PCM1794 vs. PCM1798 in the real world? - Page 2 - diyAudio
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Old 14th May 2007, 04:27 PM   #11
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Join Date: Oct 2006
Thanks for the reply Dougl, I wasn't knowing output of pcm1794 was constant current ouput.

I've read that bipo, jfet opamp was working differently with source impédances but if it's a constant current output ... I was thinking about the new opa827 for I/V with the pcm1794.
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Old 9th December 2016, 08:43 AM   #12
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folks, some badguy on the internet just swaped the 1798 and put the 1974 into their DACs, the mod's done. WoW, sounds too simple...
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Old 12th December 2016, 02:37 PM   #13
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I built an experimental DAC which utilized the PCM1794A with passive resistor I/V that then directly fed my power amp. My extensive experimentation with the board caused the PCM1794A to fail, so I replaced it with a PCM1798 out of curiosity. I did not notice any obvious subjective difference, but, of course, A/B comparison wasn't possible. Objectively, I had measured 0.022% THD with the PCM1794A via 75 ohm resistor passive I/V, but have not got around the performing a distorion test with the PCM1798. According to their data sheets, the differences between the two devices are digital filter stop-band supression level, output current which is approximately twice the magnitude for the 1794A, and a 3dB better SNR for 1794A. Otherwise, they are pin-compatible.

My suspicion is that the 1794A internally contains two converters per channel to deliver that 3dB improvement in SNR, and so, consequently features twice the output current of the 1798. Having a greater output current magnitude can be a benefit or a problem, depending on the I/V circuit employed. With passive resistor I/V, higher output current magnitude enables lower I/V stage output impedance, which can be a benefit if the I/V then directly drives a transformer, for example. However, greater output current magnitude may place significantly greater demands on a active virtual-ground type of transconductance I/V circuit, which has to handle the output current magnitude. The lower the signal magnitude the more linear will be most active circuits.

In short, I've been equally happy with the real world sound of the PCM1798 as I was the PCM1794A. If device cost difference is a significant issue, I think you will be satisfied with the 1798. If you plan tomdo such a swap, don't forget to scale the I/V circuit gain, which is usually set by a single resistor per channel, no matter whether the circuit is passive or active.
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Last edited by Ken Newton; 12th December 2016 at 03:06 PM.
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