Some of the most important circuit technology in audio is also the simplest. Take for example the constant current source.
If we look at a simple single stage amplifier it is pretty straightforward: a single transistor (Q1) and a few resistors. Depending on the value of those resistors, we can get a certain amount of gain when we input an audio signal—small signal in and we get at the output a larger version of that signal. Easy peezy. This is the basis of how we started designing discrete amplifier circuits.
Add a second transistor (Q2) and another few resistors and that single-stage amplifier becomes a differential pair. Feed the output of that differential pair into a third transistor (Q3) and its associated resistors and voila! We have made a simple discrete op amp. Lastly, add a few more transistors (Q4 and Q5) at the output of this simple amp so that they provide more current (and protect the sensitive amplifying transistors before it), and now we have a great sounding discrete op amp: the original basis of all PS Audio phono and analog preamplifiers. (The two diodes D1 and D2 set the bias for the little output stage).
Good sounding, yes, but with a very simple addition, this circuit can sonically open up and sing like you would not believe. In fact, the simple addition of yet another transistor turned this wonderful little circuit into something rather extraordinary (at the time).
This was the 1970s at the very beginning of our journey and all this discrete analog stuff was heady indeed. We were pioneers foraging our way through the weeds of uncharted territory. A fellow engineer or audio nerd would whisper in our ear about some new discovery (like bypassing electrolytic caps with small film caps) and everyone in our tight little circle of nerds would pounce on it to see if a difference could be heard. If it mattered, it became audio gospel.
What we knew was that those first two transistors that make up the input diff pair were not ideal. Because the transistors were connected to the power supply through mere resistors, the rising and falling input/output signal forced those transistors into constantly fluctuating with every string pluck on a recording. What we would love is a way to have a constant current (like a class A bias) running through those two devices. If that were possible, we could fix the little circuit's operating point for whatever sounded best and be assured it would never change with louder or softer music.
That was the holy grail.
Someone whispered the answer in our ears. "Shhhh....a constant current source will do it. A single transistor, a couple of diodes and a resistor or two and voila!"
No shit. Wow.
Look at the first diagram. See R2 (47K)? If we were to replace that resistor with a specially configured transistor, we could set the level of constant class A operation on the diff pair. Here's something similar.
This is a bit simpler because of their use of an LED instead of two signal diodes to bias on the transistor, but the idea is the same. The LED as fed from the resistor RB turns on the transistor so it starts drawing current. The lower resistor, RE, determines just how much current is constantly flowing through this transistor. Simple.
Today, nearly 50 years later, this is just the way you do it. We would never consider using a simple resistor that lets the current flop around with the musical signal.
But,. back then, it was a miracle!
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