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.
The original article is at elecdesign.com
This may useful to some of you.
Eric.
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.
The original article is at elecdesign.com
This may useful to some of you.
Eric.
Re: Re: Ooops.......
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.
janneman said:
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.
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.
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.
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.
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.
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
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
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.
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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