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Is a preamp needed?

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#42
That is exactly what I do with my RME. I use a passive preamp that was custom built with excellent attenuators and switches
The only downside is that you lose remote control of volume if your passive doesn't have that feature. I have an excellent DIY Slagle AVC preamp but I don't use it because I really depend on a remote control these days.
 

bennetng

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#43
Thanks. Well, I was planning on using my ADI-2 to directly control volume of a pair of NC-400 monos. I might just abandon that idea then.


At full volume Schiit Sys is transparent. As we lower the level on Sys, naturally the measurement noise as a percentage rises leading to the graphs that you see. In other words you lose signal to noise ratio with passive volume controls.
You also decrease SNR when using passive volume control.

Read page 63:
http://www.archiv.rme-audio.de/download/adi2dac_e.pdf

Actually I am not surprised that DacMagic+ mentioned in the first page shows digital spuriae in spectrum when using the internal digital volume control:
https://www.audiosciencereview.com/...measurements-of-cambridge-dacmagic-plus.6887/

While the measurements are not bad especially when considering the age, SINAD and DNR are about 20dB behind SOTA DACs these days. With a bit of Googling I also found that it uses Wolfson (now Cirrus Logic) WM8740 24-bit DACs. However AKM/ESS and some of the newer products like CS43198 are 32-bit DACs and have higher precision when calculating digital volume levels, and if you are using software volume control at 32/64-bit float precision with dithering, there will not be digital spuriae up to the noise floor of the best DACs (~22 bits) these days. See this video:
https://www.audiosciencereview.com/...y-of-software-volume-control.5922/post-132803

I am not saying a passive preamp cannot improve SNR under ALL circumstances, however, if it does improve, it just means the DAC has higher analog noise than the preamp, and it has nothing to do with digital artifacts.
 
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Schackmannen

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#45
Thanks. Well, I was planning on using my ADI-2 to directly control volume of a pair of NC-400 monos. I might just abandon that idea then.
I see no reason not to use the ADI-2 DAC as a preamp, in it's lowest reference level of 1 dBu its balanced output has a residual noise of 1.16 uV which is extremely low, even lower than the Benchmark LA4 (<1.9 uV) which to my knowledge is the best analog preamp there is (or at least one of the very best). While it's theoretically true that a DAC loses SNR as you turn down the volume, it doesn't matter since if the noise isn't audible with the volume turned up it's not gonna be audible with the volume turned way down since the noise is at a constant level.
 

MRC01

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#48
You also decrease SNR when using passive volume control.
Read page 63:
http://www.archiv.rme-audio.de/download/adi2dac_e.pdf
...
SNR will drop at lower levels, but how much? Some methods drop more than others. The RME discussion on p. 63 is a bit of a straw man argument, as it pertains only to conventional analog potentiometers. There are other alternatives, such as a stepped attenuator using metal film resistors. Alternatively, one could drive the full scale DAC output into an active analog stage having less than unity gain (instead of a typical analog stage, which has fixed gain at unity or greater with attenuation through adjustable resistance).

There are several ways to do it, and Amir's measurements of several different devices show that some work better than others at preserving SNR at low levels.
 

VintageFlanker

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#50
Thanks. Well, I was planning on using my ADI-2 to directly control volume of a pair of NC-400 monos. I might just abandon that idea then.
Nope.
The purpose of Auto Ref Level is well explained in the manual. It achieves the max SNR possible at most output voltages.
My NC250MP had about the same input sensitivity than NC400. I used the ADI-2 around +1 to +7dBu Ref Level with RCA (say -5.5dB max) for high listening volume. And on Ref Level -5dBu at moderate levels.

For instance, +1dBu at -3dB volume gives a SNR of 110dB. -5dBu at -8dB gives you 102dB SNR...etc. So no, SNR is not an issue using ADI-2 DAC as preamp. (If Auto Ref Level). Of course calculations for XLR will be different (twice the voltage but slightly higher SNR)

Subjectivly speaking, then: The ADI-2 DAC, as preamp, is a dream to use and listen.
 
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#51
I found this thread had useful feedback on this topic.

https://www.audiosciencereview.com/...ttenuation-before-power-amp.5844/#post-131431

@bennetng makes the point that it is best if you can do the attenuation in software with 32bit or 64bit floating point calculations.

He goes into much more detail in his guest post on Archimago’s blog.

http://archimago.blogspot.com/2019/06/guest-post-why-we-should-use-software.html

This is all new to me, but I have been furiously reading around this topic for the last few weeks.
 

bennetng

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#52
I found this thread had useful feedback on this topic.

https://www.audiosciencereview.com/...ttenuation-before-power-amp.5844/#post-131431

@bennetng makes the point that it is best if you can do the attenuation in software with 32bit or 64bit floating point calculations.

He goes into much more detail in his guest post on Archimago’s blog.

http://archimago.blogspot.com/2019/06/guest-post-why-we-should-use-software.html

This is all new to me, but I have been furiously reading around this topic for the last few weeks.
Thanks. One of the reasons that I care about this issue so much is because some mastering engineers started to use a conservative mastering technique which permanently damage the sound quality of distributed music.

While some individual products (e.g. RME) has some unique methods to optimize SNR and some historical DAWs like Pro Tools TDM introduced some headroom into their 48-bit (with 56-bit register to take care of clipping) fixed point audio engine, a lot of typical DACs just use the standard fixed point volume control.
 

MRC01

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#53
Overall, a big part of preserving as much SNR as possible at low volumes is about the analog stage having just enough gain, no more than needed. A traditional analog stage having gain fixed at unity or greater, isn't ideal, because when you attenuate it, you're turning down the signal while the noise remains constant. With typical op-amps, open loop gain is ideally infinity, so closed loop gain depends on how much negative feedback, or the ratio of the R1 and R2 resistors. Lower gain has higher negative feedback, more stable, lower noise, lower distortion, and higher bandwidth. Turning it down by swapping the gain-feedback resistors to reduce gain, should be quieter than a higher gain circuit attenuated by a resistor.

Reading the RME manual posted earlier, it seems like it does exactly this with its "Ref Level" settings. It reads as if these are 4 different settings for its internal gain-feedback loop, each 6 dB apart. Whichever you select defines the maximum output, which you can then attenuate digitally. For example: if you want the output at -18 dB from full scale, you can select the highest Ref Level and digitally attenuate by -18 dB. Or, you can select the lowest Ref Level with no digital attenuation. This gives a higher SNR than the first. Its "Auto Ref Level" feature appears to optimize the output SNR for any given volume setting by automatically select the lowest "Ref Level", with the corresponding least amount of digital attenuation.

This is a good way to reduce noise at low volume settings, but it can be taken even further. Instead of having only 4 steps, why not have 50 or 100 steps, and eliminate the digital attenuation entirely? Always use the full scale output from the DAC chip, and attenuate it by changing the gain of the analog stage in small (say, 0.5 dB) increments.
 

MRC01

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#56
... Subjectivly speaking, then: The ADI-2 DAC, as preamp, is a dream to use and listen.
From what I read here, I agree. RME's "Ref Level" feature optimizes its output for whatever level is being used, from full scale to the low to moderate levels we actually use during listening. That's really neat. Some of the best DACs measured here (such as the Matrix Element X) do not do as well at low output levels.
 

VintageFlanker

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#57
From what I read here, I agree. RME's "Ref Level" feature optimizes its output for whatever level is being used, from full scale to the low to moderate levels we actually use during listening. That's really neat. Some of the best DACs measured here (such as the Matrix Element X) do not do as well at low output levels.
For everybody to see how it works:


With of course my power amp off.
 

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