PCM1791A - what OpAmp's? - diyAudio
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Old 13th March 2010, 12:17 AM   #1
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Default PCM1791A - what OpAmp's?

I have those DAC's in my receiver and in my DVD-player (Denon both).
Those are followed by some LM833 (in the receiver) or BA15218(5.1)/NJM2068(stereo) in the DVD-player...
So I want to swap them with something else.
Starting point: LM833 has 4 nV/sqrt Hz, 7V/uS.
My pick:
1. LT1028 (0.85 nV/sqrt Hz, 11V/uS)
2. LM 4562 (2.7 nV/sqrt Hz, 20V/uS)

Now, I know that the I/V stage is located inside the 1791 DAC, and it might not matter too much but I have the itch to do it.
TI recomens for this DAC the OpAmps: OPA2134 (8 nV/sqrt Hz, 20V/uS) or 5532 (5 nV/sqrt Hz, 9V/uS).

Any toughts about this? Did anybody experiment with changing OpAmps on voltage-output DAC's? It is worth the trouble (chips are SMD)?

Last edited by SoNic_real_one; 13th March 2010 at 12:20 AM.
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Old 13th March 2010, 12:26 AM   #2
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Well, with those cmos opamps inside the 1791 already , I just wouldnt worry.
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Old 13th March 2010, 12:36 AM   #3
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Now... the DAC's are not sounding that bad. I was hoping to get more from them.
But, I guess, I either belive you and leave them as they are or... work my way in something that I might regret.
To build another DAC section altogether is temptig but obvious more expensive and maybe imposible (wonder if there are other PCM direct compatible with PCM1791A in SW mode?).

Last edited by SoNic_real_one; 13th March 2010 at 12:54 AM.
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Old 13th March 2010, 12:54 AM   #4
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There are plenty underrated DA chips outthere, but I guess the 1791 just isnt one. BTW I can't be bothered to mod this pcm1704 thing with 5532 IV, Im goin to sell the whole thing. I for one beleive 1704 is way overrated.
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Old 13th March 2010, 12:50 PM   #5
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The OpAmps that I have there are used for the third-order filter and conversion from balanced to single-end, so I think that some improvement can be made there.
Also in the receiver I have oane more OpAmp for front channels (NJM5532) that is used for the +6dB boost needed for HDCD decoding. I think that can make a difference too.
Am I wrong?
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Old 13th March 2010, 02:17 PM   #6
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That hdcd dnr extension wont trigger, ever.
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Old 13th March 2010, 06:48 PM   #7
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It is not for the extension. The HDCD normally has to be 6dB amplified after decoding to sound at the same audio level as when you play on the CD undecoded. It is switched "ON" all the time while HDCD signal is on.

I did the swap. Tryed both of the above. SOIC8 cips.
LT1028 runs HOT. I don't think it oscillate since the sound was ok, but, did't care to measure for real. Running to hot for my taste - and for the analog power supply I guess. And the pain with the single-dual converter... Too much!
LM4562 was warmer to touch than the original but in an OK range. Sounds great - I did swap the filter (and the +6dB) on the front and surround and compare with the back channels via stereo mode (as much as I can). Tested with SACD and HDCD - To my disbelief, I can hear a slight improvement on the high-end, clarity even. Nothing on the noise - it was "silent" from the start anyway and the difference between 2.7 and 4 nV/sqHz is anyway small. Maybe if I turn the volume to the max... but I can't do that easily with the Denon AVR 3805 in my livingroom.

All in all, three OpAmps swaped latter - I am happy. Thanks to National Semi, Linear and their free samples

Last edited by SoNic_real_one; 13th March 2010 at 07:18 PM.
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Old 13th March 2010, 07:34 PM   #8
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Why would they throw a whole bit away ? 0.1dB is lost for the DF function. And 6dB is the transient boost that never happened, neither the slow roll-off filter-swap.
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Old 13th March 2010, 10:30 PM   #9
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They don't throw away nothing. Listening to a HDCD via a regular CD player will produce a compressed analog audio that will be percieved as "louder" than the case that is run via a decoder - the decoder un-compresses (expands) the signal, so it will be perceived as not so loud.
To compensate that, HDCD license requires that the decoder has a mechanism to amplify the analog audio by 6dB so it will pe percieved as having the same loudness as previous case.
I think this was done initially so the people don't perceive HDCD decoders as weak and maybe associate them with low quality.

Last edited by SoNic_real_one; 13th March 2010 at 10:35 PM.
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Old 13th March 2010, 10:42 PM   #10
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Quote:
Originally Posted by SoNic_real_one View Post
They don't throw away nothing. Listening to a HDCD via a regular CD player will produce a compressed analog audio that will be percieved as "louder" than the case that is run via a decoder - the decoder un-compresses (expands) the signal, so it will be perceived as not so loud.
To compensate that, HDCD license requires that the decoder has a mechanism to amplify the analog audio by 6dB so it will pe percieved as having the same loudness as previous case.
I think this was done initially so the people don't perceive HDCD decoders as weak and maybe associate them with low quality.
thats what Im talkin about , this is garbage, never worked in practice, simply because record producers never embedded those transient flags in the first place.

Last edited by tritosine; 13th March 2010 at 10:45 PM.
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