Differential Balanced Sound Quality

So if I start with a conventional amplifier block with an inverting and non-inverting input and a single output. A single ended amplifier input might be a 100Ω series resistor followed by an RF filter & DC blocking capacitor, in non-inverting mode the input impedance is set by the resistors to ground at the input so it’s not difficult to maintain a high input impedance alongside a low thermal noise from the series resistor. The actual impedance of the non-inverting input is so large that it can be pretty much ignored.

If you take the above example and feed the cold signal into the inverting input, the series resistor on the cold input will dictate the maximum input impedance as the current will be flowing into the virtual ground at the summing point. So 100Ω is now out of the question. You might for example choose to go with 10kΩ series resistors on both inputs, that’s 20dB more thermal noise than 100Ω.

@pragmasi 


There's no such thing as 'cold signal'. There's non-inverting and inverting. What you're describing (as seen in your link) has to do with an opamp which has differential inputs but a single-ended output. A differential amplifier always has dual outputs. Look again at the diagram I linked:
http://www.atma-sphere.com/en/resources-understanding-our-circuits.html

When I look at your amplifier I see that you have two outputs and I suspect that is the source of confusion... at what point does the cold signal get inverted?.. or does it connect to the negative speaker terminal?

Really, I think this 'cold signal' thing is confusing you. Both inverted and non-inverted signals are 'hot'. They must both be treated the same way. If you are referring to the inverted signal (for example the minus output of a phono cartridge) it gets inverted at the output of the device to which it was applied as an input.

There's no such thing as 'cold signal'.

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 doesn't confuse me and I don't need to be patronised.

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. 

The conversation flow of this thread is about the pro's and con's of balanced connections and what I have said is correct.

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.pdf
with 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.).

I apologize if you see this as being patronized, that was not my intent!

Cool... no problem.

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.

Absolutely, agreed... I was talking about the topology shown in the first image.

IME the statement is only true if opamps are used *and* the internal circuitry is single-ended

Yep... op amps are the usual way of doing it and that's what I was talking about.

BTW love the look of your amps and I can see the passion you put into them.

Thanks for all the discussion folks.  It's always nice to read a thread that actually educates, or at least presents the opportunity...