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Old 13th June 2011, 07:12 AM   #21
gaetan8888 is offline gaetan8888  Canada
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Hello Thorsten

The OPA637 need a minimum gain of 5 but better with a gain of 10 to work good.

How can we use it in a IV amp ?

Thank

Bye

Gaetan

Last edited by gaetan8888; 13th June 2011 at 07:16 AM.
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Old 13th June 2011, 07:22 AM   #22
ThorstenL is offline ThorstenL  Germany
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Hi,

Quote:
Originally Posted by gaetan8888 View Post
The OPA637 need a minimum gain of 5 but better with a gain of 10 to work good.
That is not entirely correct. The OPA637 needs a NOISEGAIN of 5 (this is not neccesarily the same as the actual gain) and only at high freqiuencies.

Quote:
Originally Posted by gaetan8888 View Post
How do you manage to use it in IV amp ?
The usual way?

For a typical TDA1541 CD-Player from Philips/Marantz remove the 15-33pF compensation capacitor between pin 5 & 8 and connect 10nF (low inductance) from the inverting input to ground. The OPA637 will then work without any issues (as the noisegain is 5) and any additional problems caused by the very fast current steps from the DAC are mitigated.

While I derived this principle by my little old self, it was described before by Scott Wurcer in the Application notes for the AD797 somewhere, it was discussed several times here at DIYA, which is how I became of Scott's work on this.

Ciao T
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Old 13th June 2011, 07:23 AM   #23
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by gaetan8888 View Post
How can we use it in a IV amp ?
Use what I call a 'gain stealing' network. A series RC to ground from - input raises the loop gain only at high frequencies whilst keeping it at unity for low DC offset. I see T suggests 10nF but does not mention any value for R. You'll want to watch the settling behaviour though on your scope - its easy to get the R and C values wrong.
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Old 13th June 2011, 07:32 AM   #24
gaetan8888 is offline gaetan8888  Canada
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Hello Thorsten and abraxalito

Maby a simulation with LtSpice or Tina-Ti

Btw, is that higher value of cap for compensation will reduce the slew-rate of the 637 ?

You are both living in asia, I think, what time is it there ?

Here it's 3:30 am, I have to go to sleep.

Thank

Bye

Gaetan

Last edited by gaetan8888; 13th June 2011 at 07:35 AM.
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Old 13th June 2011, 07:37 AM   #25
abraxalito is offline abraxalito  United Kingdom
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3.30pm where I am.

You can try that LTSpice sim but in my experience the models aren't that close to reality, particularly in terms of dynamics. If you want good results you'll end up developing your own models and that takes longer than building and testing.

Nice job on the CD-77 btw Thorsten. I just wonder about one thing - the little labels on all but one of the ICs (not the TDA1541) in the Six Moons pic. Their purpose is?
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Old 13th June 2011, 07:54 AM   #26
gaetan8888 is offline gaetan8888  Canada
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Hello

And about the slew-rate of the 637 with the higher value of that compensation cap ?

Thank

Bye

Gaetan

Last edited by gaetan8888; 13th June 2011 at 07:57 AM.
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Old 13th June 2011, 08:03 AM   #27
abraxalito is offline abraxalito  United Kingdom
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The comp cap Thorsten is talking about is the one needed to make a 5534 stable in unity gain. OPA637 has no such facility to compensate it for unity gain. With the external cap he's suggesting you don't lose any of the 637's better slew rate.
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Old 13th June 2011, 08:17 AM   #28
gaetan8888 is offline gaetan8888  Canada
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Hello abraxalito and Thorsten

Since, in the Thorsten's example for those cd players, the opamp are use in inverting input, that 10nF cap from the inverting input to ground are just an input filtering to filter rf frequency ?

Thank

Bye

Gaetan

Last edited by gaetan8888; 13th June 2011 at 08:27 AM.
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Old 13th June 2011, 08:24 AM   #29
abraxalito is offline abraxalito  United Kingdom
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Perhaps its best for Thorsten to answer that one. Myself I shy away from putting naked capacitors on the output of DACs - that's why you see my suggestion has a series resistor too. With the series R, the HF noise gain is clearly defined - so the R can be chosen to be one-quarter the value of the feedback resistor. Then the idea is not RF filtering rather opamp stability.
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Old 13th June 2011, 08:48 AM   #30
Marcelinho is offline Marcelinho  Netherlands
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Here you can find a schema for a ad811 i/v. Resister R2 is the key here. (All in dutch though). All inspired on Walt Jung's work.

Toon onderwerp - Philips DAC-960 modificatie • forum.zelfbouwaudio.nl
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