PCM1704 Vref & ServoDC, 12 years and still a secret? - Page 3 - diyAudio
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Old 9th February 2012, 10:54 AM   #21
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Hmmm. I tend to stay away from curent feedback Opamps...
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Old 9th February 2012, 12:26 PM   #22
regal is offline regal  United States
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Originally Posted by SoNic_real_one View Post
Hmmm. I tend to stay away from curent feedback Opamps...
oscillation?
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Old 9th February 2012, 12:30 PM   #23
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Yes. I found out that is easyer (for me) to control a fast voltage feedback OpAmp.
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Old 9th February 2012, 01:21 PM   #24
regal is offline regal  United States
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Yes. I found out that is easyer (for me) to control a fast voltage feedback OpAmp.
thats my fear as well, I have zero experience with them
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Old 9th February 2012, 02:16 PM   #25
von Ah is offline von Ah  United States
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Originally Posted by SoNic_real_one View Post
Even if that would be the 6-th DAC in my house...
Well, which ever one you like the least I could easily dispose of it for you.
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Old 9th February 2012, 03:19 PM   #26
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Hey, it's a cheaper obsession than collecting cars
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Old 9th February 2012, 07:57 PM   #27
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There are few areas that need attention with CFB amplifiers. Keeping them stable is not easy... I suppose some experience with RF designs could help. Apart from outstanding sound in I/V application, they are quite educational in a way that everything has to be almost perfect to get them perform adequately.... and that is a challenge. Latter, VFB amplifier can be dropped in if that is preferable; they will also benefit from optimised “surrounding environment”... but, once you try CFB you will not go back to VFB... and very son you will realise that various OP amplifier will not be considered any more as devices that will colour the sound, but as a silicon gain stages…

My suggestion is to start with LME(can’t remember the number…) CFB Op, then move to 844 and finish with 811. Each step forward will require additional effort in circuit design optimisation to keep them stable. If you can get 811 to run stable -> you’ll know you’ve done (a fair few) things right.

Boky
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Old 10th February 2012, 03:08 PM   #28
regal is offline regal  United States
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Thanks for the info.

On a side note I guess some have taken offense with my comments on the first post that says: "Folks have resorted to high distortion high bjt count open loop transimpedance I/V that really never caught on."

Now About the best you can do with these circuits is -80db distortion from what I can tell the first BJT is -90db , maybe you can cancel some even harmonics subsequently. But I run a tube headamp after the DAC (which is about -70 db at my listening levels), Just seems that starting at -80db isn't ideal for my setup.

I guess in the past consumers didn't "like" the sound of CFB opamps in thier DAC's, but for feeding probably the most linear singleended feedback free single active device amplifier possible they may prove to be a good match.

Everyone has a different situation, I try to keep that in mind.


Also it looks like I have a line on a JT no3 PCB
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Old 10th February 2012, 07:40 PM   #29
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The PCM1704 has polarity invert/non-invert built-in, so balanced operation is very easy. I actually think the 1702/1704 has more sonic potential in parallel than in differential. Theoretically should be the opposite, but my ears say parallel is better.
I think that theory actually supports your ears in this case. Two independent DAC outputs, after conversion to voltage, will produce a net 3dB improvement in SNR whether configured differentially (serially) or in parallel. In the differential config., the signal will be doubled (+6dB), but the noise will also increase by 3dB for a net change of +3dB. In the parallel config. the signal remains the same (0dB), but the noise is reduced by -3dB giving a net SNR change of +3dB.

Okay, so then, how might theory support the difference you hear? I'd argue that it has to do with the way harmonic distortion is handled in each configuration. Differentially, distortion will be correlated with itself, much the way the desired signal voltage having a 100% positive correlation with itself produces a doubling in signal amplitude. With harmonic distortion, however, the even order products will have a negative correlation and tend to cancel each other. A fact which helped popularize push-pull and symmetrical amplifier topologies. The odd order components wil have some degree of positive correlation and will tend to reinforce each other.

In the parallel DAC configuration, although distortion is correlated, the even order harmonics are not cancelled but neither are the odd order harmonics reinforced. Remember how the signal voltage itself does not increase in this configuration? So, I'll speculate that you are experiencing an affinity for even order distortion products over odd order.

Certainly, however, there are many other system variables which could have accounted for your listening preference, depending on whether those variables were kept constant in your comparative evaluations.
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Last edited by Ken Newton; 10th February 2012 at 07:45 PM.
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Old 10th February 2012, 08:49 PM   #30
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I don't understand why the "even order" would cancel from one DAC with the other. They are still in phase one with the other (from DAC to DAC), no?
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