The term is commonly used both in text books and the professional recording environment as shorthand for the inverted signal in a balanced pair..
It is and many people are confused by it! I apologize if you see this as being patronized, that was not my intent! When I am writing anything here, I am assuming that others are reading along. For that reason I try to avoid getting too technical.
If you do a quick google search on differential amplifier you will see that by far the most common use for this term is exactly what I have described and this is what is used in the majority of equipment with balanced inputs.
OK. Here is just exactly that:
https://www.google.com/search?q=differential+amplifier&client=ubuntu&hs=WBe&channel=fs&a...Take a look at the first hit (this is an images search). The first image shows both what you’ve described **and** what I described in the same image, the latter of which does not have the Johnson noise issue.
If the device has a transformer at its input, this noise won’t occur. If an opamp is used, it might or might not be used that way- depending on if the input is more of the instrumentation variety rather than a single opamp device (and the reason for not doing that would be if the internal circuitry of the preamp or whatever is fully differential; at that point you’ll need opposing outputs from the first stage which a single opamp can’t provide). We recently had a McCormick amplifier come through the shop; its balanced inputs were executed as I described in one of my prior posts (and as you see in that first Google image), using dual matched input transistors, probably similar to a MAT-12
https://www.analog.com/media/en/technical-documentation/data-sheets/MAT12.pdfwith a common emitter circuit.
Our conversation started with you stating that a balanced input would be noisier due to the resistor noise:
The bit that a lot of people are unaware of is that balanced inputs are actually noisier than single ended (RCAs etc.) in that they utilise relatively high value resistors that introduce their own (johnson) noise into the signal.
IME the statement is only true if opamps are used *and* the internal circuitry is single-ended (in this case, if succeeding opamps are used but with the signal only applied to one input, I would regard that as single-ended. Another way to look at that is look at the volume control; if it has one deck for each channel then its single-ended. This thread is about ’Differential Sound Quality’ so I have to assume that circuits/products that are in fact fully differential are included.).
