[b0ck3n]

I'm wondering if those educated can help shed some light on this.
 
In an audio component chain, source > DAC > amp > headphone, what carries through to the next stage, and what is bypassed? Say for instance that the source, a CD-player or DAP, has 0.05% THD and the amp has 0.005%, are all of these values added on to a total THD at the speakers or headphones? How does it work with output impedance, does that carry through or is that bypassed by the last (amp) stage?

 

[High_Q]

If you look at the numbers, DAP has an order of magnitude higher value of THD, so in comparason the amp's THD is negligable in comparason. To optimize, go with lower THD DAP.

 

[b0ck3n]

The numbers are there just as an example - the real question is whether they are additive to the final result.

 

[khaos974]

You should assume that in 99% of the cases, the THD of the electronics is going to be buried in the THD of the transducers. Transducers regularly have 1% THD, electronics regularly go under 0.01%. THD has nothing to do with impedance.

 

[b0ck3n]

As alot of people cite the fact that amp X has a lower THD value than DAP X as a deciding factor in using an amp with their DAP. If it only adds to THD that arguement doesn't work.

High output impedance skews the frequency response of headphones that don't have flat impedance curves. I'm asking whether each piece of equipment in the chain adds the effects of its specific output impedance to the signal, or whether a <1 Ohm amp can be used to bypass a higher output impedance of a DAP.

 

[EthanWiner]

Quote:

The numbers are there just as an example - the real question is whether they are additive to the final result.

You're getting the right advice, especially to keep in perspective that gear having small amounts of distortion matters little when followed by something else having much more distortion. But to answer your specific question, distortion does accumulate. However, it's not that simple because there are different types of distortion (THD and IMD), and it's possible for different devices to have different distortion spectrums.
 
--Ethan

 

[b0ck3n]

Thanks for clearing that up.
 
What of output impedance then? Is it important for all stages in the chain, or just the last stage that the headphones are plugged into? Can you solve the problem of a 10 Ohm DAP with a 1 Ohm amp?

 

[khaos974]

As alot of people cite the fact that amp X has a lower THD value than DAP X as a deciding factor in using an amp with their DAP. If it only adds to THD that arguement doesn't work.

High output impedance skews the frequency response of headphones that don't have flat impedance curves. I'm asking whether each piece of equipment in the chain adds the effects of its specific output impedance to the signal, or whether a <1 Ohm amp can be used to bypass a higher output impedance of a DAP.

Actually, my argument that the THD of electronics matter little is valid when the amplification section of the DAP (or any amp for that matter) is working within it's nominal power rating. If you are using inefficient headphones (HE-6 I'm looking at you), you can easily go over that limit. Another addendum is that I'm talking about sensibly designed solid state amplification sections, I've seen DAP that were not sensibly designed or tube amp with a very large amount of distortion.

I'm mostly skeptical about how most portable amps are designed, manufacturers ofter only provide the barest specs.

As far a impedance and skewering up frequency response, you are mostly right, you need a headphone output with a low impedance (<2 ohm) to get the best of your headphones. A sensibly designed amp should have an input impedance of at least 10 kOhms, thus it should be able to accept output devices with relatively large output impedance, 10 or even 100 ohms outputs shouldn't be major issues. The high input impedance makes the output behave closely to a pure voltage source, the larger the ration, the better it is.

 

[Anaxilus]

Quote:

Thanks for clearing that up.
 
What of output impedance then? Is it important for all stages in the chain, or just the last stage that the headphones are plugged into? Can you solve the problem of a 10 Ohm DAP with a 1 Ohm amp?

IME yes.  For example using my DACPort on it's own yields some audible 'issues' w/ the 20 ohms of my 5 driver, 3 way ES5.  But, if I add my 4G Arrow at <1 ohm output impedance and use the DACPort as my pre-amp the 'issues' are largely resolved as far as my ears can tell.  Not true of my 2.2G Arrow under the same circumstances with it's 10 ohms.  If anyone would like to show some math proving my ears wrong I'd welcome it.  
 
This obviously isn't the whole story as a 75ohm interconnect is supposed to be ideal throughout your chain under ideal circumstances.   
 

 

[Sylverant]

Quote:

You should assume that in 99% of the cases, the THD of the electronics is going to be buried in the THD of the transducers. Transducers regularly have 1% THD, electronics regularly go under 0.01%. THD has nothing to do with impedance.

And if your running an analog line out signal to a pair of speakers with 10% THD?
 

 

[khaos974]

Exactly what I'm talking about, you analog line out is unlikely to have much distortion, on the other hand 10% distortion for speakers, while quite a horrible figure, is fairly believably, especially for non hi-fi stuff.

 

[b0ck3n]

What spurred the question was someone arguing that the low distortion value of an amp made a massive improvement to sound quality from a source that had alot of distortion - as it bypassed the distortion of the source, which now I've learned isn't the case. Thanks.
 
For me, output impedance is of importance as a BA IEM user. I've got my whole life stuffed in my iPhone but I worry that it's output impedance is affecting my IEM negatively. I was planning on solving the problem by using a DAC or amp with low output impedance, but if the negative effects of my iPhone's output impedance have already been applied to the signal an amp isn't going to solve the problem.

 

[khaos974]

What spurred the question was someone arguing that the low distortion value of an amp made a massive improvement to sound quality from a source that had alot of distortion - as it bypassed the distortion of the source, which now I've learned isn't the case. Thanks.
 
For me, output impedance is of importance as a BA IEM user. I've got my whole life stuffed in my iPhone but I worry that it's output impedance is affecting my IEM negatively. I was planning on solving the problem by using a DAC or amp with low output impedance, but if the negative effects of my iPhone's output impedance have already been applied to the signal an amp isn't going to solve the problem.

If your source has difficulty in driving the headphones you use because they require too much power and is thus distorting, using a portable amp is useful because the load of an amp (at least 10K) is very different from the load of a pair of headphones. If your source is plain crappy, and distorts naturally (quite rare but still happens with noname DAPs), then an amp won't help with the distortion.

As far a I know, iPhones and iPod are reasonably good and should distort with any IEMs, though I don't know about their output impedance.

 

[JRG1990]

The distortion of an amp various in how hard it is being driven, speaker amp manufactures use the distortion rating from 1watt and it's a very different much higher rating when the amp is at maxium output, probley the same for headphone amps.
 
if your worried about the output impedance of your iphone, then get a lineout dock for it and a headphone with has a output impedance lower enough you don't have to worry about it, something like the fiio E7 which has a output impedance of 0.13ohms.

 

[xnor]

b0ck3n,
THD adds up like this:
 
0.01% THD => 20*log10(0.01 / 100) = -80 dB
0.001% THD = -100 dB
 
sum = 10*log10(10^(-80/10) + 10^(-100/10)) = -79.957 dB => 10^(-79.957/20)*100 = 0.0100496% THD
 
or sum = 10*log10((0.01/100)^2 + (0.001/100)^2) = -79.957 dB
 
 
For driving your headphones only the output impedance of the amp is crucial.
DACs can have outputs with several hundred ohms and expect a 'load' of several kiloohms, because they are supposed to output the signal (voltage) only and not power.
Of course, if you load any component down (i.e. drive a low-ohmic load) it will cause a lot of distortion which will propagate through the rest of the chain (see calculation above).
 
But, as others have noted, the transducers will most likely dictate THD (around 0.01% with high-end headphones) in your chain.