HI
Higher Class A bias would not provide any performance benefit over the current quad complimentary output stage we currently use.
james
Bryston’s Proprietary Output Stage
When the transistor was first invented, it functioned only in one polarity. That meant that there was asymmetry in amplification circuits, resulting in distortion of the signal. Later, the other polarity of transistor was developed, making it possible to have a symmetrical, ‘complementary’ circuit, thus reducing distortion.
Unfortunately, these opposite polarity transistors are not exact matches to each other. They have differences in bandwidth, differences in threshold Voltage, and differences in the way their respective gains track both Voltage and current changes. Thus, there continued to be small variations in symmetry, revealing subtle but audible amounts of distortion, even in supposedly ‘complementary-symmetry’ amplifiers. These distortions were worse with increasing frequency, giving a characteristic haze or graininess to transistor amplifier sound.
In most of the amplifier circuitry, the above asymmetries can be compensated for with proper design, but the output stage of a power amplifier is in direct contact with the speaker load, and thus experiences large variations in both Voltage and current with the signal. It is thus subject to ‘worst-case’ conditions for the asymmetrical distortions left in these opposite-polarity transistors. It was for this reason that Bryston developed the Quad-Complementary output stage. This nomenclature stems partly from the fact that it requires at least four transistors to assemble the final section of the output stage, one of each polarity on both sides of the push-pull output section.
In this way, it became possible to eliminate almost all of the remaining asymmetry in the output stage of an amplifier, because each transistor is paired with another of its opposite number. This creates what amounts to a compound device, displaying the mixed characteristics of both. Thus, the upper and lower halves of the output stage match each other’s dynamic characteristics exactly, at both high and low levels. Signal distortion is virtually eliminated.
This circuit also displays advantages in some other areas, like faster response and lower input drive current for the same output power. Those characteristics give the amplifier lower distortion in all areas, but especially in the important high frequencies. Thus, a Bryston amplifier does not display the characteristic high frequency haze or grain often heard in other transistor amps.
