The pcm1794a datasheet I/V converter and how to improve it. - Page 12 - diyAudio
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Old 1st September 2012, 04:18 PM   #111
smms73 is offline smms73  Portugal
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The differential output cancel mainly the 2° harmonic .
And that is the benign distortion of them all .
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Old 2nd September 2012, 12:36 AM   #112
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It seems there more than one route to ascend to audiophile heaven .

My solution is to build the two IV converters, and decide for my self.
I'm committed to the transformer route, since I just received my two Lundahl LL1517's yesterday. I don't know what else I'd do with them if this doesn't sound good (like I can tell... haha).

I just need to get my test gear together so I can look at the hidden dragons way down there in the -130dB range.

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Old 2nd September 2012, 02:09 AM   #113
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dream on ...
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Old 2nd September 2012, 09:34 AM   #114
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Did anybody notice that the output current of the PCM179x devices are specified as negative?Since they call it output current,a negative value would go into the pin, but i do not believe it's that way.The current comes out of the pins if it's true that the voltage after the first opamps is always negative.
Am I missing something here or what?
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Old 2nd September 2012, 10:52 AM   #115
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Yes, the voltage in the output of U1 and U2 are always negative. You have a table of the output voltages of the opamps in page 25 of datasheet.
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Old 2nd September 2012, 11:05 AM   #116
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I'm committed to the transformer route
The transformer produce mainly 3° harmonic distortion and the higher the frequency the lowest the distortion.
I think that with a transformer you will have lower noise and 2° harmonic but a little bit more of the third.
The best is for you to bild it and let us know your results.

I have a emu1212m sound card, what are the sound card that you are thinking to buy ?
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Old 2nd September 2012, 11:21 AM   #117
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Is the LME49990 ok with you? It's really awesome on paper.
The 49990 its a very good opamp but with a GBW of 110 Mhz is not a good option for using a buffer at unity gain.
But I have to measure the phase of this buffers, in the simulator it seems very good. I am thinking using the OP2134 (gbw 8Mhz) because I have a lot of them.
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Old 2nd September 2012, 11:21 AM   #118
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PCM179x runs with positive single supply.
so their Iout pins would be open drain of Pch mosfet.
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Old 2nd September 2012, 12:42 PM   #119
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Quote:
Originally Posted by smms73 View Post
The transformer produce mainly 3° harmonic distortion and the higher the frequency the lowest the distortion.
I think that with a transformer you will have lower noise and 2° harmonic but a little bit more of the third.
The best is for you to bild it and let us know your results.

I have a emu1212m sound card, what are the sound card that you are thinking to buy ?
Since I have a laptop, I only have USB or Firewire digital connections to external devices. So, I need an external "sound card". I've decided that I'm going to get TI PCM4222EVM evaluation board, which is super duper ADC. The board has a serial connection that can be configured for I2S, so I bought a miniStreamer USB bidirectional 12S <-> USB interface. So, I have the ADC side taken care of once I build the box for the board and USB interface. The PCM4222EVM has been used a lot by musicians on message boards like GearSlutz and GroupDIY. I'm also building Pete Millett's sound card interface.

On the source side, right now I have one of Victor's super low distortion 1kHz oscillators (too many zeros to count).

Otherwise, right now I have a simple Behringer UCA202 ADC/DAC with is pretty good. I use TrueAudio's TrueRTA (1/24th octave real time analyzer). I also have other spectrum analyzer software. With an Express Card USB 3.0 and the Behringer, the loop back shows noise at about -110dB if I'm lucky. I'd like to make it to -130dB, with this new gear. I think I may run the ADC on battery power. Someone posted their results with the PCM4222 here on DIYAudio and the noise floor was extremely low. We shall see, fingers and toes are crossed.

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Old 2nd September 2012, 12:44 PM   #120
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Originally Posted by smms73 View Post
The 49990 its a very good opamp but with a GBW of 110 Mhz is not a good option for using a buffer at unity gain.
But I have to measure the phase of this buffers, in the simulator it seems very good. I am thinking using the OP2134 (gbw 8Mhz) because I have a lot of them.
OK, then I should just run the simulator with the boring old NE5534... haha.
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