From: Kalkor (kalkor@kalkor.com)
Date: Thu Mar 18 2004 - 16:21:34 MST
<Walter>
Are there any electronics experts around here. I need to know if connecting
the sound cards from 2 different PCs through a Y-cord into 1 sound system
(external amp, sub, etc.) will cause any crosstalk or impedance problems. I
thought about using a switch, but I would rather not.
BTW, I'm not sure what crosstalk or impedance really mean, but I know
enough
to suspect they MIGHT be relevant.
Thanks in advance for any advice,
Walter
<Kalkor>
So, I'm going to assume you're talking about the SPEAKER OUT line on your
sound cards, the one that's labeled GREEN. This is not a line-level output,
it is a higher amplitude to drive speakers. It does not require a
high-impedance input to plug into, as it is designed to go into an 8- or
16-ohm speaker system most likely. (8-16 ohm is low impedance)
So, with this in mind, you are probably safe to plug this SPKR OUT line into
the LINE IN on some piece of stereo amplification equipment, but bear in
mind that LINE IN usually wants line-level, which is a much smaller
amplitude signal, and the LINE IN on your stereo is going to be HIGH
impedance. Bottom line: turn down the sound card input before you plug a
SPKR OUT to a LINE IN.
Ohm's Law, which states that current is proportional to voltage, and
inversely proportional to impedance (or restistance), is represented as
I=E/R (I=Current in Amperes, E=Volts, R=Resistance in ohms).
To explain impedance, I'll use the analogy my first electronics teacher came
up with: Voltage (E) is water pressure, Current (I) is rate of flow, and
Resistance (R) is inverse of diameter of hole. Increase resistance, at the
same voltage, and you decrease rate of flow. Increase voltage, at the same
resistance, and you increase rate of flow.
So plugging a spkr output into a high-impedance, line-level input will only
result in less draw on the supply circuit. The opposite situation, plugging
a line-level into a speaker, will simply result in max current draw but of
course, amplitude remains the same and you wont be able to drive a speaker.
(not enough water pressure to open the valve at the end of the hose? taking
the analogy too far...)
Now, you may or may not have a line-level out on your sound card, it should
be labeled BLACK (?). If you have this, you should try using it first to
plug into a line-level in (you mentioned preamp, they want a low level
signal to start amplifying hehehehhe).
So, crosstalk. This is an induction issue, when you have modulated or
unmodulated signals being induced somewhere there are already other signals
present. Especially in a semiconductor, because then you have mixing (sum
and difference frequencies, etc) and the effect is that you can add audio
artifacts to other audio signals for instance.
This is more of a radio problem and you shouldn't encounter it.
So, the final question is whether to plug two devices into the same
line-level input. Since the input is high impedance, you should not have a
problem with excessive current draw from the supply devices. However, you
now have three devices hooked together in parallel, one high impedance and
two possibly low? The devices have the potential to damage each other. My
suggestion would be to couple both signals into the input via inductance (an
audio transformer) to prevent a problem. Or better yet, do like I do in the
living room:
I have one cheap bookshelf stereo system with one stereo input. If I'm using
the puter to watch movies in there, I plug the puter into it. If I'm using
the dvd player or game machine or something else, I plug THAT in there. I
have to mess with the volume control on my devices of course; some are
line-level, and the computer is spkr out. Oh well, just a teensy bit of
extra work ;-}
I hope my rambling makes a little sense; I just finished finals and indeed
am completely DONE with my courses now.
See you guys around some!
Kalkor
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