opamp vs none opamp dacs??
Oct 26, 2009 at 8:45 PM Thread Starter Post #1 of 9

oldson

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i see that some higher end dacs dont use opamps.
what are the pro's and con's?
 
Oct 26, 2009 at 8:47 PM Post #2 of 9

dura

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I wonder as well; I was led to believe discrete ampstages sound better, but hearing good things about opamp DACs here like the Meijer StageDAC I'm starting to wonder.
 
Oct 26, 2009 at 8:55 PM Post #3 of 9

oldson

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Quote:

Originally Posted by dura /img/forum/go_quote.gif
I wonder as well; I was led to believe discrete ampstages sound better, but hearing good things about opamp DACs here like the Meijer StageDAC I'm starting to wonder.


stagedac is on my shortlist.
i understand that there may not be a definitive answer.
i was hoping for a deneral "in theory" answer.
 
Oct 26, 2009 at 9:17 PM Post #4 of 9

BobMcN

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There are different types of DAC's and different types of output stages.

NOS (non-oversampling) tend to do something simple, like just using a cap as an output stage. These can sound very smooth, but tend to get congested on complex music and don't necessarily have any kick to them.

Cheaper DAC's generally use op-amps, since they are inexpensive and easy to implement. The good news here is that if they put them in a jack, you can easily replace them with a better op-amp and get a dramatic improvement in sound.

There are many expensive DAC's that use op-amps also, but their output stage tends to be far more sophisticated and the sound shows it. I would not presume to replace op-amps in those since you are paying for their tailored sound.

In general op-amps can add a punch and dynamic to the sound that may not be there otherwise. They have more PRAT (pace, rhythm and timing)

Then there are DAC's that use discrete output stages, some using tubes and some not. These can be a mixed bag, but the expensive designs get that PRAT and the texture and the smoothness and all the good things that each of the other approaches bring to the table. But you have to pay for that level of engineering and expertise.

Enjoy,
Bob
 
Oct 28, 2009 at 5:05 AM Post #5 of 9

ford2

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And the most important point of all will you be able to hear a difference.
You need a very revealing system to start picking very slight differences,not to forget very revealing ears.
 
Oct 28, 2009 at 9:16 AM Post #6 of 9

bearmann

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You shouldn't forget that there are at least two different kinds of DAC chips. You can find DACs that have (differential) Voltage Output (like the popular PCM1793) and there are DAC chips with (differential) Current Output (PCM1796, PCM1794 and many more) .
The difference: With Current Output DACs you need directly after the DAC a so called "I/V Stage". In this section the different Current levels at the output of the DAC (which represent the audio signal in mA and not in V) have to be converted to equivalent voltage levels -> "I to V Conversion".
There are numerous different approaches on how to convert the current to voltage: active, passive, (active) discrete, (active) opamp based, ... this has a tremendous effect on the sound quality and is one of the reasons why it's futile to compare different DACs solely on what DAC chips they are using.
Most "cheap" DACs use opamps in the I/V stage which is perhaps not the best solution. (at this point my background knowledge ends... there are probably quite some DACs with very well built opamp based I/V stages)

In contrast, the (differential) Voltage Out DAC has some kind of I/V stage built in and you just need an opamp/whatever to amplify this signal. The design tends to be more simple and the use of opamps is very convenient.

That's at least how I understood it...
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best regards.
bearmann
 
Oct 28, 2009 at 12:09 PM Post #7 of 9
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There are a number of DACs around using OPAMPs where people experimented not only with rolling the OPAMPs, but replacing them with "discreet OPAMPs", or HDAMs as they are otherwise known. The result is almost always better SQ, though there are a few more expensive OPAMPs that come pretty damn close. The only high end DAC I've tried with OPAMPs in it is the Assemblage DAC 3, and it sounded rather like the OPAMPs it used, the OPA627, famous for having a rather warm and "tubey" sound. Certainly the OPAMPs I've tried all have a slightly different tonal flavour. I'm not sure how physically possible it is to have an opamp-based circuit that measures as flat as a high-end discreet DAC, nor if it can be as detailed.
 
Oct 28, 2009 at 2:33 PM Post #8 of 9

Pars

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Many of the discrete stages I have looked at, including the one I have built/used and the other that is in progress do not use feedback, so will measure worse than an opamp stage which (by definition of opamps) has to employ feedback (and usually lots of it). I think that the discrete stages sound much better than any of the opamps I have tried however.
 
Oct 29, 2009 at 2:52 AM Post #9 of 9

WesMiaw

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I'm a vendor, so feel free to take my words with a grain of salt.
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Some of the claims of discrete circuits over op-amps are just marketing. But designing a discrete circuit gives you more control over what is happening and so as a engineer you have more freedom to make it do what you want, how you want. There is a lot of difference between op-amps, if you just look at their measurements, so it's pretty obvious why swapping can make a difference.

Between current and voltage DAC chips, current output chips are preferred in higher-end implementations because it is much easier to control current than to control voltage. So the chip itself is likely to do much better. But in the end, you still need to convert to voltage, so this is sort of like the op-amp versus discrete trade-off I mentioned above. There are lots of ways an engineer can choose to implement the I/V conversion.

It's not fair to categorically say one implementation technology is better than another. Particularly because not everyone is looking for the same sound. You can achieve excellent results with any approach, but whatever you pick is going to have limitations of some sort.
 

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