times and under all conditions. It also reduces inter-channel crosstalk that can blur the sound and impair the correct sense of where instruments, 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 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 31 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 31 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 31 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 31’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 cascaded 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 solid state amplifiers.
