From: "Paul Vandenbulcke" Date: Wed, 14 Jan 1998 09:30:29 +0100 Subject: Re: high and low (about impedances)
- -----Original Message-----
>From: "Romain CHAMBONCEL" >Date: Tue, 13 Jan 1998 04:56:49 -0700 >Subject: high and low > >I read archives again on mics and amps and I'm a bit confused with impedances. >What are the effects of impedance mismatch (high in low-Z and low in high-Z) ? >What typically are Z types of guitar boxes input and output ? >On a guitar amp, what typically are impedances types of LINE and INPUT inputs ? and >on a PA ? >What are the bads of low-Z mics ? >How, if not specified, can I know if a mic is low or high-Z ? >Plugging my mic in an overdrive pedal and then in the PA, I've noticed that, >when bypassing the effect, I had to readjust gain on the PA, has this something to >do with impedance mismatch ? >Any help appreciated to definitively clear my mind on this subject. >Romain, Saint-Denis, France.
There are several 'facts' playing when talking about impedances :
First some general information. When there must go energy from a source to a destination, men use lines between the two.
Each line has characteristic impedance. I.e. a coax may be 50Ohm or 75Ohm, depending wether it is for networks or for antennasignals.
Each source is not perfect (an ideal source wouldn't have series impedance, like a battery does : the tension becomes less as the current increases), but every source can be imagined as series combination of an ideal source with an impedance. This is called 'the Thevenin equivalent'. The imedance mentioned, is the output impedance of the source.
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. So there stays the output impedance of the driver and the input impedance of the receiver. Here you have a common resistor divider. It's in fact a very simple thing, but a little difficult to explain in 'text-mode'.
Imagine the scheme we have : we have a source (microphone) with his series output impedance and the input impedance of the receiver. The current start flowing from the source through the output and inputimpedances. So we will have a tension over those two impedances, proportional with the impedances themselves. How bigger the impedance, how bigger the tension.
The tension over the input impedance of the receiver is the usefull tension. It is this tension that will be handled in the amplifier.
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.
3. Captured noise
When both input- and output impedances are higher, the line is more 'floating' and subject to noise, caused by dimmers, interrupting currents, clicks,... A compromis is (used in audio systems) impedances of about 600 Ohms (just to have an idea).
These are some considerations. I have not the intension to be complete, but I only hope to have given you more insight in the matter.
