stereo to bridge amplifier transformation

If you bridge you reduce the load impedance seen by the amp by a factor of two, so if the amp is only 4-ohm capable it will drive 8-ohms bridged, but not 4.

And by ground-referenced I presume you mean output is ground referenced.

Class D amps usually aren't ground-referenced, and even if they are might not bridge depending on the design.

The normal opamp inverter circuit can be used, nothing exotic is needed, but if you want differential inputs a fully differential input amp makes sense.

There are ways of combining amps as a bridge if the inverting input of the input stage is accessible - some chip amps do this.
I am not sure what you are asking, could you make a sketch of what you intend. If you have a differential output, from your DAC then I am sure that you could feed each into an amplifier, and it would appear as being bridged. When bridging amps, all you are really doing is driving each amplifier differentially, i.e. out of phase with the other amp. If the amps are already internally bridged, then I don't think you will achieve anything.
Looking at your data sheet, absolutely possible when connected as shown on the first page. That amplifier is fully differential and will be driving each of your power amps, so it will produce 4 x the power of a single power amp. However, the output impedance needs to be doubled else the signal may exceed the safe operation of the amp because you will double the output voltage swing.
Another limitation is the SOA (safe operating area). Most output transistors can handle much more current <50V rail supplies.
(below) NJW0281 (DC) can only do 1.5A @ 70V rails , 40-50V is a safer 3-4A per device.


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Here is a practical example. The output of the master amp is fed via a 33K resistor to the inverted input of the slave amp, while the slave amp's input is shorted to ground. The master and slave amp are identical.
In short, the output of the master amp is fed through the same resistor value as is in the normal feedback path. This is a very simple way of achieving a bridged amp, and many commercial amps get away with a simple bridged/non-bridged switch on the back of the amp. I am not sure but think that Bryston did it this way as well. You can also find an example of this method in ESP's website.
Take what Ostripper says above seriously. The amplifier has to have the capability to run the higher current, else you will simply fry them.


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There may have been some tweaks over the years, but attached is the original schematic. There is really no need to bridge this amplifier, it was just something interesting that was asked on this forum many years ago. I tried it, it worked and that was it, been there done it.

Before anyone comments that I should have done this or that, I am not revisiting something that has been archived since 2010, and I happen to have the data at hand on an old CD and just shared it for interest’s sake.

Sorry this amp was never featured here but on DIYAudio Turkey


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Something I should mention from experience over the past 50 plus years. No matter how good you think a typical amplifier sounds, raw power even if you never go above 10 watt always sound better. You remain in the linear portion of the amplifier and it is just better sound, I don't care if it is a $30K jewellery 20 watt amplifier that boasts fantastic specifications - power beats everything.
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Transfoemer based solutions sound best to me, as the dont introduce self noise, isolate the ground path and provide excellent distortion performance on par with most solid state solutions, given the transformer design is decent.
That's a big 'given', measured in $ and weight - an opamp's cheap, an opamp's small/light, an opamp doesn't need magnetic shielding.

But the ground isolation is an attractive prospect, though you have to worry about impedance matching a lot more with transformers.