[penmarker]

Referring to the title above, has it been done? For instance, maybe with 2 Lehmann Black Cube Linear clones.

I googled around for a few minutes and found this thread from a few years ago
https://www.head-fi.org/f/threads/can-a-single-ended-amp-be-truly-balanced.515737/

[snip...]
"Balanced" headphone drive is more often about opposing polarity signals to each driver and is less often clear about the balanced impedance (which is arguably inconsequential in headphone drive) - in loudspeakers amps that drive opposing polarity to the loudspeaker terminals are call "Bridged Amps"

Locally we also use "balanced headphones" (or balanced headphone cabling) to distinguish between the standard TRS and any of the several cable connectors that give separate connections to each of the 2 headphone driver's 4 wires - however nothing says that just because you have 4 wires available that you have to apply "balanced" opposing polarity drive signals - but most here would consider it cheating to call a amp "Balanced" that wasn't Bridged (or use balanced output xfmr) even if you could only plug in "Balanced (cable/connector) headphones
[snip...]

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.

So I guess what I'm asking is has anyone made a balanced input, amplification, and output amp with common mode rejection from two (or more) single ended amps?

 

[Rroff]

Not quite the same but my current DIY amp is loosely similar to the PBA - takes an unbalanced input and outputs push-pull "balanced" (but not inverted) with somewhat overkill complete electrical isolation of the channels (even the power supplies have galvanic isolation from each other) - works a treat with my HD600s and a couple of other headphones I've modified for separate ground returns for each driver.

 

[penmarker]

Not quite the same but my current DIY amp is loosely similar to the PBA - takes an unbalanced input and outputs push-pull "balanced" (but not inverted) with somewhat overkill complete electrical isolation of the channels (even the power supplies have galvanic isolation from each other) - works a treat with my HD600s and a couple of other headphones I've modified for separate ground returns for each driver.

How did you go around doing that?

 

[Rroff]

The source ground connects to both power supply grounds (I did consider looking at other ways to do it but there isn't much to gain really) and each power supply has a (transformer) floating ground. The output channels each use a separate dual opamp one side acting as the gain the other as the active buffered ground.

Those that have actual audio/electronics engineering knowledge would probably have a fit but it sounds great to me.