Analog attenuation: why you need analog preamp/headphone amp

[yummy]

thought I've posted this but actually not.. sorry, age is a trikcy thing

let alone headphone amps, it's kinda preamp + power amp but much much lesser powerful, so I only talk about preamp here, when you have a basic idea you will underatand the thing's about noise mainly (driving to good or not are not disscussing here, those related to power amp, power amp is another big fat problem, the last puzzle we have no idea how to get it done in super low distortion whatsoever, and high thd it might not sound bad at all, tubes like 300B having "good" distortion that sounds kinda yummy)

everything has noise (residual noise), the noise of AP is around 1uv (uwt, BW unknown, guess 200k)

let's say that your DAC is 5uv noise, preamp 2uv (which is pretty good)
noise calculating through the chain is complicated, here just make it simple, not accurate, only taking as an example

going analog
DAC (5uv) -- preamp (6db gain, 2uv) -- power amp (12db gain, 20uv) -- transducer
assume that gain section is in the input stage in this preamp
5uv 6db = 10uv, and (attenuation) -40db to listening level = 0.1uv
plus the noise in preamp itself 0.1uv + 2uv = 2.1uv, 12db gain = 8.36uv, power amp noise 20uv = 28.36uv
(with high-sensitivity earbuds or high-sensitivity horns you might need to attenuate like 60db to listening level)

headphones
DAC (5uv) -- headamp (0db gain, 2uv) -- transducer
5uv -40db = 0.05uv, 0.05 + 2 = 2.05uv

digital
DAC (5uv) -- preamp (6db gain, 2uv) -- power amp (12db gain, 20uv) -- transducer
5uv +6db = 10uv, 10 + 2 = 12uv, 12uv +12db = 47.77uv, 47.77 + 20 = now you have 67.77uv noise
you go digital volume you could only pull down the output level, but the noise remains the same, won't change the noise level, output level smaller, but noise keeps the same

headphones
DAC (5uv) -- headamp (0db gain, 2uv) -- transducer
5uv + 2uv = 7uv

above our preamp/headphone amp is only 2uv, smaller than (5uv) the DAC's
what if
our preamp/headphone amp having 10uv
DAC (5uv) -- preamp (6db gain, 2uv) -- power amp (ignore) -- transducer
5uv x2 -40db + 2uv = 2.1uv

DAC (5uv) -- preamp (6db gain, 10uv) -- power amp (ignore) -- transducer
5uv x2 -40db + 10uv = 10.1uv

in here although we're both using analog attenuation, but if the preamp/headphone amp is already having more noise than the DAC itself, it's meaningless, the output from your preamp/headphone amp is actually more noisy than the output from the DAC, in this case, you don't need preamp/headphone amp, just use digtial volume could actaully get less noise than adding noisy-than-DAC preamp/headphone amp

like this
DAC (5uv) -- no preamp (6db gain, 10uv) -- power amp (ignore) -- transducer
5uv digital volume, stays at 5uv

so, choosing preamp/headphone amp, you have to pick ones that is less noisy than your DAC's
if your DAC is already like 2uv, you gotta need super good preamp/headphone amp to achieve lesser noise
and preamp/headphone amp should be at 0db gain considering the noise (only consider the noise here)


the things about digital volume is that they could use algorithms to "mask" your ears to let you feel that is actually not so lossy, the digital volume in foobar2000 isn't so bad, even doing -20db you won't feel much different, but going -60db digital with high-sensitivity earbuds would be a no go, usually digital volume sounds weak, sounds pruned, bad digital volume are those in-chip ones, like AKM digital vol in the DAC.


how to calculate DAC residual noise (not accurate but useful in case of lacking top notch analyzers)
you need to know the output level
and SNR or DNR

convert the otput level into dBu format
2v rms = 8.239dBu
4v rms = 14.26dBu

say, SNR like 122db here (usually awt, awt is cheating but popular)
yeap, balanced output technically having double noise
122 - 8.239 = 113.761
122 - 14.26 = 107.74
(or more formal 14.26 - 122 = -107.74 whatever you like)

-113.761db = 0.0002051% = 2.051uv
-107.74db = 0.0004102% = 4.102uv

so low output level (like 2v rms) and high SNR (if dreamingly 135db) at the same time is a monster

Awt will improve the result by 2db to 5db (usually), if you get Awt result and want to "guess" the uwt result (not accurate man), like 122db under uwt it might be 117db - 120db


/
passive preamp have THD problem, best passive preamp are those transformer based ones (passive preamp also have noise but good ones like transformer one only had noise on the level of a series of resistors, the wires in the tranny),
but distortion pretty high, like -60db around 20-500hz, -80db at 1KHz best case (most of them are 2nd distortion)



//ps, I just came across the gear list of Amir, all I can say is good choice, scientific, especially the main gears
Levison preamp flagship/ex-flagship are the best measured preamps in the market, still are, some could use latest op-amp to build better-specs simpler preamps but could you beat Levison in all-around full test I'm in doubt.
power amps are so variable I will skip here, every famous manufacturer release their mono amps.


Ravel is the most scientific designed speakers, I won't say it's the most good listening one but Ravel is based scientific fact, like textbook level, and they have gaint absord rooms to test every top line speakers (super super rare), persoanlly I might choose modern JBL but that's personal choise, JBL is not as scientific as Ravel.
they don't use the best fancy drivers but they do everything to the best based on many supported solid facts.

 

[yummy]

actually I've been using a more accurate way to calculate the noise, could apply to preamp too, but need to know the output level under that specified SNR

get output level in v rms
get SNR or DNR

like SNR = 122db, level = 4v
-122db = 0.0000794%
0.0000794*4 = 0.0003176 = 3.176uv

122db could adjust to 117-120db to "rough guess" the uwt result

 

[solderdude]

What is the purpose of this exercise ?

why you need analog preamp/headphone amp

is easily answered.
You need a pre-amp if the input signal is not enough to drive the amps behind it or when you need a pre-amp to buffer the source.
You may need it for volume control, maybe even when certain connectivity or functionality is needed.
If this is not the case an attenuator COULD be fine.

You need a headphone amp when the source you are using cannot drive the headphones properly.

Noise is really only a problem for high sensitive IEM's connected to gear not suited to drive them.
There are solutions for that.

Most recordings have much much much higher noise levels than the background noise of a DAC.

Also consider that noise of a DAC is not only the quoted self-noise of that DAC (which can be very low).
Noise levels can increase greatly due to harmonic distortion in which case the background noise calculations are moot. (SINAD can lift a glimps at a certain level). When playing music the noise levels of the DAC thus increase and make it problematic to calculate.

Passive pre-amps, those with (auto)transformers, not only have gain/attenuation but at the same time have different output resistances and input resistances. Input resistance can become way too low for low frequencies.
With most active preamps the input R and output R are a constant and FR and distortion is lower (unless really crappy design)
Passive attenuators have a varying output resistance which can become quite high depending on the used volpot and position.
This could result in treble roll-off when the load is rather capacitive.

And .. no ... LDR's don't improve things either

 

[NTomokawa]

I'm not a headphone user.

For speaker amplification, I believe the dedicated preamplifier went out of style by the 1980's.

Unless, of course, somebody is using monoblocks, with a preamplifier to control them. That, however, is approaching the area of "audiophoolery"...

 

[garbulky]

I have no idea what's happening here