From: Ian Graham Date: Thu, 15 Jan 1998 11:03:04 +1030 (CST) Subject: Re: harp-l-digest V3 #265
> Each destination has an input and when you put a tension on this input, a > current will flow in this input. So, this input may be imagined by a > resistor or an impedance. This is the input impedance. > > Now, we have the consequences of this : > > 1. Reflection > > When the input impedance of the receiver is not the same as the > characteristic impedance of the cable (line), there will be reflection : a > part of the energy will be reflected through the cabel to the source. > When the output impedance of the source is not the same as the > characteristic impedance of the cable, the reflected energy wil be reflected > again. > > So it's evident that the energytransfer between source and destination is > optimal when both are adapted on the cable. That's the reason why you have > to put a special resistor divider when splitting your antenna signal to two > television sets. > > This may seem very simple, but there are very extensive theories behind this > all. > > This reflection is more pronounced with increased frequencies. For audio, > you wont have to bother. The next effect is dominant. > > > 2. Energy > > With low frequencies (<100kHz), the characteristic impedance of the cable > (wich is dependend on the frequency used) is very large, so you can almost > ignore it.
Mic cables are not transmission line cable, (i.e. 50 or 75 ohm co-ax), but merely shielded audio cable. None of the above applies.
So there stays the output impedance of the driver and the input > impedance of the receiver.
Correct.
> > In the case the imedances are matched, they are the same and so, the tension > over the two is the same. So half of the energy is over the input impedance > and usefull. > > When working with a low impedance mic and a high impedance input, all energy > will go into the receiver. The gain wont have to be set as high as in > previous case. > > When working with a high impedance mic and a low impedance input, all energy > will stay in the mic. >
Now we're getting somewhere. Except with mics and guitar/harp amps, we're not concerned with power (energy) transfer so much as avoiding a load on the mic, thereby maximising the voltage present at the amp input.
To summarise:
Lo-Z mics have low output voltage. They will work into a Hi-Z input amp, provided that the amp has a ton of gain. Lo-Z input amps will also work, providing there is enough gain. (Like mic mixers) The average guitar amp has not got enough voltage gain to be driven sufficiently with the average Lo-Z mic.
Hi-Z mics output much more VOLTAGE (not power) than Lo-Z mics. They will work into a Hi-Z input guitar amp with average gain. The reason they won't work so well into a Lo-Z input is that the low impedance input will "suck the power" out of the mic, and reduce the available voltage, as well as changing the dynamic chracteristics of the element by a damping effect.
I tried to keep this non-technical, but it pops up so often, I thought I would try to dispel the impedance myth.
