jeffmeh;368171 Wrote: > Thanks Dave and Clive. Buffer to boost, attenuator to attenuate.... I > missed the obvious notion that the impedance mismatch is not always a > case of a source with output that is too hot for the target input.
Impedance is almost never at issue in this case. Maybe I can clear up some of the confusion: ** Line-level interconnection (ie analog RCA or XLR cables): You should have a low impedance output (eg 100Ω) driving a high impedance input (eg 10,000Ω).* The exact numbers do not matter, you just need the receiving end to be much larger (>10x) the transmitting end. You are NOT trying to accomplish "impedance matching" here, nor are you trying to achieve "maximal power transfer". Instead, the goal is to have a stiff/strong source driving a weak/loose input. The goal is to transfer information (low frequency voltage), not power. (Notice that I have not mentioned anything about signal levels, more on that later). ** High frequency connection (eg s/pdif, video, UHF, etc): Here all interconnecting parts must be impedance matched.* That basically means that any cable, connector, or input must "look like" a 75 ohm resistor from the perspective from an incoming wavefront, for the brief period of time that it takes that wavefront to propagate through the thing. If any connector or cable does not have a matched impedance, the signal becomes distorted, resulting in for example blurred video. The goal is to ensure that 100% of the energy emitted by the transmitter is received by the receiver, not because transferring power is the final objective, but because it is necessary to maintain signal integrity. ** Power amp driving speakers: here you want an extremely low output impedance driving a load of 4 to 8 ohms.* An amplifier's output might be labeled 4Ω for example, but this does not mean that the amplifiers output impedance is 4Ω. It means that it's designed to drive a _load_ of 4Ω. You have to "match" the load to the amplifier in the sense that you are connecting the correct amount of load, but this is NOT what is meant by "impedance matching". In this case the goal is not "maximal power transfer", but efficient and sufficient power transfer. If the amp were actually 4Ω you would find that is gets very very hot. The outputs should be much less than 1Ω. (It may surprise you that "maximal power transfer" and "maximal efficiency" are not the same thing, nor are they even consistent with each other. That's a little more than we need to get into here, but you might find a hint by contrasting the goals of high frequency transmission). So what happens in each case if you get it wrong? Line levels: if the receiving device has a low input impedance (in which case it is probably defective) or if the transmitting device has a high impedance, what happens is that the signal becomes attenuated - like a weak person arm-wrestling a strong person, the change in arm position (voltage) is reduced for a given amount of exerted force on the part of the weak person. This mainly just results in a reduction in volume, but may also increase distortion depending on the characteristics of the transmitter. Using passive attenuators increases the effective impedance of a source, but in most cases should not cause problems. High frequencies: if the impedance of the receiver is too high, the signal will overshoot and kind of slop all over the place. If it's too low, it will undershoot because energy is being reflected back to the transmitter. In either case you get a distorted signal. In situations where you must interface with a different impedance, you can use transformers to make them match. Power amp: If the speaker impedance is too high (not stiff) then then you would have to turn the volume up much higher to increase the output voltage until you reach a reasonable listening level. Even at low volumes you may have problems if the amplifier is not seeing the load it expects. Imagine swinging a sledge hammer into a sheet of styrofoam - it doesn't stop like the way want it to. On the other hand, if the speaker impedance is too low (stiff) this can also damage the amplifier because it will be trying to drive much more current than it wants to, for a given signal level. Tube vs solid state amps differ here in that tubes are more likely to be damaged driving by too weak of a load, whereas solid state is more easily damaged by too strong of a load. Back to the original subject of this thread... taking the above into account, can anyone suggest a practical scenario that would benefit from this product? I can't. If a line level source device is too weak to accurately drive a >10K input, then it is quite simply defective. -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=56068 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
