Connecting the 205 Directly to a Power Amp -- Part 2
In my post above I talked about "impedance" considerations in connecting the Analog outs of the OPPO UDP-205 directly to a Power Amp -- no pre-amp in between. Now I want to talk about Volume.
A key difference in connecting directly to a Power Amp is that the Amp is not likely to offer Volume control -- or call it "Gain" control. That means you will be relying on the Analog output Volume control provided by the 205 itself.
Power Amps have different "Gain" characteristics, and if the Gain of the Amp is too large (and your speakers too "efficient") you will need to use a very low Volume setting in the OPPO to achieve a comfortable listening level. That raises the question whether a low Volume setting in the OPPO affects the audio quality. And if so, how low is TOO low?
For folks new to this, an important concept here is that the Volume control in a device like the OPPO does not "amplify" the output. The 205 is designed to put out a "full scale" signal. That is, the max Volume setting of the 205 (a Volume of 100 -- or the Output "FIXED" setting, which does the same thing) puts out the Analog audio signal the player is DESIGNED to put out. It's very BEST signal.
Instead, lowering Volume below 100 "attenuates" the full scale output -- lowering the output volume below that of the "best" signal. But how does that affect audio quality? Does it even make a noticeable difference? There are two considerations: Dynamic Range and Noise Floor.
The first takeaway here should be that Dynamic Range is not a problem in the 205. Here's how we get to that result.
The Analog output Volume control in the 205 is implemented at very high quality -- in the 32-bit DACs of the player. That is the Dynamic Range of the Volume control itself is 32-bit. But the max dynamic range of the content you can PLAY in the OPPO is 24-bit. That gives us 8 bits extra.
Each bit of precision is equivalent to 6dB. So that says you could lower Volume by -48dB below full scale and have ZERO effect on the Dynamic Range of the Analog output stage for the content this player can play.
But wait! There's more!
The Noise Floor of the 205 is -120dB below full scale. That is to say any portion of the content which is recorded -120dB, or lower, below full scale is going to be "in the noise".
The maximum potential Dynamic Range of a 24-bit digital audio stream is 144dB. So the lowest 24dB of that will be "masked" by the Noise Floor. What's left gives the maximum perceivable Dynamic Range of the Analog outs: 120dB.
Combining these two results says you could lower Volume a whopping -72dB and still have no effect on the perceived Dynamic Range (i.e., after amplification to a comfortable listening level through a high quality Amp).
So what's the problem? The PROBLEM is that -- and this is another key concept -- attenuating the output signal does NOT also lower the Noise Floor! So if you set volume -72dB down (a Volume setting of 28 in the OPPO), you will have lowered the signal you WANT to hear 72dB closer to the Noise Floor. Or to put it another way, you've raised the Noise Floor to -48dB!
When you put that Analog output signal through a high Gain Power Amp to get things back to a comfortable listening level that Noise Floor -- now only -48dB below the loudest portions of the content -- ALSO gets amplified. And may very well become noticeable, particularly in a quiet listening room.
So that raises the question of how high is too high for the Noise Floor? There's no pat answer for that. But as a straw man, let's suggest that for high quality listening you don't want the Noise Floor to be closer than -90dB below the loudest portions of the content.
As I said above, the Noise Floor of the 205 is -120dB below full scale. So that gives us 30dB of Volume control to play with!
In the 205, Volume is implemented as -1.0dB per step below Volume 100. So that says you should TARGET a setup where you can get a comfortable listening level without having to lower Volume in the 205 below Volume 70. Higher is better, but 70 is fine.
If you decide you prefer to be more or less conservative regarding this Noise Floor straw man, you can do the easy math to adjust that TARGET Volume to taste.
So what if the Gain in *YOUR* Power Amp is so high that Volume 70 in the OPPO wakes the neighbors and knocks plaster off the walls? Well that says that your Power Amp is not, by itself, a good "match" for the *DIRECT* output of the 205.
One solution, of course is to put a pre-amp between the OPPO and your Power Amp. The pre-amp provides Volume control and so you can leave the Analog output Volume of the OPPO at 100.
Another solution is to invest in some high quality "attenuators" which you will put, in-line, between the OPPO and the Power Amp, so that you can keep Volume in the OPPO at 70 or above (assuming our straw man example here) while achieving a comfortable listening level.
Now does that meant the signal is going to turn to crap if you lower Volume below 70? Of course not. You may very well want to use lower Volumes when you are not doing "critical listening" for example -- perhaps late at night depending on the persnicketiness of the neighbors. In most such cases you will probably NOT notice a decrease in audio quality.
But if you TARGET a setup where you are commonly using, say, Volume 30 for critical listening then you are not using the hardware of the 205 to best advantage.
ETA: Things Change!
The Official 0922 firmware for the 205, released September 25, 2017 in the US, changed the way the Volume control works.
Rather than -1.0dB attenuation per step below Volume 100, as described above, it is now refined to -0.5dB attenuation per step.
See this post:
Thus to achieve -30dB attenuation (the straw man suggested in the math above), you would set Volume to 40, instead of the Volume 70 used in the math above.
That says that starting with this firmware, you can achieve the same level of safety margin regarding Noise Floor if you target a comfortable listening level of Volume 40 or higher -- instead of Volume 70 or higher as described above.