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Old 12th July 2008, 12:54 PM   #21
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Hello again Redrabbit, that stage is certainly novel and seems to be an unusual configuration of a differential output stage employing cross coupled feedback loops. At first glance you tend to think uh oh, the load is being driven with in phase signals instead of antiphase. Closer examination reveals that the lower stage takes its feedback signal from the upper stage output, which is inverted. Similar remarks apply to the "inner" feedback loops. Nothing jumps out at me as being wrong; just different. These stages are notorious for their general instability and require extremely close matching of resistor values. This is best done by laser trimming on the chip as in the single chip products by AD, Burr Brown, THAT etc.

I think you are likely to get some good advice on Wayne's forum. I became his 100th member the other day!

Cheers Keith
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Old 12th July 2008, 02:00 PM   #22
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Thanks Keith for some insight on the above OP471 circuit.
The pin numbers are wrong though, 12 & 13 are reversed.

=RR=
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Old 12th July 2008, 08:11 PM   #23
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I think that there's a second error in the ADI drawing in addition to the pin numbers of the top op amp.

The + and - symbols are also wrong on the bottom op amp. The pin numbers are correct, it just shows the inverting an non-inverting symbols wrong.

If you compare it to the SSM2142 CCOS circuit, drawn correctly below, you'll see it:

Click the image to open in full size.
Image Courtesy of Analog Devices, SSM2142 Data Sheet


Here's the original Thomas Hay 1980 AES figure:

Click the image to open in full size.
Image Courtesy AES, Preprint 1723

BTW Thanks Keith.
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Old 12th July 2008, 11:43 PM   #24
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I tried building this 2 years ago, and it never worked.....at a time when I knew less than now (which is will be more, in a few years, as long as they don't cut the power grid. )
I think Wayne is correct......one sign is that the inner 10k feedback resistors would/should be on the negative input side of the lower opamp...???

I planned to re-do this at some point, because I have several ceramic OP471's.
(or, the OP470 , a low-noise version)

=RR=
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Old 13th July 2008, 03:49 PM   #25
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FWIW the Thomas Hay cite is closer to the OP471 circuit (despite the error) than the SSM2142. The OP471 and Hay inputs use a non-inverting follower to buffer the input. The FB resistors reference to ground. Hay probably serves as a better model to fix the ADI error than the 2142 does.

It would seem, based on casual observation only, that the output balance in the corrected OP471 cite and the Hay version would not have the sensitivity to input source resistance that the SSM2142 does. Source impedance doesn't become part of the gain equation and balance like it does with the 2142.
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