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Old 30th April 2004, 11:58 AM   #11
Alien8 is offline Alien8  Belgium
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Hmm, seems I was off a bit.

I was judging that based on the scope pics from the tnt site, I don't know the datasheet so I really couldn't say.

Thanks for correcting me John !
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Old 30th April 2004, 02:57 PM   #12
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Hi John

What do you mean by "some form of differential summer"? Is there a way to do this using only passive parts?

Quan
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Old 30th April 2004, 03:03 PM   #13
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Quote:
Originally posted by quantran
Hi John

What do you mean by "some form of differential summer"? Is there a way to do this using only passive parts?

Quan
you have to know the impedance -- but an RC filter will be of some help.

you might want to take a look at the FDNR (frequency dependent negative resistor) threads -- or the spec sheet for the AD713, or Burr-Brown (Texas Instruments) spec sheets on low distortion, low noise anti-aliasing filters.
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Old 30th April 2004, 03:47 PM   #14
JohnW is offline JohnW  Hong Kong
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The only way I know of to differentially sum with Passive components, is to use a transformer, but IMHO these are worst then opamps.

You could increase the order of the passive filter between the DAC outputs and the differential sum Opamp, there by reducing the “RF levels” seen by the Opamp input stage, then take the audio output directly from this Opamp - so there’s only one Opamp in the signal path.

I have always found the AD711 to be a well-behaved Opamp in the presents of RF.

See Cirrus Logic App Note AN48 for 2 Pole filter with differential input.

John
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Old 1st May 2004, 01:35 AM   #15
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Thank you all for your valuable advices. It seems that one opamp on the signal path is unavoidable.

I've looked at the BB application notes for FDNR. It looks promissing and not too complicated for me to build. I have one spare pair of OPA2604. I will try it.

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Old 2nd January 2005, 03:28 PM   #16
klitgt is offline klitgt  Denmark
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I would like to try to take the signal directly from diff. summing IC512 at pin 6 before R543 and leave out the LPF (IC513) and output buffer (IC514) completely in my Denon DCD3560.
But to avoid the worst HF I want to make a passive LowPassFilter. Can I do that just by replacing R544 with a cap? And which value should it be?
I plan to use OPA627 for IC511 and IC512. Any points of view on this project?
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