When we use the term ‘true balanced’ we mean a circuit that is balanced from input to output, as opposed to one that is balanced only on its input, output or both. Why is this important? Because true balancing a circuit means we not only take advantage of noise reduction on the input, but we carry forward that same benefit to artifacts of the amplifier circuit itself, such as distortion; something a balanced input/single ended design cannot do.
Here’s the circuit I showed you in yesterday’s post.
On its inputs (labeled Input 1 and Input 2) we benefit from the elimination of common noise as would be expected from any balanced amplifier. This is because the differential amplifier, made from J1 and J2, rejects anything common to both its inputs and amplifies only differences between the two. But note the use of a second differential amplifier following the first and composed of transistors J3 and J4. This second differential amplifier works in exactly the same manner as the first, but noise is not what we expect it to reduce. Instead, amplifier two will reduce any common distortion or non-linearities of the first amplification stage. And every amplification stage has distortion and non linear products, typically produced in equal amounts between the two transistors that make it work. So following this first stage with another differential amplifier is a very smart thing to do. Plus, there is yet another benefit. We get the outputs already balanced for us.
Note where we see the nomenclature Output 1 and Output 2. Like our balanced inputs at the beginning of this arrangement, we now have a fully balanced output without any extra circuitry added to achieve it. Add a couple of buffers on the output to drive low impedance loads or tricky connecting cables and you have a fully balanced circuit from input to output that is simple, quick and extremely good sounding.
There are other topologies that achieve this same level of true balance, but few I am aware of that are as simple and clean.