Halo A 21 Amplifier Build Quality - Continued
There are many manufacturers of home theater electronics who place a high emphasis on cosmetics in order to make their gear look as though it where of a higher quality from the outside, though most of them never expect anyone to look under the cover. Parasound is not one of those commonly found cosmetic only companies as the build quality of the outside of their amplifiers are transposed to their inside. When removing the cover of the Halo A 21 and A 51 amplifiers, what becomes most apparent at first glance is their massive toroidal transformers with independent secondary windings for each channel. In essence this feature means that each channel has its own devoted power supply. Combining the transformer with four 80 volt 25,000 microfarad capacitors ensures that the Halo amplifiers are capable of providing plenty of reserve power when the source content requires low impedances and maximum current. Such a design reveals how Parasound justifies their specs with sufficient high quality components implemented within a well conceived design. I also noticed that all of the resistors were 1% tolerance parts thereby ensuring excellent stability for the signal path.
As you can see from the above pictures, there is minimal wiring within the Parasound amplifiers. This means that the signals are carried directly from the connectors to the circuit board which can greatly reduce the possibility of noise infiltration. The A 21 and A 51 also sport internal and external gold plated connectors which tend to be an uncommon feature.
The amplification sections of the A 21 and A 51 were equally as impressive as the other portions of Parsound's design. All of the transistors are discrete as opposed to integrated circuits. The input stage uses matched pairs of JFETs and the driver stage uses matched pairs of MOSFETs which tends to give a less harsh and colored resulting sound. These two stages give the amplifier its sonic signature while the output stage gives the amplifier its high current capabilities. Parasound also implement bipolar transistors in the output stage since such a design typically results in very rugged high current capability.
One of the most noteworthy
operational functions of the Parasound A 21 and A 51 amplifiers are their Class
A-A/B operation. The input and driver stages operate in pure Class A meaning
that the transistors are turned on full all of the time. In essence there are
two banks of transistors in the output stage, one of which is devoted to
positive voltage and the other devoted to negative voltage. The point where the
positive output transistors turn on while the negative transistors turn off (and
vice versa) can cause nasty higher-order odd harmonic distortion (3rd, 5th, 7th,
9th, etc.) for which even very small amounts have been demonstrated as a primary
cause of listening fatigue. In these Parasound amplifiers both the positive and
negative transistors for each channel are always fully on – the definition of
Class A operation - up to 8 watts output per channel. So, for the first 8 Watts
of power, which exceeds the typical average power requirements for music, the
output transistors are always on. For greater than 8 Watts, the transistors are
in class A/B operation where they are partially on when they're not up for duty
and fully on when they are up for duty.