Thank goodness we have choices in the devices we design into an amplifier; not all are best for a given application.
If you were to ask me to list the worst devices to build a power amplifier output stage a vacuum tube would be first. And there are a number of reasons why.
Let's remind ourselves what the task at hand is: moving a loudspeaker driver back and forth in response to a musical signal. A small change at the input is translated to a larger change at the output. Or said another way, a power amplifier uses a little bit of power at its input to produce a larger power at its output. This occurs when the small input power supplied from the source of music connects the loudspeaker to the amplifier's power supply in varying degrees. It is the same concept as controlling a water faucet: a small change to the water valve results in a big change at the output of the faucet. And it is the action of the aforementioned valve I am concerned with.
Connecting the power supply of an amplifier to a loudspeaker demands an accurate, proper valve. The best device for the task is one that provides a high input and low output impedance, preferably operating in the current, rather than voltage domain. Tubes are best as voltage amplifiers, transistors as current amplifiers. It's why in the upcoming BHK Signature we opted to choose the device best suited for each activity: a tube in the front end for voltage amplification, transistors in the back end for current.
In my research for similar design concepts I found a few attempts at similar hybrids. Surprisingly, few got it right. Most flipped the formula on its head in a bewildering fashion: transistors on the input, tubes on the output. Bizarre.
I am convinced some designs exist to be different while others strive for perfecting musical truth.
