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Old 23rd December 2011, 02:03 PM   #31
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Originally Posted by SoNic_real_one View Post
I guess ALL the DAC manufacturers are stupid because they ALL recomend OpAmps as I/V (with virtually zero impendance on DAC outputs). Their engineers are smart enough to design a whole DAC, but they cannot figure that a resistor is a better choice for I/V? Or is a world-wide conspiration?

If resistor was sounding better, it would be in the datasheet. I guess the "do-it-on-the-knee" companies and users on the net are the true specialists...
Yes, and, minimizing the virtual ground impedance is the primary reason they recommend an op-amp based i/v. However, the practical benefit of a near zero impedance virtual ground depends upon the specific D/A converter utilized.

Let me ask you a simple thought question. Suppose the current output of some given converter is not audibly or measurable degraded (although it may be theoretically degraded) by a non-zero virtual ground of, let's arbitrarily say, 50 ohms? Which objective technical parameters of that 50 ohm passive resistor i/v would not be superior to those of any op-amp based i/v implementation? Please list such parameters, i.e., THD, IMD, noise, slewing limit, PSRR, bandwidth, settling time, phase margin, etc.. I'll wait...
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Old 23rd December 2011, 07:12 PM   #32
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Problem is... there is not such thing. Current sources inside DAC are derived from switched capacitors at low voltage. They are not ideal current sources. Not at the level of 20-22 bit resolution.

Also, resistor voltage noise can be higher that the one of the OpAmp. Settling, phase, slew and bandwith (they are all the same thing) can be worse due to capacitance following it (even buffers have some parasitic capacitances). Also, due to the fact that you don't have any proper filters following that I/V (because OpAmps are devil's work) or OS (because that is also complicated and it's just another conspiration) - the resulting DAC's trully suck in the THD department.

Do you really think that the engineers are morons (from all the Europe, USA and Japan) and didn't take ALL those aspects in consideration? That any DIY without any formal training know more than them?
The manufacturers would LOVE to sell a product that is cheaper to incorporate in designs, especially if it is better that the more expensive alternative. Any of the DAC manufacturers would at lest mention that as a posibility in their data sheets if it was even remotelly fesable.

The reality is that they DID TRY it in their labs, mesure them (with tools that probably cost as much as the house you are living in), listen to them (yes they are doing that too) and the results where less than the alternative that you see in data sheets.

Resistive I/V is just kids toy. Same like when I was kid and would make "radios" from a potato (variable capacitor) and germanium diode or shaving blade. Looks cool, but still a toy.

Last edited by SoNic_real_one; 23rd December 2011 at 07:17 PM.
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Old 23rd December 2011, 10:42 PM   #33
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I can tell you that at least in the Philips lab the red pencil was put to good use AFTER the engineers had done their job...
A long time ago I tried to find out some design details and talked to one of the last engineers from the CD design team and was astonished to hear that the bean counter dept cut so many good design ideas just because they would save some cents here and there.
Many design aspects were just implemented to satisfy consumer expectations, SQ was not the most important one.
Not arguing against what you are saying but what engineers know is best is def. not always what is sold in the end
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Old 23rd December 2011, 10:48 PM   #34
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Quote:
Originally Posted by SoNic_real_one View Post
Problem is... there is not such thing. Current sources inside DAC are derived from switched capacitors at low voltage. They are not ideal current sources. Not at the level of 20-22 bit resolution.

Also, resistor voltage noise can be higher that the one of the OpAmp. Settling, phase, slew and bandwith (they are all the same thing) can be worse due to capacitance following it (even buffers have some parasitic capacitances). Also, due to the fact that you don't have any proper filters following that I/V (because OpAmps are devil's work) or OS (because that is also complicated and it's just another conspiration) - the resulting DAC's trully suck in the THD department.

Do you really think that the engineers are morons (from all the Europe, USA and Japan) and didn't take ALL those aspects in consideration? That any DIY without any formal training know more than them?
The manufacturers would LOVE to sell a product that is cheaper to incorporate in designs, especially if it is better that the more expensive alternative. Any of the DAC manufacturers would at lest mention that as a posibility in their data sheets if it was even remotelly fesable.

The reality is that they DID TRY it in their labs, mesure them (with tools that probably cost as much as the house you are living in), listen to them (yes they are doing that too) and the results where less than the alternative that you see in data sheets.

Resistive I/V is just kids toy. Same like when I was kid and would make "radios" from a potato (variable capacitor) and germanium diode or shaving blade. Looks cool, but still a toy.
The op-amp i/v will never have lower noise simply becuase it has an resistor in it's feedback loop. Which means an op-amp based i/v will always have greater noise than an equivalent passive resistor i/v.

The issue, once again, isn't the technical competance of the engineers at T.I., or Analog Devices, or whomever. The issue is that all current type DAC chip outputs, theoretically perform better into a near zero impedance virtual ground. The simpliest way that I know to implement a minmal impedance virtual ground is by feedback amplificstion. The lowest cost high open loop gain feedback amplifers are op-amps. So, it's rather simple to understand why engineers of large microchip vendors recommend op-amp based i/v circuits. It seems rather clear that the primary design objectives for commodity (op-amp) based i/v circuit are a minimal virtual ground impedance, and lowest static THD.

In addition, you assume that those DAC vendor engineers have put much thought into the external i/v circuit. I can you tell you from my own 10 year experience at a major semiconductor vendor that such often wouldn't be the case. The engineers involved are mostly concerned with the performance and customer support of the device the company is trying to sell, and in getting that device out to market as quickly as possible. Those two realities very often don't lend themselves to great expenditures of thought and energy on optimizing external circuitry, particularly, over the subjective aspects of audio performance. This all has NOTHING to do with their technical competence, so, you can stop attempting to conflate technical competance with subjective listening results.
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Last edited by Ken Newton; 23rd December 2011 at 11:02 PM.
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Old 29th June 2013, 07:40 PM   #35
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I know it is such an old but brilliant and short thread, but I am really curious if any of this thread participants can add anything to the facts posted ? Have they got any extra facts, details, personal experience ?
It looks like it was mainly discussion of I/V stage and not the dac chips , and specificly opamp based vs passive one.
Pardon me if I am wrong, but it is worth getting back to.

Any comments ( or all been said) ?
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Old 30th June 2013, 01:33 AM   #36
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I haven't chipped in up until now, but here's my 2c.

I used to take the view, rather like SoNic that THD and SNR told us pretty much all we needed to know about a DAC's performance. If you come at DAC design with this assumption then opamps and presenting the lowest impedance to an I-out DAC are uppermost in your mind.

After I began playing with DACs and listening to how they sound I did find there was a difference between the various types. And when I played around with opamps in I/V I found there was a definite improvement (more clarity, better soundstage) in having a faster opamp and in a lower value of feedback capacitor. Neither changed the THD to a significant extent from what I recall.

On my listening journey I eventually abandoned opamps and S-D DACs altogether and nowadays I only design with passive I/V and multibit DACs as subjectively I very much prefer the sound. Use of passive I/V allows a steep passive filter to be employed between the DAC and subsequent amplifier stage. In the past few months I've been engrossed in passive filter design and inductor selection.

My hypothesis for why passive I/V and passive filtering delivers tbe best sound is that, rather than THD, its IMD we need to minimize for the best subjective effect. IMD measured with a very large number (>100) of tones, not the traditional 2 tone measures.
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Old 30th June 2013, 05:59 AM   #37
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Do you have any examples of good implementation of passive one ?
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Old 30th June 2013, 10:07 AM   #38
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My blog would be a place to start - I have some passive filters shown on there, also one of the chips I've tried as post-amplifier which is a little unusual, an AD603. This part comes with its own volume control which sounds transparent to my ears.
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Old 30th June 2013, 10:32 AM   #39
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Will have a look
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