Many amplifiers do generate internal noise that is easily audible if you are direct driving horn compression drivers in a multiamped system.
Haha Greg, you seem to know more about my system than I myself. So, all still 100% correct ...
(and still no noise)
Anyway, very nice reasoning about the "attenuation" in the PCM1704. Maybe we should talk adding / subtracting currents, but okay.
Without analysers at hand, or pictures of it, all is theory of course. Or at least maybe
it is for you. I mean (and for example only), just try to pull a plot of the noise line from an amp from somewhere (internet) that is 100% straight (up to say 96KHz). So there it starts. Btw, I think I already said that the Sauermann does this. Same for the GainClone.
That is one part only. A next is your attenuator, or preamp for the worst "solution".
Along with this I can only try to tell you (listening would be so much better
) that this noise line tells all. When that turns into something uneven (lumps, bumps, slopes up / down) the music is killed. It is just a measure for that. And not to forget, I worked for months on it until I had to give up (if only the PCM1704 could be voltage regulated).
It is a bit more difficult to relate this to the "analogue attenuation anyway" theory you applied so nicely, but let's say I don't see that happening.
It's just that by careful design BB keeps any filtering to a higher frequency than we will ever worry about.
That neither, but that's another story (what "filter" ... where ?).
So for example, when I attenuate digitally with 141dB, I have the (btw 16/44.1 !) signal just sticking out the noise, and this noise does not look differerent from no signal being there at all. Notice though that this could be the opposite situation of "attenuating" and I never really wondered what comes first in the 1704 (full signal which is divided, or no signal which is added, but the bi-polar etc. stuff should tell a few things).
Now, with my mentioned -21dBFS signal, the noise line still does not change, but, a few harmonics are there of course (that's just the THD from the chip, and losing bits if you like).
Only when I attenuate less than 21dBFS the noise line starts to rise linearly which in my case will be caused by the means of (Arc Prediction) filtering. But, this noise is still completely "horizontal" (straight). This latter only applies to the audio band, or more when higher resolution is in order (meaning : after fs/2 the noise level rolls off). Well, look here :
To keep in mind : The FFT depth used here (and further pictures) makes the noise look 20dB lower than realilty. So it's here at -140dB really.
Anyway, this is what I call a straight noise line. Measure this behind my GainClones and nothing changes except for the gain which is added plus a small 3rd harmonic (sticking out with 9dB IIRC).
This is 1000Hz at -0dBFS signal at -3dbFS (16/48 Arc Prediction upsampled to 24/192 (and *not* to 24/768 which the NOS1 can digest). All still straight.
(notice this is up to 10KHz (just grabbed an existing picture from somewhere)).
Here you see what I talked about regarding the fs/2 thing. The 20 dB added must be from my filtering (I see this as random HD resulting in white noise) with possible reactance in the gain stage.
Each dB of attenuation will drop the noise floor linearly, until it gets under the inherent noise (this is what I talked about in my earlier post).
Here you see the proof of the fs/2 thing, since this is a 24/96 signal.
Ok. As you can see, noise lines are not sooo straight as I implied they should be, but this is software impeded. To my ideas this can't harm because it "goes on with the flow" and further more it is as straight in the audio band intended (fs/2).
Now add any attenuation means (of which I don't have pictures, or at least not at hand). On of the means I obviously tried was your type of attenuator (I won't name the brand to not discredit well respected people), relais regulated. It was the worst of all I tried. Look at the first picture again, but now imagine lumps all over like you see beyond my fs/2 pictures. But then in the audio band of course. Spikes all over the place, and of course varying per chosen attenuation (btw, digitally attenuating with the ladder DAC shows different harmonics per attenuation level just the same - just saying, but in normal "expected" fashion).
Maybe you can dig up plots from your attenuator, or otherwise go out on Google and find nice straight noise lines which are not 100 times smoothened (look at the thickness in my plots).
Anyway, this is measuring. But before that comes listening. Ad then what's audible is visble at measuring. It just is so. In all situations I tried (up to one-value fixed resistors in all kind of setups, yes, also right in front of the amps).
Any means of transformer (I/V) ? it just can't work (with a TVC ahead as a rough one). I went as far as buying a most official LDR for USD 450 or so, and then to think this was for two "ch" only (so not even able to work in differential mode which would add another 200 or so).
In my case you can also say that I chose the wrong chips of course. But in the end not, because doing it digitally is just the most okay and really creates the best sound by far.
And not to forget, in the 1704 situation 90% of the work in this context goes to the I/V itself, which is a pain ... (but very much "volume" related).
Aaanywayy ... no noise here with dual GainClones and compression drivers. Haha.
Oh, this was about the Sauermann. Well, as said, same story. Same noise line too (and I say this is RARE).
Thanks for sparring a bit.