Influence of DC offset on bias

Of your amplifiers, I'm only familiar with the circuit in the GAS amps . . . and both are fairly representative of a common-practice solid-state amp with an emitter-follower output. The complimentary diff-amp is kinda fancy but by no means unique, and the Ampzilla's bootstraped outputs are typical of many higher-powered amps of the era.

So to answer your question, small amounts of Vos are really only possibly of concern to your woofers, in terms of a static displacement in the voice-coil position or eventual loss of magnetic flux. While there is indeed a small difference in current between the halves of the output stage as a result of a DC offset, the optimum bias point in a Class B amp is dependent on the voltage between the output transistors' emitters, not the current through the output stage. And yes, they are sometimes the same thing in practice, but if you were to change the value of the emitter resistors, the optimum bias current value would also change. And since the bias generator sets the voltage between the two halves of the output stage (not relative to ground), the presence of an offset doesn't change the transfer function of the output stage.

The main issues with output stage biasing are how well it's maintained with variations in temperature and line voltage, and how much an increase in signal current affects the linearity in the crossover region. And these issues span everthing from circuit design, the characteristics of the transistors themselves, the layout of the wiring and circuit board, and the physical packaging and thermal design . . . there are no universal solutions.

Hi Csontos . . are you experiencing differences in timbre or level between channels on some of your amps? If so, given the age of the GAS stuff, the first thing I would be looking for is dried-out electrolytics used for coupling caps, especially at the input and in the feedback ladder.

Well, bias of course affects distortion performance, and I have seen many, many amplifiers where the final quescient bias level when warmed up varies with how much load there is on the amplifier when its warming up, and ends up being very different from the value and conditions specified by the manufacturer in the biasing procedure. These thermal subtleties seem to be lost on a great many engineers, and a manufacturer's audiophile "brand reputation" seems to have very little correlation with competency in this area.

As far as offset goes, most modern amps *shouldn't* exhibit a change in distortion performance as a result of an offset adjustment. But many early solid-state designs, including the GAS amps, don't have a current-mirror on the input diff-amp, which means that the balance of quescient current between the diff-amp pair is highly dependent on component tolerances . . . and offset adjustment. And even-order distortion products rise pretty quickly as the diff-amp becomes unbalanced.

With a distortion analyzer, oscilloscope, and a bit of experience, one can easily see the different effects of different distortion mechanisms on the amplifier as a whole. But without this, you're pretty much just "stirring the soup" of whatever flaws the amplifier has, and getting whatever floats to the top.