SA said:
A vendor setting up a system for us asked a question that I don't know the
answer to.
Does anyone know the impedance on the output (speaker or headset) jack of
most pc audio cards? We use mostly Dell Optiplex's with I believe Soundmax
cards in them. I know very little about audio. Is the impedance on most
soundcards the same, or do they vary a lot?
Thanx
Sound cards are not really intended for driving out a lot of power.
A typical card is intended to drive a 32 ohm load (headphones) on the
green LineOut jack. The signal level is "line level", which might be
1 volt AC RMS or so of signal. The headphone jack on the front of the
computer, may be tied into that as well, or the front headphone jack
may have its own 32 ohm drive.
Sound cards may have multiple channels. They could be 5.1 or 7.1 for
example. The green jack is the one that has the 32 ohm drive. The
others may be intended for higher impedance loads, which typically
might be referred to as "600 ohm". The voltage level is still 1 volt RMS,
but the other jacks aren't intended to drive headphones.
To give an example of where this is important - Zalman makes a
5.1 set of headphones. It contains multiple elements mounted inside
the headphones. The headphones come with three stereo plugs.
Now, you'd assume you could plug them into just any 5.1 sound card
output. All three plugs on the Zalman are 32 ohm loads. When you do
that, only the elements tied to the green plug work properly.
The others outputs are too weak, due to the 600 ohm drive.
Higher impedance is weaker, so the 600 ohm output is weaker than the 32 ohm
output available on the green jack.
http://c1.neweggimages.com/NeweggImage/productimage/26-501-002-02.jpg
To solve that problem, Zalman made their own sound solution, which
has three 32 ohm jacks on it, perfect for the job. The problem
could also be solved with three stereo headphone amplifiers (so
all channels matched, and each pair of channels had its own
mechanical volume control knob).
A typical stereo system, has 10000 ohm input impedance, a relatively
high impedance. That does not significantly load a 600 ohm output
or a 32 ohm output. So any amplified computer speakers, or your
home stereo, will not cause the amplitude of the signals to be
reduced. But, if you try connecting 32 ohm headphones, to all the
audio jacks on the back of the computer, the 600 ohm ones will not
give satisfying volume.
So the best case scenario, is an audio jack is rated for 32 ohms.
The signals like "Center/LFE" can be weaker, like 600 ohms.
If you look at a datasheet for the sound chip itself, the
actual output impedance of some of them, is around 1 ohm.
But that doesn't mean you can use the chip for arc welding -
the chip might go into current limiting, if you attempt
to abuse it, and the result would be a heavy dose of
distortion. The 1 ohm output impedance is a small signal
kind of rating, more of academic interest than anything
else. The 32 ohm rating is the one to pay attention to,
as that one is a "full power" rating.
As long as your vendor uses amplified equipment, with line
level inputs using 1 volt RMS levels, you should be fine.
There are more expensive sound cards, consisting of
things like Wolfson DACs for output, and some of those,
lack a buffering element on the output. The thinking in that
case is, adding an opamp to the output, would degrade
the fine specs of the Wolfson. The problem is, the DACs
in that case, have extremely high impedance outputs,
of perhaps 100 K ohms or higher. The instant that any
kind of electrical load is connected to them, the
frequency response of the signal is shot, and so is
the amplitude. Even connecting your stereo directly
to such a device is a mistake (the output would sound
like a $5 AM band radio). Those require a preamp
of some sort, to match the 100 K from the DAC, to the
heavier 10 K loading of a stereo system. That is a
pretty extreme case, but is to illustrate that there
are some pretty goofy sound configurations out there,
if you look hard enough. What you find in a Dell,
is not likely to be like that.
How long has your vendor been in the business ?
HTH,
Paul
