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Old 5th July 2008, 10:38 PM   #1
ctong is offline ctong  United States
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Default Connecting Pass D1 analog stage to a differential DAC

Are the Iout+ of the DAC supposed to be connected to the in+ of the D1 IV stage? In many players with opamps for IV conversion the signal is inverted, that is, when the current from Iout+ of a DAC increases the output voltage of the IV stage decreases. Even the demo boards of DACs are connected this way. In the D1 stage the output voltage is in phase with Iout. So I am wondering whether In+ should be connected to Iout-. Has anyone encountered this issue?
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Old 5th July 2008, 11:29 PM   #2
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The D1 I/V stage does not invert the signal.
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Old 6th July 2008, 12:30 AM   #3
ctong is offline ctong  United States
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Quote:
Originally posted by Nelson Pass
The D1 I/V stage does not invert the signal.
Thanks Nelson. So should the DAC's Iout+ be connected to the In+ of the D1 I/V stage?
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Old 6th July 2008, 01:29 AM   #4
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Quote:
Originally posted by ctong


Thanks Nelson. So should the DAC's Iout+ be connected to the In+ of the D1 I/V stage?

Yes, match the polarity.

-David
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Old 6th July 2008, 02:40 AM   #5
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Quote:
Originally posted by dw8083



Yes, match the polarity.

-David

That means the DAC manufactures suggest inverting the polarity. I don't know what is going on.
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Old 6th July 2008, 02:49 AM   #6
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The DAC manufacturer probably assumes that you will use a
conventional I/V conversion by op amp with an inverting summing
junction.
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Old 6th July 2008, 03:19 AM   #7
ctong is offline ctong  United States
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Quote:
Originally posted by Nelson Pass
The DAC manufacturer probably assumes that you will use a
conventional I/V conversion by op amp with an inverting summing
junction.
I checked the datasheets for pcm1738 and pcm1792a and the summing junctions suggested by TI are non-inverting. So as a whole the analog section is inverting.

I connected the D1 I/V stage that I just finished (without the follower buffer) to the Iout- of my Denon 2900. It sounds fine and I cannot tell it is inverting. I will try to connect it to Iout+ and see whether I can tell the difference. Is there a audible difference for the two polarities?
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Old 6th July 2008, 05:12 PM   #8
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Whenever I've flipped the inverting switch on a DAC, I have never been able to hear the difference. This is true for several DAC's in my listening room. The manual for my ADCOM GDA700 says something like (paraphrase): You may or most likely may not hear a difference, the reasons are esoteric and beyond the scope of this manual.

-David
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Old 6th July 2008, 06:23 PM   #9
ctong is offline ctong  United States
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Quote:
Originally posted by dw8083
Whenever I've flipped the inverting switch on a DAC, I have never been able to hear the difference. This is true for several DAC's in my listening room. The manual for my ADCOM GDA700 says something like (paraphrase): You may or most likely may not hear a difference, the reasons are esoteric and beyond the scope of this manual.

-David
I guess I probably won't hear any differences either. But I still find it odd for a manufacturer to recommend an inverting analog stage.
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Old 6th July 2008, 07:21 PM   #10
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Quote:
Originally posted by ctong
I checked the datasheets for pcm1738 and pcm1792a and the summing junctions suggested by TI are non-inverting. So as a whole the analog section is inverting.
No, they are inverting. What you see in their circuit is that two
outputs for each channel go to inverting I/V converters and then
are differentially amplified with phase inversion. Two inversions
make for non-inversion, so the polarity at the output of the DAC
chip reflects the actual phase. If you run it into a D1 I/V, then that's
the phase you get at the output.
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