for their superior linearity and low dielec-
tric absorption. Semiconductors are selected for supe-
rior 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 select-
ing parts makes the difference between a very good
amplifier and an outstanding amplifier.
The Power Supply
The heart of the power supply is a 2.4 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 51 power transformer employs multiple inde-
pendent 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 instru-
ments, dialogue and effect are positioned.
Each channel
’s +/- 80 Vdc B+ and B- supply rails use
high-speed rectifier diodes and four enormous 8,200 uF
electrolytic filter capacitors, chosen for their low
Equivalent Series Resistance (ESR) and dielectric absorp-
tion. 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 51 is first turned on, there is a significant
amount of in-rush current required to charge the enor-
mous 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 cur-
rent by approximately 50%. Once they heat up, they
essentially become a jumper with zero ohms resist-
ance. However, the A 51 goes one step further for this
circuit. After the NTC resistors have done their job of
suppressing in-rush current a gold contact relay auto-
matically 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 51 is in full operation.
Audio Circuit Path Topology
Parasound’s circuit topology is a hybrid of carefully
chosen discrete transistors that result in superior per-
formance 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 transis-
tors 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 51’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.