[DeadTrying]

I read this in the Portable Audio Section and it didn't seem to be getting any answers. I figured you guys might have a better idea of what a line out actually is (and what it is NOT) and how it should be implemented (curcuit-wise).



Quote:

Sweet Spot Said Man, all of this talk is frustrating. I'm tired of people claiming to know what the answers are, when their only proof is fancy words and pictures of amps, DSP stages and signal paths. Seems to hardly make any sense unless we know a few basic things such as: What is considered by the experts in the audio world, and not some DIY audio geeks (no offense, I'd gladly wear that title) to be the meaning of a line out. What does the term encompass, and what are the hacks that can make one "fake" ? Till I know that, I consider everything to be heresy. quote: -------------------------------------------------------------------------------- The iPod and miniPod technically have line-outs without a dock if you buy a Sik Imp/Din cable. the line-out does bypass the internal iPod amplification. -------------------------------------------------------------------------------- This confuses me. What about the cable enables it to bypass the line out ? Wouldn't the signal have to be traveling on a different path all together in order to bypass the line out ? Since the DSP stuff is still being processed, and making it through to the final stage, it would seem that this is not the case, but I dunno. Now, the question I have being an IHP owner is, if the IHP's op amp is what's shaping the sound characteristics (warm, bright etc..) then by all means if the "line out" is real, then using an external amp and having the IHP up to 40 (as people have claimed to be the way to a real line out) would enable the external amps op amp section to take over in that dept, thus the IHP's overall sound characteristics would be totally different right ? Somehow, I don't see this happening with the iHP though. Another question. If a port is to be considered a "real" line out, to what extent is it being powered by the internal amplifier ? Is it just the op-amp which is bypassed ? Or does it depend on the configuration, meaning that there's more than one way to skin a real line out ? Can someone with a NJB3 please plug some high and low impedance cans into the line out and tell me what the results are ? I'm just curious. (Wish I still had mine now) Blah.

Original Post Here

 

[Dane]

Line out is how analog equipment communicate source signals. A line out is designed to drive a load of around 100k Ohms and has a maximum voltage swing of around 2 Volts.

The chain typically looks like this:

media -->
read -->
digital processing (including EQ) -->
DAC -->
pre driver --> (line out)
volume control -->
power amplifier -->
head phones

The headphone output is designed to drive much tougher loads around 100 Ohms and also has a larger voltage swing (important for high impedance phones).

For a protable device like the iPod the above chain is all build into one box making it harder to determine what is what.

You see, the line out and the headphone outputs are designed for different purposes and components are selected accordingly. Also, the line out is "purer" since it is tapped out earlier in the signal chain and bypasses the volume and power circuits.

Now it may be that some players can make the headphone output kind of work like a "line out" by fiddling with the volume and what not. But in this case you have your signal going through two volume control circuits and two power amplifiers which, everything else being equal, is sub optimal.

All processing happening before the DAC in the chain should not affect the quality or "purity" of the signal.

 

[DeadTrying]

Quote:

Originally posted by Dane The headphone output is designed to drive much tougher loads around 100 Ohms and also has a larger voltage swing (important for high impedance phones).

Hi Dane...thanks for your excellent reply!

Can you carify somethign for me. 100Kohms =100,000Ohms >100Ohms right?

Then why do you say "headphone output is designed to drive much tougher loads around 100 Ohms" or by 'load' are you referring to something else?

Sorry..I am pretty clueless about electronics metrics.

 

[Dane]

Yep 100k Ohms = 100000 Ohms

Ohm is a measurre of resistance, higher ohms means higher resistance, which is easier to drive.

A nail has a resistance close to zero Ohms, which is why you would find it pretty difficult to drive

Now, you may be confused now since you might have heard that high impedance (= resistance) phones are considered harder to drive. In fact they are not. The problem is rather that most portable equipment cannot deliver the required voltage swing due to their internal 3 Volt (not sure here, but it's in that range) battery.

 

[DeadTrying]

[size=small]Ah-ha!!![/size]

Then Dane, maybe you can answer me one last question...

I'm trying to establish a connection between sound quality and power usage to figure out an ideal output specification for a portable unit.

The units are amps, and volts for power output which easily convert to mW and Ohms for headphone input

The problem arises when you convert the mW and Ohms to into db and some other 'unit of sound quality' which has not been given a name yet!

So basically what do you think a good output spec for a portable unit would be. Would it be ok to have a 10 mW/channel 16ohm headphone out and a 2.5mW/Channel 64ohm line out (would the latter be too little mW?). I'm basically trying to keep the power usage constant to to get the best sound i can out of a portable.

Also what would you say the thresh-hold is.. i mean.. whats the lowest in terms of Ohms that most Audiophiles would settle for in a portable?

 

[Dane]

Dead, you're gradually moving towards the limit of my knowledge (I'm new at HeadFi), but I'll give it a try:

"I'm trying to establish a connection between sound quality and power usage to figure out an ideal output specification for a portable unit."

I dont think you can establish such link, sound quality is about more than power. But of course there's a lower limit, but to be honest I dont know that limit. The 3G iPod has a headphone power of 60mW and it's considered to be quite good.

"The units are amps, and volts for power output which easily convert to mW and Ohms for headphone input

The problem arises when you convert the mW and Ohms to into db and some other 'unit of sound quality' which has not been given a name yet!"

You cannot convert like this since the efficiency of the headphone also determines how meny dB you get at a given power level. And again "sound quality" cannot be extracted this way.

"So basically what do you think a good output spec for a portable unit would be. Would it be ok to have a 10 mW/channel 16ohm headphone out and a 2.5mW/Channel 64ohm line out (would the latter be too little mW?). I'm basically trying to keep the power usage constant to to get the best sound i can out of a portable."

I really dont know this. But w.r.t. the line out I dont think you should be concerned about the spec of that line out, as long as it has one (and it's a real one).

"Also what would you say the thresh-hold is.. i mean.. whats the lowest in terms of Ohms that most Audiophiles would settle for in a portable?"

I had a question about this in another thread: What is the difference in terms of sound quality between low and high impedance phones. I got a technical answer, which I already knew. I'm still confused about this, why do manufactures build high impedance phones? What's the advantage?

 

[mkmelt]

In a traditional amplifier there are several stages of amplification prior to the final stage or power stage. These stages may include starting with a buffer stage (primarily to match impedance between the source signal and the amplifier, followed by a voltage stage (provides voltage gain but no appreciable current delivery and therefore no appreciable power), next there may be tone control amplifier stage (may be switchable and defeatable), followed by a driver stage and finally a power stage. Very high power amplifiers may have several power stages cascaded together.

The voltage amplifier stage, also called a line stage, is where a line-out signal can be tapped. This is before the tone control stage or any of the driver stages or the final power stage. In a receiver or integrated amplifier, the tape output signal is usually tapped from the line stage and is therefore a true line out signal. The volume control is not in the circuit path of the lineout stage, so you will need an amplifier with a level control to control the volume of your speakers or headphones. Line out signals are theoretically superior to other output signals because they will have less noise and distortion than signals tapped from the later stages.

In a portable CD player or MP3 player, wher the amplifier is contained on one or a very few integrated circuits, separating the output of the integral voltage amplifier from the final drive stage may not be practical, because the chip may only have connections for input and final output.

As far as power available from the headphone jack, as units have gotten smaller and lighter power levels have dropped. A good headphone output from a home CD player used to provide a minimum of 100 milliwatts at 32 ohms. Today's portable players make due with between 15 and 30 milliwatts of power at 32 ohms.

These portable players can't make equivalent power at high impedances such as 120 - 300 ohms because the power supply of these miniscule amplifiers chips is usually based on no more than 1.5-3 volts. Even with a voltage doubler circuit, the supply voltage is not adequate for delivering rated power into a high impedance load. The aftermarket small portable headphone amplifiers typically operate at higher voltages, at least 9 volts. The better examples of these amplifiers can deliver at least 100 milliwatt into a headphone load from 32 ohms up to 300 ohms. This is enough power to drive most headphones to satisfying levels without clipping the amplifier and resulting in audible distortion.

 

[john_jcb]

Mkmelt, you are on a roll today. Well thought out posts that are very enjoyable to read. Thanks

 

[Ticky]

Yes, thank you Mkmelt. Your well written and easy to understand explanations is greatly appreciated by this non-engineer audiophile aspirant. I've copied your explanations into my "Audio Terminology Dictionary."

Thanks!

 

[lindrone]

Quote:

-------------------------------------------------------------------------------- The iPod and miniPod technically have line-outs without a dock if you buy a Sik Imp/Din cable. the line-out does bypass the internal iPod amplification. -------------------------------------------------------------------------------- This confuses me. What about the cable enables it to bypass the line out ? Wouldn't the signal have to be traveling on a different path all together in order to bypass the line out ? Since the DSP stuff is still being processed, and making it through to the final stage, it would seem that this is not the case, but I dunno.

Seems like no one addressed this specific confusion... the 3rd generation iPod has pin-outs for a true line-out on the "dock connector" on the bottom of the iPod.. not the headphone output. So yes, they are indeed two different signal path.

The dock for the 3rd gen iPod has a mini jack on the back of it that's connected from this line-out. The Sik Imp/Din cable that was mentioned in the post is a cable that connects into the dock connector and gives you a mini-jack. It's basically a portable line-out from the dock connector without having to carry the dock around.