Something quite new
So now that we know how conventional amplifiers and switchable amplifiers work, what is it we did differently in the BHK Signature Mono amplifier that started this thread? We built a parallel amplifier to turn the stereo BHK Signature into the Mono BHK Signature.
A parallel amplifier simple means you take two channels of an amp, left and right, and feed both inputs the same signal, then tie the two outputs together to feed a single loudspeaker. Seems simple, right? As I had described in an earlier post, it isn’t so simple and almost never done because it can cause major problems. In a solid state amplifier, if you tie the two outputs together – red to red, black to black of both channels fed the same input – you piss off the power amplifier and have poor results. Why? Feedback. Each amplifier channel has its own feedback loop that compares what has gone into the input of the amp and what comes out, making corrections for any differences. When you tie two amplifiers in parallel the feedback loops fight each other. The exception to this rule is a tube amplifier with an output transformer. In this situation the output of the two transformers can be paralleled to good advantage because of the transformer isolating the two.
But none of this applies to what we actually did in the BHK Mono and the differences are what I believe result in the extraordinary sonic improvement, beyond what we would expect when going from a stereo to a mono build.
Bascom H. King, the designer of the BHK Signature decided to take a very different course than had ever been done before.
Instead of paralleling just the outputs and managing the feedback problem I previously described, he fashioned an entirely new topology that pairs every component, not just the outputs. So, for example, the input vacuum tubes are tied together acting as one. This is a technique I have seen used only in moving coil preamplifiers to lower noise. Many moving coil phono preamplifiers parallel multiple devices (usually transistors) together. Each time a new device is added, the noise floor drops. So, for example, adding one device in parallel drops the noise in half. But getting another such drop requires two more in parallel, then four more, then eight more and so on. But noise was not the main goal in BHK’s thinking, as the noise level of the amplifier is already extraordinarily low. Rather, sound quality was foremost on Bascom’s mind when pairing tubes, drivers and outputs together in a configuration new to power amplifiers.
And just when I thought I’d seen every possible design twist in amps, along comes BHK and shows us once again, clever design with the goal of better sound, wins every time.