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Old 22nd January 2004, 03:26 PM   #1
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Default Lowering Buffer Opamp Distortion

In the December edition of Electronics World magazine, Cyril Bateman makes reference to and measurements of a buffer stage capable of driving a 575R//10.2nF load at 4V RMS at vanishingly low distortion levels.

He quotes single op-amp buffer stage and dual op-amp buffer stage distortion figures for several common op-amp types as follows..........
TL072CP - 0.01266% vs 0.00041%
NE5532AN - 0.00032% vs 0.00022%
MC4558TPI - 0.00228% vs 0.00062%
AD712JN - 0.00088% vs 0.00032%

He also quotes figures for various physically separate op-amps as output amplifier and various op-amps as the feedback buffer.
Part of his conclusion is that the heating of the output stage of one channel of a dual op-amp affects the other channel, and 2 op-amp packages can work better - as good as 0.00010% at 5V RMS into 575R//10.2nF load with AD811AN output and OPA2134CPA nfb buffer.


Quote:
This device improves state-of-the-art circuitry by about 20 dB over currently available buffers (Fig. 1). Two amplifiers make up the system. The second amplifier is contained in the feedback loop of the first amplifier and configured for a gain of one.

Note that VOUT is approximately equal to VIN. The voltage at TP2 is similar to VIN as well. Consequently, the error be-tween the noninverting terminals and inverting terminals of the first amplifier is the same as the error between the noninverting and the inverting terminals of the second amplifier. Therefore:

VOUT = VIN + VERROR1 − VERROR2

or

VOUT = VIN
The original article is at elecdesign.com

This may useful to some of you.

Eric.
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Old 22nd January 2004, 05:38 PM   #2
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Interesting new idea. I believe the numbers, as we have also obtained THD in the order of 0.0001% with the different topology (in real circuit, not simulation).
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Old 22nd January 2004, 06:15 PM   #3
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Default Ooops.......

The circuit.........
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Old 22nd January 2004, 06:40 PM   #4
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Default Re: Ooops.......

Quote:
Originally posted by mrfeedback
The circuit.........

Yes, active feedback was pioneered by Jerry Graeme of Burr-Brown, late 70's IIRC. When opamps started to be so cheap, this became a easy upgrade.

Jan Didden
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Old 22nd January 2004, 07:03 PM   #5
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Default Re: Re: Ooops.......

Quote:
Originally posted by janneman



Yes, active feedback was pioneered by Jerry Graeme of Burr-Brown, late 70's IIRC. When opamps started to be so cheap, this became a easy upgrade.

Jan Didden
I suppose you are only referring to active feedback using op amps,
since otherwise I can once again refer to a certain book from 1968,
which I have happened to mention a couple of times recently, and
where active feedback loops with BJTs are presented. In fact, they
seem to be considered so obvious that thay mostly just occur
among the exercises.
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Old 22nd January 2004, 07:11 PM   #6
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Default The last to know...........

Mr. Jung has been doing this for over a decade I believe and there are of number commercial products using this approach including some mid-fi lines like Adcom and PS Audio. Poor Mr. Bateman doesn't get out much, does he. Using 20 and 30 year old op amps as examples is cutting edge as well. Heating from output stage affects the input diff pair ( on single op amps too) and the effect has been mentioned in some of Analog Devices ap notes. Techniques used in designing fast pnp transistors in op amps can actually make this problem worse for some of the newer op amps. Most op amps are designed to drive back terminated coax cable now so a lot of attention is paid to this in the newer op amp designs. There are other distortion mechanisms as well and biasing op amp output stages with a current source is old hat as well. Am I the only one who reads the Audio Amature (audioeXpress) and op amp data sheets and ap notes? This is old news.
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Old 23rd January 2004, 01:06 AM   #7
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Default Hi Fred.......

This testing is an extension of Cyril Batemans 'Capacitor Distortion' series and is thrown in the last article in the series.
Yes, I have seen this technique previously but this is the first time that I have seen distortion figures for commonly used and cheap 'industry standard' op-amps in this configuration.
He does mention much more modern op-amps and gives performance figures also.
I know this is old hat to pros, but may not be to DIYers, hence posting this kind of info here for the possible benefit of all.

Eric.
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Old 29th January 2004, 01:06 AM   #8
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I designed an amp six years ago called the Lifeforce which used this principle.

It's standout was brilliant imaging, and wonderful musical intimacy.

My implementation, not entirely original just as Fred says, was quite complex, but it sure worked well.

While there is distortion cancellation due to the conjugate nature of the inputs at the inverting and non-inverting nodes (a variation on feedforward), when used with global negative feedback the lower order distortion is cancelled more effectively than the higher orders; not really the optimum.

This mimics the problem of eliminating crossover distortion; the high order artefacts are just too fast.

Cheers,

Hugh
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Old 29th January 2004, 08:55 AM   #9
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Default It's All In The Spectrum......

Hi Hugh,
Thankyou for relating your observations.
Yes I had thought about the effectiveness (more so the maybe lack there of) of higher order harmonics cancellation, but have not tried this circuit arrangement for myself yet.

Of course it is a p.i.t.y. that typical blanket distortion figures do not reveal the spectral nature of the distortions.
It is also a big p.i.t.a. that much lowered but skewed harmonic characteristics can be subjectively sonically worse than the original condition !!!!!!

Regards, Eric.
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Old 8th January 2009, 04:16 PM   #10
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Anyone tried this for opamp buffers?
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