O is correct - when we speak of the impedance of a source, we are talking about what is effectively a series resistance "in" the source. For a power amp driving speakers, you want it as low as possible. See diagrams here: http://www.kpsec.freeuk.com/imped.htm. Also, highly recommend reading: The Art Of Electronics.
But getting back to the subject of line-level audio, there's an example here if we suppose we're using passive attenuators, i.e. a resistor divider. Let's say you have a typical 100 ohm source and you want to divide its output voltage level in half. One way to do this would be to put a resistor divider on the output, say two 500 ohm resistors (for a load of 1K). Now you have an output with a higher impedance because the source is not as "stiff", it's going through this high resistance that you've added in order to create the voltage divider. However, it's not exactly half, because the divider itself is also acting like a load, pulling down on the source's 100 ohm impedance. The voltage division is effectively as if you have a 600 ohm resistor and a 500 ohm resistor. But also, the new output after the divider has a higher Z because the load is being driven through that additional resistance of the divider. Alternatively, since there is some impedance in the load, you could just hang a single resistor to ground on the output, and this will create a voltage divider in conjunction with the source's 100 ohm impedance. But now the source is working much harder because it has to drive that relatively low impedance, not just the amplifier's high Z of 10K ohm. This will significantly increase the distortion level of an op-amp output. If you've followed all this so far it should make sense why you'd want a low source Z and a very high load Z for a line level signal. You're not trying to transmit gobs of power as with a speaker output, nor are you trying to deal with reflections as with a high-frequency signal. Instead, you are just trying to get signal information (a voltage level) from A to B as accurately as possible (low noise, low distortion). This is best achieved by a "stiff" or strong (ie low Z) source feeding a high Z load. The reason line-level sources have a 100ohm impedance instead of close to 0 is to provide some short-circuit protection for the output. And in Transporter, this impedance is in fact provided by a series resistor. By contrast, for speakers, you are trying not just to move information, but to actually get work done, and for that to happen your load must have a fairly low impedance: a few ohms, instead of many Kilo-ohms. Otherwise, the voltage required to make the load do anything would be ridiculously high. Also, now that you're dealing with significant power transfer, you want a source impedance as close to zero as possible, because otherwise a bunch of energy is being wasted in the source. -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=46347 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
