[wellylug] HW specs - Line In/Out and radios etc.

[Sam Vilain]

On Mon, 2009-01-26 at 15:08 +1300, Carl Turney wrote:
> Hello,
> 
> Any one know the voltage (etc) specs (e.g. compatibility) on the Line In 
> and Line Out of PC sound cards and laptops, particularly in relation to...
> 
> - the 3mm stereo plugs on external PC speakers, and earphone sockets on 
> transistor radios etc.
> and
> - the RCA in and outs of home stereo systems
> and
> - those 6mm professional-studio-type headphones.
>
> I had thought that Line Out and Headset were quite different, but my new 
> Acer laptop manual gives the impression they are the same.
> 
> Am thinking about a variety of connections in this regard.  And vaguely 
> recall burning out some sound components, probably by porting headphone 
> out to line in, many years ago.  (May not have involved computers at 
> all, at the time.)

You remember correctly.  The fixed line-out voltage is 200mV "consumer"
and 2V "professional" (RMS).  That's a 10dB difference so you know when
you're dealing with gear outputting the higher level because the sound
is MUCH louder switching between gear.  Some amps have a "low level"
switch to select this.

The expectation with these outputs is that they are driving a high
impedance load, eg, 47kΩ.  So if you plug them into a low impedence
load, like a 16Ω headphone, then it will not be able to drive it.
Probably the bass will disappear and it's possible it might even damage
the driving circuit.

Headphone outputs are similar, except variable voltage and able to drive
power into much lower impedance loads.  ie, they have an amplifier as
well.  While I think I might be forgetting something to do with
inductance, say you were driving a 64Ω pair of headphones at 200mW (loud
for headphones), then you would be looking at about 3.6V output (RMS).
But most headphone outputs from computers and ipods won't do that.  A 2V
RMS output will drive 32Ω headphones at 125mW, which is plenty loud
enough - depending on the sensitivity of the headphones it may even be
unbearably loud.  So you can see that it's quite possible that a
headphone output is suitable for a line output, as the range of output
voltages is often within the range which a regular component will
output.  Nevertheless, some input circuits will indeed fry if they get
much more than the consumer output level, let alone more than the 2V
professional level.  Headphone outputs on discrete audio units, like a
tape player or amplifier usually will be able to reach these high
voltages.

So.  To figure out what levels to use;

  1. connect an external component to one input on your amp, start a
song playing

  2. start the same song playing at roughly the same time on your
computer, with the volume set to being 100% (or 0dB on Alsamixer - make
sure it's not positive) on the PCM, and the "master" set very low (eg,
-40dB).

  3. switch between the two inputs and adjust the "master" volume on
your computer until it sounds about the same level.  When you start
getting into the ~3dB range it becomes very difficult to perceive it by
ear.  If your amplifier has a level needle or equaliser, then this can
be very useful to get it "just right"

After this you will know what setting on your master output is "full
volume".  Then just make sure that your mixer is set to control the PCM
level (usually the default) and not the master, so that you don't
overdrive the inputs.

Or just plug it in and turn it up until it sounds right, if it sounds
like it's distorting even with the stereo on a low volume then the level
is wrong.  Turn down the master until it sounds clean, turn it down a
little bit more and then turn the stereo up and it should sound right.

HTH,
Sam.

ps. there's probably an idiom about knowing this stuff, similar to
playing a good pool game ;-)


> Thanks very much,
> 
> Carl
> 
>