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Old 22nd June 2013, 11:43 PM   #40551
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Originally Posted by john curl View Post
... there remains a critical 'weak link' audio reproduction. It is not necessarily digital, itself, BUT the interface between converting the output of a first class D-A, to a final audio signal that would go either to a quality preamp or a first class power amp.
This is usually done with IC's, and pretty good IC's at that. However, problems remain.
First, the IC's are often loaded with a difficult load, due to gain reduction requirements.
This makes the IC's put into a less than optimum situation, and I think that this is a real problem. I hope to research this in future.
You will probably be surprised to hear that fully complementary, j-fet input, open loop OTA's have been used in that place some 20 years ago
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Old 22nd June 2013, 11:59 PM   #40552
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Yes, Rick. They could be the 'KEY' to better sound, with the best digital playback, in my opinion.

Last edited by john curl; 23rd June 2013 at 12:04 AM.
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Old 23rd June 2013, 12:01 AM   #40553
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I'm not surprised, but the 'latest stuff available to me has 'loaded' IC's. It may well be that 20 years ago, digital was still the weak link, today I suspect it is not so.
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Old 23rd June 2013, 12:38 AM   #40554
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I think Barrie Gilbert remarked that opamps are about the worst things to use for I-V conversion with the ill-named current-output DACs. But although I saw the remark, I don't recall that he suggested an alternative.
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Old 23rd June 2013, 12:44 AM   #40555
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In the 2/92 issue of The Audio Amateur, Walt Jung shows on pages 16 and 17 how good the AD811 is for an I/V converter. And I know John will like this, the 811 has on open loop bandwidth of 21Khz.

I feel that the 811 is an excellent IC for an I/V stage. I like to use some LC filtering in front of it to lower the RF going into it from the DAC chip.
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Old 23rd June 2013, 12:59 AM   #40556
bcarso is offline bcarso  United States
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Originally Posted by Rick Miller View Post
In the 2/92 issue of The Audio Amateur, Walt Jung shows on pages 16 and 17 how good the AD811 is for an I/V converter. And I know John will like this, the 811 has on open loop bandwidth of 21Khz.

I feel that the 811 is an excellent IC for an I/V stage. I like to use some LC filtering in front of it to lower the RF going into it from the DAC chip.
I think you are on the right track with a current-feedback opamp, as the inherent inverting input impedance is so low to begin with, and feedback enhances it. The DACs that are misleadingly described as "current output" I've remarked elsewhere should be called "DACs whose outputs should be terminated in a very low impedance". It is unfortunate that few manufacturers reveal what their output impedance really is, and in particular how much it may be code-dependent.

Brad
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Old 23rd June 2013, 01:07 AM   #40557
jcx is offline jcx  United States
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Quote:
In the 2/92 issue of The Audio Amateur, Walt Jung shows on pages 16 and 17 how good the AD811 is for an I/V converter.
yea no progress in op op amps in 20 years, no sir, never happened...

nor would anyone think to put a feedback cap across the I/V R - making the low Z of the output dominant at high frequency with VFA I/V - or write up the advantages say back when AD was still building DACs in potted boxes...

Last edited by jcx; 23rd June 2013 at 01:12 AM.
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Old 23rd June 2013, 01:15 AM   #40558
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Rick, thanks for your input. I agree that your 811 is probably one of the BEST op amps for I to V conversion. It is designed for lower impedance loading, which is necessary.
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Old 23rd June 2013, 02:17 AM   #40559
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Thanks John. On page 17 Walt shows that the dynamic impedance of the AD811 as an I/V is .85 ohms.
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Old 23rd June 2013, 02:38 AM   #40560
bcarso is offline bcarso  United States
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Originally Posted by jcx View Post
yea no progress in op op amps in 20 years, no sir, never happened...

nor would anyone think to put a feedback cap across the I/V R - making the low Z of the output dominant at high frequency with VFA I/V - or write up the advantages say back when AD was still building DACs in potted boxes...
It's a bit more complicated than that.

To start with a low impedance open-loop, and then enhance that with feedback, is an attractive approach. For some reason a great many people are not thoughtful about this, and continue to apply voltage-feedback opamps to this particular task. Indeed, virtually all of the app notes I've seen are still using voltage feedback amps. Yes, the results are acceptable, just suboptimal.
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