[robrob]

I think most of us think of a "truly balanced amp" as having three properties all of which reduce crosstalk:
 
1. Balanced input meaning identical impedance in the positive and negative inputs (so common ground between the left and right channels cannot be used).
 
2. Balanced amplification meaning complete separation between left and right channels so no common ground in the amplifier circuitry (dual differential or push-pull amplification is normally used).
 
3. Balanced output meaning balanced (identical) impedance in the positive and negative transmission lines (so common ground cannot be used). The primary benefit of balanced output is to take advantage of common mode noise cancellation.
 
Each of the 3 can be implemented independently:
 
An amp can have unbalanced (common ground) input with balanced push-pull amplification with balanced or unbalanced output.
 
Some amps have balanced input with ground referenced single-ended amplification with balanced output.
 
The schematic below shows a single-ended amp with balanced output. The KT88 is definitely being used as a single-ended amp but the speaker side of the output transformer is not grounded so it acts as a fully balanced output and would offer matching impedance on the + and - speaker outputs so it would offer common mode noise cancellation. (But if you used a coax for speaker wire and wired the + to the center conductor and the - to the shield you would lose common mode noise cancellation due to conductor impedance mismatch. The coax has unbalanced impedance.)
 
 

 
Here's a push-pull amp with common ground (unbalanced) output. Note the ground symbol on the speaker tap at far right:

 
In layman's terms, the opposite of push-pull is single-ended and describes amplification circuitry. The opposite of balanced is unbalanced (or common ground or single-ended). It's used to describe how circuits or components are connected.

 

[audiowize]

   
2. Balanced amplification meaning complete separation between left and right channels so no common ground in the amplifier circuitry (dual differential or dual push-pull amplification is normally used).

I'd be interested in seeing that drawn out.  Since a balanced cable interface implies equal impedances between hot/ground and cold/ground, in the instance where this impedance is not infinite, there will be a common ground connection.  OTOH, I could build an amplifier with input transformers that accept an unbalanced connection but breaks ground current loops, then have those transformers feed a pair of single ended amplifiers with floating grounds, then have balanced output transformers.
 
Such an arrangement would fulfill your definition of "Balanced Amplification", while the vast majority of differential amplifiers would not. 
 
Since balanced vs. unbalanced applies to a cabling interface, amplification is defined as balanced/unbalanced solely based on this criterion.
 
If you want to talk about what's going on inside the amplifier, then you have to move into amplification design topology. 
 
 
 

 

[robrob]

  I'd be interested in seeing that drawn out.  Since a balanced cable interface implies equal impedances between hot/ground and cold/ground, in the instance where this impedance is not infinite, there will be a common ground connection.  OTOH, I could build an amplifier with input transformers that accept an unbalanced connection but breaks ground current loops, then have those transformers feed a pair of single ended amplifiers with floating grounds, then have balanced output transformers.
 
Such an arrangement would fulfill your definition of "Balanced Amplification", while the vast majority of differential amplifiers would not. 
 
Since balanced vs. unbalanced applies to a cabling interface, amplification is defined as balanced/unbalanced solely based on this criterion.
 
If you want to talk about what's going on inside the amplifier, then you have to move into amplification design topology. 
 
 
 

 
This is what I meant when I said, "complete separation between left and right channels so no common ground in the amplifier circuitry's direct signal path (dual differential or push-pull amplification is normally used)." This amp is "fully balanced" in that it has balanced input, output and no ground in the direct amplification signal path.
 

I'm trying to keep it relatively simple so the average audio enthusiast can see the difference between a single-ended and push-pull (double-ended) amplifier and realize the input and output can be balanced or unbalanced with either type amp.

 

[audiowize]

Q1/Q2 are tied to ground, T2 is tied to AC ground, this will very likely be a common ground between channels. 
 
This is the failure of understanding - that balanced/unbalanced has to do with signal interface and ground, which doesn't apply well to the inner workings of an amplifier.  We use differential, push pull, etc. to describe these modes of operation, not balanced/unbalanced. 
 
 I will propose that your definition of "fully balanced" is by no means universal, and may really only exist on this forum.

 

[robrob]

 Q1/Q2 are tied to ground, T2 is tied to AC ground,

 
The grounds are not in the direct signal path like a single-ended amp.
 
Quote from my post above:

 In layman's terms, the opposite of push-pull is single-ended and describes amplification circuitry. The opposite of balanced is unbalanced (or common ground or single-ended). It's used to describe how circuits or components are connected.

 
I would think most of us would agree the common meaning of "fully balanced amp" means balanced input and output and amplification that maintains separation between the 4 signal paths meaning no common ground in the amplifier's direct signal path.

 

[Steve Eddy]

I would think most of us would agree the common meaning of "fully balanced amp" means balanced input and output and amplification that maintains separation between the 4 signal paths meaning no common ground in the amplifier's direct signal path.

I would call that a common misunderstanding rather than a common meaning.

se

 

[audiowize]

   
This is what I meant when I said, "complete separation between left and right channels so no common ground in the amplifier circuitry (dual differential or dual push-pull amplification is normally used)." This amp is "fully balanced" in that it has balanced input, output and no ground in the amplification signal path.
 

 

Draw the AC current loops inside the transformers - they share a ground ^_^

 

[robrob]

  Draw the AC current loops inside the transformers - they share a ground ^_^

I don't consider that ground to be in the direct signal path.

 

[Steve Eddy]

I don't consider that ground to be in the direct signal path.

As opposed to what? The indirect signal path? If so, what exactly do you consider the indirect signal path to be?

se

 

[audiowize]

As opposed to what? The indirect signal path? If so, what exactly do you consider the indirect signal path to be?

se

 
Thank you Steve!
 
Rob - you can't make up a definition of a new type of amplifier by using another definition that also doesn't exist. 
 
(Also, I can make you a single ended headphone amp that's push-pull)

 

[Steve Eddy]

Thank you Steve!

Rob - you can't make up a definition of a new type of amplifier by using another definition that also doesn't exist. 

(Also, I can make you a single ended headphone amp that's push-pull)

No problem.

The whole notion of "signal path" has always been a bit squidgy in consumer audio. It's often thought to be only the horizontal lines in a schematic. This notion gave rise to the "shunt" type attenuator that's been popular with the DIYers. It's employed by using a single fixed resistor in series, and then a variable resistor in shunt. The idea being you can use a single, very high quality resistor for the series element and a lesser quality variable resistor for the shunt because the shunt resistor isn't "in the signal path" and so won't have any meaningful effect on the sound.

Of course this grossly over simplified notion of "signal path" overlooks the fact that the output voltage of such an attenuator is the result of the current that's flowing through the lower quality shunt resistor!

se

 

[audiowize]

Yeah, whatever signal current flows through the series element will also flow through the shunt element... (unless a gnarly load is placed at the output)

 

[robrob]

As opposed to what? The indirect signal path? If so, what exactly do you consider the indirect signal path to be?

se

It's my understanding the center taps of the transformers have no current or voltage, they just provide a ground reference. What I mean by "direct signal path" is the schematic's outside loop from push tube/transistor--through the output transformer to the pull tube/transistor and back.

 

[Steve Eddy]

It's my understanding the center taps of the transformers have no current or voltage, they just provide a ground reference. What I mean by "direct signal path" is the schematic's outside loop from push tube/transistor--through the output transformer to the pull tube/transistor and back.

Yeah, but that schematic is rather over simplified and among other things does not include the power supply.

se

 

[Steve Eddy]

Yeah, whatever signal current flows through the series element will also flow through the shunt element... (unless a gnarly load is placed at the output)

Yup. And as per Ohm's Law, voltage is equal to current times resistance, in this case the resistance is the shunt element.

se