Like I said, point proved. I rest my case.
Chris. The first comment was aimed at Julf.... and yes of course you can deliberately "unbalance" the balanced input to give improved CMRR with a specific known input impedance. Not exactly ideal for a universal balanced input! The single op amp version is best kept for internal use in a piece of equipment where it can be driven from known low source impedances from a previous op amp stage.
The main argument was whether or not a single op amp balanced input was a good solution. I have proven that it is not.
You seem to think my previous post proves this point ^.
I can't see why you think that.
If so, you've seem to have ignored most of my post and picked on one small item which was included as a throwaway but technically accurate comment and not relevant to the real world maths
The engineering/maths is quite simple - balancing an input inherently provides CMRR (even if it's
not perfectly balanced)
Refer back to the equations in Putzeys' document. The performance/CMRR of any balanced input connection is dependent on the
balance of the input. It doesn't matter what this balanced device is - whether it's the input loading of an instrumentation circuit, an opamp, or a pair of transistors. Or even balancing the earth connection.
Balancing an unbalanced input is better for CMRR than leaving it unbalanced.
I prefer unbalanced to balanced in general for the reasons we started with... ie unless the circuitry itself is completely balanced it means adding op amps to the signal path.
As previously, you don't need to add opamps to get a balanced circuit.
Most so called balanced units are internally unbalanced. They then add op amps to the output to make it balanced just before it goes to the XLR socket. Usually the signal then goes through more op amps at the other end to convert it back to unbalanced where it then continues through unbalanced circuitry.
"Most" circuits doesn't mean it's the only way. Extra opamps are not needed.
No, you haven't. You have pointed out, and I agree, that it is not perfect. It is still much better than unbalanced. This is the fact you keep trying to avoid.
No I'm not avoiding it al all I'm saying the opposite is true. Unbalanced is better than balanced.
You're saying that
no CMRR is
better than having CMRR.
And as I have " proven" (to use your terminology), nothing needs to be added to a circuit that uses an opamp, and only the input transistor needs to be added to a discrete circuit. I even posted the circuit diagrams. Do I have to do it again?
You can post your circuits as many times as you like and will remain wrong as, apart from anything else, we are arguing different things.
Not wrong. A balanced impedance differential input is ... balanced.
If you take a circuit which has differential inputs and present the signal across the inputs with balanced impedances, then it is ... balanced
You are mixing up two separate things - fully balanced circuitry (that you seem to be talking about) and balanced/differential connections (that the OP is about). The latter only needs a differential input, not a fully balanced circuit all the way through the amp.
Differential and balanced are the same thing. It's just semantics.
It's important semantics. It's very easy to have an unbalanced differential circuit. Both are needed. Lots of circuit blocks are differential but aren't balanced.
If I was going to use balanced I would use completely balanced discrete circuitry from input to output and not bastardise it by repeatedly converting the signal from balanced to unbalanced and vice versa with op amps!
You're implying that deliberate conversion backwards and forwards between single ended and balanced is being advocated, and of course it's not. The point is that every time you can make the interconnection balanced (impedance) you improve CMRR, even if the circuit block is nominally single-ended.
If you haven't done so yet, please do go and read the Bruno Putzeys document. His balanced volume control stage is particularly interesting. And then come back and tell us what is wrong with the document.
(As Putzeys clearly states the (balanced) circuit included in the document is a proof of principle and not a production design, so let's stick to his maths and the balanced behaviour of the circuit.
There are quite a few of these preamps out in the wild)