Unnamed feedback method explored.

[Mr Evil]

The schematic I posted can be used for a real audio amplifier, if it's inverting. If it's non inverting then it requires a slight rearrangement.

Yes, you can use two different amplifiers to do it. In fact it needs the two amplifiers to be different. The main amplifier can be anything; the error amplifier should be something wide bandwidth, linear and it can be low-power. An op-amp would be a good, simple way of doing it. A highly linear class-A amp would be ideal though.

[name]

In Electronics&Wireless World 1987 V2 such circuits with 3 Amps was published. It improves the bandwidth and accuracy significantly.

[janneman]

Evil,

It is similar, but not identical to the Hawksford error correction, explored elsewhere in this forum.

Hawksford also takes the difference between input and output of a (gain = 1) stage, but adds this EXACT difference to the source.
This exact difference is significant. In Mr. Evils' graph after correction, you still see some remnant of the xover distortion. In theory, this can be made arbitrarily small but not zero, by arbitrarily increasing the gain of the correction amp, as has been shown, but you quickly get into stability issues.

The beauty of the Hawksford system is that by copying the error to the source, the error in theory becomes zero.
Of course, there's no free lunch here either, because the limiting factor becomes how accurately you can set the exact times-one adding of the error to the source.

Feedback is a fascinating subject! Think what all those non-global-feedback guys are missing

Jan Didden

[PMA]

Originally posted by Mr Evil
After error correction it looks like the picture attached to this post.

So has anyone made anything like this before, or read about it? Any drawbacks I haven't thought of (increased complexity and care needed for stability is all I can think of)?

I have seen your pictures. It works but I have to comment a little bit. The image (curve) that we can see after error correction is still highly distorted. The correction circuit must be able to remove cross-over distortion to that extent that it is invisible for the oscilloscope. Then you start with spectral analysis and will see your results.

Regards,
Pavel

[Upupa Epops]

Nothing new on this planet . Circuit based on this principle was published at JAES maybe twenty years ago ( I don't remember author's name ).

[Mr Evil]

Originally posted by name
In Electronics&Wireless World 1987 V2 such circuits with 3 Amps was published. It improves the bandwidth and accuracy significantly.

If only I was still at university, where they had a large backcatalogue of those. Maybe my local library will have some.

Originally posted by janneman
Evil,

It is similar, but not identical to the Hawksford error correction, explored elsewhere in this forum....

Indeed, I've just been reading some of his publications. Very interesting, and again, something that seems as though it should be used a lot more than it is. The difference is that it is feedforward, where this is feedback.

Originally posted by PMA
I have seen your pictures. It works but I have to comment a little bit. The image (curve) that we can see after error correction is still highly distorted. The correction circuit must be able to remove cross-over distortion to that extent that it is invisible for the oscilloscope. Then you start with spectral analysis and will see your results.

Regards,
Pavel

Yes it's still distorted. That's because it started of with an unholy amount of distortion in the first place. I've attached a fourier transform of the simulation results matching the two posted earlier. You can see that distortion is reduced by about 20dB for all harmonics, which is the gain of the error amp, as mentioned before.

Originally posted by Upupa Epops
Nothing new on this planet . Circuit based on this principle was published at JAES maybe twenty years ago ( I don't remember author's name ).

I didn't think it would be new, since it is an obvious step to take with feedback. I'm just surprised I haven't seen it before. I was thinking there might be some compelling reason why not to use it.

[janneman]

Eric, (?)

A possible reason that it is not used more could be that the traditional feedback gives the same reduction of 20dB if you add 20dB gain to the main amp.
And don't underestimate the power of tradition. We (the engineering world) is so much used to using normal feedback that you have to come up with something truly spectecular like saving 50% in manufacturing to even get them to take notice.

Jan Didden

[Mr Evil]

PMA got me thinking about the limits of the error correction, so I tried increasing the gain of the error amp to 40dB. Here's a fourier transform of those simulation results, showing a 40dB reduction in distortion. Now it's brought the distortion down to almost reasonable levels. Even harmonic distortion is off the scale, below -140dB.

The second graph is the same test performed at 20kHz. Here you can see the effect of the reduced bandwidth of the main amp (1/10th the bandwidth it was when the error amp only had a gain of 10). Odd harmonic distortion is still reduced by the same amount, but the effect on even harmonics is not as big anymore, with 2nd harmonic only just below -120dB, which is about the same as without error correction at all.

Originally posted by janneman
Eric, (?)...

I'm not called Eric!

[janneman]

Originally posted by Mr Evil
[snip]I'm not called Eric!

Sorry ! It's just that I am not accustomed to calling someone Evil to his face
Any hint?

Jan Didden

[PMA]

Originally posted by Mr Evil
PMA got me thinking about the limits of the error correction, so I tried increasing the gain of the error amp to 40dB. Here's a fourier transform of those simulation results, showing a 40dB reduction in distortion.

Yes, this is what I would expect from the error correction. According to my experience it is possible to get 40 - 50 dB distortion reduction, and this is worthwhile. The combination of output stage error correction + NFB leads to excellent measured and sonic results. There are several advantages of an error correction: it reduces high harmonics caused by cross-over distortion, reduces and linearizes output impedance, extends bandwidth (reduces turn-on and turn-off spikes and phase shifts).