In the comments section of yesterday’s post on amplifier bias, a great question was asked.
“What differences would I hear between a small amount of class A bias vs. a lot?”
It’s a great question but one that’s hard to answer. You cannot simply compare the sound of one model of amp with standard bias vs. another model of amp with higher bias. The two designs are so dissimilar that the degrees of bias would be swamped by the basic topological differences in the first place. For example, the BHK has a vacuum tube for its input stage and same sex N-channel MOSFETs for its output. No other amplifier I know of has this design—thus any comparison with another amp of different bias levels would be invalidated.
In order to properly answer the poster’s question, we would need two identical designs with different levels of class A bias to compare sound quality. I actually designed just such an amplifier, years ago. It was called The Genesis Stealth.
One of the Stealth’s features was adjustable levels of class A bias. From the front panel—in fact from the remote—users could set the output bias levels from low class A to fairly high levels of class A bias and find what suited their systems best.
What we found was interesting. With the bias levels set to their lowest point, the amp displayed power and dynamics. With the bias turned up on high—which I believe was 35 watts of class A—the sound sweetened but the dynamics and impact of the amp lessened. Users could choose which trade off they enjoyed most.
I hadn’t thought of that design for years. Thanks for the great question.
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