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Old 14th July 2005, 09:14 AM   #11
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A bit more explaining: the "rotel" way of I/V is NOT a proper opamp IV, it is a half way between passive IV / opamp output IV. Tricky.
And I did not change the NE5534 in there, because in that special application is not totally out of place, and maybe - partially - was responsible for the ROTEL's original sound character?
Strange, did You have originally NE5532 -s in place? I have there BB opamps.

The Pass D1 solution is good for single ended, as well.

Ciao, George
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Old 14th July 2005, 09:55 AM   #12
Gasho is offline Gasho  Croatia
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You say I/V is something between passive & active. That was my opinion too , but I was not shure.

Yes there was NE5534 . I put AD847 latter. It have more punchy bass, better resolution and smoother treble.
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Old 14th July 2005, 10:01 AM   #13
Gasho is offline Gasho  Croatia
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I will try with passive , and listen. Gain stage will be made with same OP for unbalanced output , and AD828(double) for balanced output. Between gain stage and I/V stage I will put coupling cap to prevent any DC from I/V to OP-s . In that way I can use smaller capacity and higher grade cap , like AN paper/cooper in oil.
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Old 15th July 2005, 03:09 AM   #14
wa2ise is offline wa2ise  United States
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I see in your diagram Click the image to open in full size.
that the manufacturer already has nearly a hundred ohm resistance from the PCM63's Iout pin 6 before it gets to a virtual ground. Resistors 301 and 303, and 302 and 304 in the other channel. This tells me that using a resistor for I/V conversion with this DAC chip is valid and should work just fine, with no distortion. Presumidly the designers of that manufacturer had access to more info from BB/TI than just the data sheet. And worked well enough when they did pre production testing. The caps 301, 303, 302 and 304 would act to divert ultrasonic currents to ground. Acting as low pass filters.

Thus you could use a hundred ohm resistor to ground for the I/V conversion, and then feed the resulting voltage into your favorite pre-amp circuit, be it transistor, op-amp, or tube to get it up to line level to feed to your audio amp.
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Old 15th July 2005, 05:04 AM   #15
Gasho is offline Gasho  Croatia
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Yes , but for passive I/V conversion , pin 6 on DAC chip must be grounded also.
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Old 15th July 2005, 10:59 AM   #16
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Quote:
Originally posted by Gasho
Yes , but for passive I/V conversion , pin 6 on DAC chip must be grounded also.

Not a good idea.
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Old 15th July 2005, 11:21 AM   #17
Gasho is offline Gasho  Croatia
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Ok, than connected od pin 6.
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Old 16th July 2005, 05:28 AM   #18
wa2ise is offline wa2ise  United States
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Quote:
Ok, than connected od pin 6.
Could you explain a little more, I'm not getting your thought that well on this.
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Old 16th July 2005, 09:40 AM   #19
Franz G is offline Franz G  Switzerland
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I am planning my own DAC, based on CS8412, PMD100 and TDA1541A and a PCL82 outputstage.

Actually I am thinking about the I/V conversion and the Rotel schematics seems to be very interesting, in the matter of the conversion AND the gain switching between CD- and HDCD mode.

I can recognize the I/V conversion in this schematics, built by R301/303 (Left channel). Together with C301/303 they form a LPF, imho.

But, I need some help, to completely understand the feedback path:

What is the gain, applied to the NE5534?

In HDCD-mode, the feedback path is quite simple: R321 1.3K in series with R307 3.3K, resulting in a gain of 47 in relation to the input-R.

But, what happens in CD mode? The feedback path is more than 1Meg in this case???

And what is the reason for C's in the feedback path in CD-mode of this opamp?

Every explanation is welcome!

Franz

P.S.
I plan to use OPA627 for this circuit.
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Old 16th July 2005, 10:35 AM   #20
Bricolo is offline Bricolo  France
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Do you have a source for PMD100?
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Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
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