LM3886 GC - "Simplest Ever Amplifier Bridging"

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I dunno if Rod Elliots simplest bridging adaptor is worse....:confused:


The distortion products of the non-inverting amplifier are being fed to the inverting amplifier: so those non-inverting-amplifier-distorion-products will cancell out. The only distortion you are left with are the distortion products of the inverting amp (and the diustortion of the distortion products, far to low to be significant I assume).

So.. You are left with a bridged amplifier having the same distortion specs as a non-bridged amp in halve the load..

versus...

When a perfect balanced signal is obtianed from a unballanced signal and this is fed to two equal amplifiers, distrotion of each amplifier alone is like driveing halve the load but.. :att'n: the even-harmonics tend to subtract from each amplifier, while the uneven-harmonics tend to sum from each amplifier... this might even be worse...


go with Elliot, that guy never failed me once....
 
tschrama said:


The distortion products of the non-inverting amplifier are being fed to the inverting amplifier: so those non-inverting-amplifier-distorion-products will cancell out.


Nonsense. The distortion (and noise) from the first amp will be distorted by the second amp. You will also get intermod products on the distortion. Its a non-linear process, not just add/subtract.
 
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OK, this sounds like an ideal job for someone with one of those spice simulator things. Sim the two designs, (they can be any old op amps in the sim, it's the principal of the thing that matters ), and see how the two distortion figures come out. Or, of course, someone with the two amp configurations could just measure them I suppose! :)
 
in theory, it works perfectly -- i put it on Multisim8 and there is no distortion to speak of:

in the real world I have found distortion of 0.01 to 0.07% with the capacitor coupled, bridged LM4780
 

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pinkmouse said:


I wonder if one of the differences between sim and real life comes down to tolerance in other components between the chips. Could this be reduced by more exact gain matching I wonder?

I guess I should put up the picture -- with the resistors matched to 0.1% there is a slight difference in gain between the A and B channels -- Gain on A is 19.67dB, and on B it is 19.77dB. I am averaging 64 samples of a 1kHz signal.
 
jackinnj said:


I guess I should put up the picture -- with the resistors matched to 0.1% there is a slight difference in gain between the A and B channels -- Gain on A is 19.67dB, and on B it is 19.77dB. I am averaging 64 samples of a 1kHz signal.


Gain matching is not an issue in a bridge configuration except for efficiency (max available output at exact matching). If one side has 100% gain and the other 50%, then the output voltage will be 150% compared with the single ended case.

Below 0.01% THD, other issues like PSU and layout are much more relevant.

Rodolfo
 
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