Owner's Manual

15
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 voltage amplifier
stage; MOSFETs (Metal Oxide Field Effect Transistors) for the drivers of a second voltage amplifier stage and
bipolar transistors for the current amplifier 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. (Ordinary bi-polar transistors are low impedance devices) 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 higher
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.
The Output Stage
The amplifier’s sonic characteristics are established by its input and driver stages. Now, the sole job of its
output stage is to deliver the enormous current and voltage from its power supply to the speakers. Bipolar
output transistors are better than MOSFETS in the output stage because of their higher safe operating area
(SOA) and inherent ruggedness. Each channel’s output stage employs four pairs of high current (15-amp)
bipolar transistors to insure long-term reliability, even with continuous high power operation and challenging
speaker loads. Lightning-fast (60 MHz) transistors respond instantly to complex demands in the musical
signal, virtually eliminating distortions that occur with “slower” transistors. Slew rate limiting and Transient
Intermodulation Distortion (TIM) are simply not an issue in the A 21+.
Class A-A/B Operation
Pure class A operation provides the purest sound. However, an amplifier operating entirely in class A
would be enormous, highly inefficient, and generate far too much heat. Class A/B combines the main
advantage of Class A with the efficiency of Class B operation. It is a compromise that reduces the heat
generated in pure class A operation and the higher-order odd harmonic distortions created in class B. In class
AB, the driver and output stages are always partially turned on, which provides a nominal amount of pure
class A operation. At higher power levels, when the musical waveform swings from positive to negative and
vice versa, each bank of transistors is allowed to rest momentarily. This resting, or quiescent time, makes it
possible to deliver high amounts of power without overheating. It also makes possible the use of passive
cooling and avoid fans, whose noise can be heard over the music. The A 21+ input and driver stages employ
pure Class A while its output stage operates with higher level of pure Class A power than many amplifiers
selling for twice or three times its price. The result is less fatiguing, more natural sound.