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"Double amping", bad?

post #1 of 15
6/23/08 at 11:29am
Thread Starter 
Guys, please tell me what I am missing.

In all DAP, analogue output of DAC is amplified to power headphones.

For DAPs that has a line out, analogue output of DAC is amplified to line level signal.

So it seems that a line out and a headphone out is not so different, but only a matter of gain in amplification.

So why is it amping a headphone out is considered "double amping" and bad, but amping the line out is not "double amping" and better?

Ok, let's assume that signal out of a headphone out is less clean because it is amplified to a greater magnitude.

Compared to a line out from non portable sources, iPod's line out is relatively weak, which in turn demands more gain from amplification for a given volume.

On the other hand, headphone out of a DAP is a more powerful signal than the line out of an iPod, from which in turn demands less gain from amplification for a given volume in relative to amplification of an iPod line out.

Noise is proportionate to the gain of amplification.

So given all things equal, why is there an advantage of amping the line out instead of the headphone out?

Of course, not all things are equal. But it seems to be that it boils down to how clean of a signal that a device can produce, rather than it is about a line out or a headphone out.

So, what am I missing?
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post #2 of 15
6/23/08 at 3:59pm
A few things are different between the two.

1) There are two types of amplification, current and voltage. I don't think there is any voltage gain in the line-out, only current.

2) Output impedance and specs of the output stage. If you're using opamps vs discrete output stages; an opamp that can drive headphones usually isn't the best for line level signals (think LM6172, AD8397, etc), and an opamp meant for driving line level signals usually can't drive headphones too well (think LM4562, AD8620, OPA627).

iPod line out is 1V, not that weak, pretty standard actually. There are lots of stationary sources use 2V, but I really don't think 1V is "low"; for almost all of my headphones (except K240 Sextetts), a unity gain buffer was enough to drive them.

Headphone out on the iPod on the other hand is very low voltage, it's meant to drive headphones, not function as a line driver.

You're also missing one more point; the headphone amp in the iPod is inside the Wolfson DAC and is actually amplifying the line level signal from it; that means you'd be 3x amplifying the signal by amping the headphone-out.

Quote:
Originally Posted by Navyblue View Post
Guys, please tell me what I am missing.

In all DAP, analogue output of DAC is amplified to power headphones.

For DAPs that has a line out, analogue output of DAC is amplified to line level signal.

So it seems that a line out and a headphone out is not so different, but only a matter of gain in amplification.

So why is it amping a headphone out is considered "double amping" and bad, but amping the line out is not "double amping" and better?

Ok, let's assume that signal out of a headphone out is less clean because it is amplified to a greater magnitude.

Compared to a line out from non portable sources, iPod's line out is relatively weak, which in turn demands more gain from amplification for a given volume.

On the other hand, headphone out of a DAP is a more powerful signal than the line out of an iPod, from which in turn demands less gain from amplification for a given volume in relative to amplification of an iPod line out.

Noise is proportionate to the gain of amplification.

So given all things equal, why is there an advantage of amping the line out instead of the headphone out?

Of course, not all things are equal. But it seems to be that it boils down to how clean of a signal that a device can produce, rather than it is about a line out or a headphone out.

So, what am I missing?
Reply
post #3 of 15
6/23/08 at 4:54pm
Hmmm...good points, Fallen. What about something like the Total AirHead, that says in its manual to connect via line-out if possible, but if you don't have it to connect to the headphone output jack and turn the volume all the way up, and use the amp's volume to control it? On mine, if I crank the volume all the way on my device (without the amp), it distorts all to hell, yet when I do the same thing through the amp, it's nice and clean. How does a portable amp take a badly distorted signal and "fix" it? Just curious...
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post #4 of 15
6/23/08 at 5:07pm
Quote:
Originally Posted by Golden Monkey View Post
Hmmm...good points, Fallen. What about something like the Total AirHead, that says in its manual to connect via line-out if possible, but if you don't have it to connect to the headphone output jack and turn the volume all the way up, and use the amp's volume to control it? On mine, if I crank the volume all the way on my device (without the amp), it distorts all to hell, yet when I do the same thing through the amp, it's nice and clean. How does a portable amp take a badly distorted signal and "fix" it? Just curious...
You can't "fix" distortion.

I'm don't know much about the Total AirHead, I have never tried one.

There are a few different schools of thought on amplifying headphone-output, mine just says "don't". Other believe turning the source to 100% to get maximum power, but others say to use 80% to avoid distortion. It's really your call in the end, whatever sounds the best.
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post #5 of 15
6/23/08 at 5:16pm
My volume on my DAP goes to 25, I just dial it back a notch to 24, but even all the way up, it sounds nice and distortion-free through the AirHead...I jsut don't understand how they do it, lol...
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post #6 of 15
6/23/08 at 5:24pm
Most of the distortion comes from what you are driving. The easier the load the less distortion. If you are using the headphone out and have it to max and then go into another amp, the amp in the source has an easy load. I don't like to use this method but it works.
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post #7 of 15
6/23/08 at 5:33pm
Even 80% power will give you a lot of distortion. You're better off using efficient IEMs or headphones than the can of worms you get with line outs, double amping, etc. My opinion is that you should have as few components as possible between the signal and your ears.
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post #8 of 15
6/23/08 at 8:30pm
Quote:
Originally Posted by jamato8 View Post
Most of the distortion comes from what you are driving. The easier the load the less distortion. If you are using the headphone out and have it to max and then go into another amp, the amp in the source has an easy load. I don't like to use this method but it works.
X2. This is how my Zune80 behaves. Even though it is known for being clean and dynamic, once the load is easier on it (connected to amp), it really sounds nice! I get the same effect if I hook the Zune80 up to my pre-amp, that my headphone amps are "tape looped" into. It allows the Zune80 to be a much better source than I ever expected from it.
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post #9 of 15
6/23/08 at 10:11pm
Long explanation for why a separate amp driven by the DAPs amplifier is almost always superior to straight DAP headphone out:

The phone out amp in a DAP behaves differently driving a separate amp than it does driving a set of phones.
The input of a typical portable amp is 10,000 ohms, of a desktop it is 50,000 or 100,000 ohms. The load of phones varies but is never more than 600 ohms in extreme cases, almost always under 100 for common DAP phones.

Volume control is voltage control. The voltage of the signal is controlled by the volume pot. The current between stages is always set by impedance. There are lots of variables to examine, but typically the amp inside a DAP is better behaved when driving a 10,000 ohm load than it is a 100 ohm load. The DAP is asked to supply only 1/100 of the current going into an amp verses straight out to phones. Most DAPs lack adequate power buffering (the rail capacitors) and are challenged supplying the current requirements of sub 100 ohm loads. They have no problems supplying the nearly non-existent current driving the 10,000+ load of a separate amp.

Of course, it is generally superior to bypass the DAPs amp with a lineout if available. Generally though, most of the improvement from using a separate amp does not depend on a line out.
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post #10 of 15
6/24/08 at 4:17am
Quote:
Originally Posted by FallenAngel View Post
iPod line out is 1V, not that weak, pretty standard actually. There are lots of stationary sources use 2V, but I really don't think 1V is "low"; for almost all of my headphones (except K240 Sextetts), a unity gain buffer was enough to drive them.
I've just ordered a DAC which has 4vRMS output. Does this mean that the output level is 4x "hotter"/louder than the 1vRMS iPod lineout? Does this voltage increase have a linear effect on the output volume?

Pardon my inexperience in this area, and thanks for the help.
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post #11 of 15
6/24/08 at 4:55am
Quote:
Originally Posted by druelle View Post
I've just ordered a DAC which has 4vRMS output. Does this mean that the output level is 4x "hotter"/louder than the 1vRMS iPod lineout? Does this voltage increase have a linear effect on the output volume?

Pardon my inexperience in this area, and thanks for the help.
Yep, that's 4 times louder and no, it does not have any "linear effect" on distortion levels.
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post #12 of 15
6/24/08 at 6:13am
Quote:
Originally Posted by druelle View Post
I've just ordered a DAC which has 4vRMS output. Does this mean that the output level is 4x "hotter"/louder than the 1vRMS iPod lineout? Does this voltage increase have a linear effect on the output volume?

Pardon my inexperience in this area, and thanks for the help.
It depends on what you mean by "louder" and it depends on what you are driving with the DAC.

Human hearing response is not linear, it is logrithmic. You actually have to increase the audio power by 10x (10db) for a perceived doubling of volume.

There is the compounding electrical issue. If you are directly driving a set of phones, not only is the voltage going up by 4x, but the current is going up by 4x as well (Ohm's Law in action). In that case, the applied power is 16 x greater, which human perception will intrepret as about 2.5 time as loud.

If you are not driving the phones but just feeding the 4 vrms into the input of an amp, the current is irrelevant and very small. Only the voltage matters. The amp will however respond to the signal and will output (if it is capable) 4 times the voltage and 4 times the current = 16 x the applied power but only the perception of about 2.5 times as loud.

Confused ?
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post #13 of 15
6/24/08 at 8:20am
Thread Starter 
Thanks guys for your wonderful inputs.

I am actually thinking of adding some "flavour" to my rather "unflavoured" sounding portable setup.

Since my DAP does not have a line out, I am wondering if I should get an iPod, though I am not a fan of anything that has a touch interface.

The last time that I tried some portable amp I wasn't so impressed. Generally speaking, most of them made the sound fuller and warmer. But I get the impression that the imaging wasn't as good (though the better ones comes close). Or is it that the analytical sound improves imaging? I'm not sure if going iPod would fix it.

With my 27 ohms ER-4P I can hear hiss out of the headphone out of my DAP, I guess this would not be a problem for an amp as the high impedance is high?
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post #14 of 15
6/24/08 at 12:14pm
Quote:
Originally Posted by bada bing View Post
If you are not driving the phones but just feeding the 4 vrms into the input of an amp, the current is irrelevant and very small. Only the voltage matters. The amp will however respond to the signal and will output (if it is capable) 4 times the voltage and 4 times the current = 16 x the applied power but only the perception of about 2.5 times as loud.

Confused ?
I'll be connecting the DAC to my Darkvoice 336i tube amp so, as you clearly explained, the current is negligible. I'm just concerned because the output of the DAC is 4 VRMS which is 4 times the power of my iPod, and am worried I'll have to attenuate the volume on my tube amp to a very low level (it's already relatively low with my iPod.)

Confused? Slightly. But your explanation helped a lot. Thanks for that!
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post #15 of 15
6/24/08 at 12:33pm
Thread Starter 
Bada Bing,

Btw, how does your M3, Bijou and DV332 compares?

I am kind of considering if I should get a DV or dirty my hand and make a Bijou. I'm also kind of worried about the high voltage and considering SS DIY design as well.
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