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,
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.
