Understanding Impedance (2003)
A simplified way of looking at impedance for audio frequencies, is to consider impedance as resistance to AC current.<p>Ordinary resistors work because they are less conductive than the wires they are hooked up with.  So as the curent flows around the circuit there is virtually no resistance from the copper wires as long as the gage is adequate, so it is the resistor that heats up.<p>Or in most electronics (other than space heaters) in place of a plain resistor there is usually a somewhat more complex circuit with resistive properties that can do other types of work as it conducts and dissipates the energy.<p>Now a good carbon resistor has a stable value measured in Ohms, and it&#x27;s just plain less conductive than copper.  You hook it up to 9VDC and it will have a known amount of current going through it depending on its Ohms.  It&#x27;s the law.<p>Hook it up to 9VAC instead, and by law the same amount of current flows.<p>The frequency of the AC could be 50Hz, 60Hz, 1000Hz, any audio frequency.<p>That&#x27;s why they call it a resistor.<p>It resists DC and AC equally.<p>But DC is simple and AC ideally is a sine wave.<p>When you&#x27;ve got a component that has much more impedance properties than resistive, then they call it an inductor.  These components are not sold by the Ohm, instead they are specified in Henrys.  With regular (DC) Ohmeters you can measure the Ohms of a loose inductor just like a resistor and get repeatable Ohm readings from identical components.  But if you want to know how many Ohms of impedance to AC power or AC signals the component has, it will be quite a bit more than the DC Ohms, and would be calculated from the Henrys and the frequency of the AC involved, on top of the DC Ohms.<p>Some of the small inductors look about like resistors on a PCB, but the big ones are usually obviously coils.  Coils which may or may not have a ferritic core.<p>The idea here is that the wire gage of the coil will have enough diameter to allow quite low resistance to DC considering a very long wire is going to be coiled up into what often seems like an excessive number of turns.<p>Well the greater the number of turns, the stronger the electromagnetic effect due to the shape of the coil itself.<p>The DC only experiences the resistance of the lengthy copper wire to hold it back.  Thats not very many Ohms of resistance from the component even though DC current does have a strong magnetizing effect as it moves through the coil, it is a static electromagnetic field.<p>But when AC comes along, it doesn&#x27;t just freely pass through a coil steadily like DC.  Nope, in the process AC has got to effectively fluctuate the magnetic field at the frequency the AC is carrying and that can be a lot of times per second.  Just to get through the coil.  Otherwise the AC will not pass through very well and this depends on frequency.<p>So the inductor is usually a coil which in operation has low DC Ohms and higher AC Ohms, and these can be good filters for audio frequencies or hum.<p>When there&#x27;s a ferritic core it&#x27;s way more difficult for the magnetic field to fluctuate, and these can have much higher Henry values for the same size coil.  They can really choke off the unwanted frequencies, and these type of inductors are usually called &quot;chokes&quot;.  AC flowing through a coil has a demagnetizing effect on a ferritic core.<p>Since impedance comes from coils that&#x27;s where you have the most matching problems in audio.  Things like guitars, microphones, speakers.<p>The preamps, processors, amplifiers and electronic boxes usually just have resistors inside on the inputs and outputs, not coils.  Exceptions being things like high-end mic preamps having transformer input, and tube amps with their transformer output.  Since transformers are coils with ferritic cores.  Most tube amps have resistive inputs though, just like solid-state.<p>Anyway a typical preamp will have the shield of the incoming signal cable grounded, and the signal conductor itself heading to the amplifier circuit.  Any resistor connecting that signal to the ground will be allowing a certain amount of your mic or guitar&#x27;s output to be diverted to ground rather than appearing as strongly at the input of the first semiconductor.  So you lose some volume at least, but that is minimized by using a properly high Ohm resistor.  Like on a guitar amp, 1 megohm is common, not a lot of current can get through a million Ohms.  There&#x27;s not a lot of current from a magnetic guitar pickup to begin with, such a Hall-effect sensor captures very little of the moving strings&#x27; energy which then has to find its way from the magnets deep inside the coil, all the way out to the outer winding where you connect it to the preamp.  Thats a lot of impedance since your signal is pure AC even though it&#x27;s not like a steady sine wave.  So a guitar pickup that measures say 10K Ohms on an ohmmeter, often has more than a megohm of internal impedance for the originating signal to overcome.  Yep, you guessed it the pickup itself is an inductor since it&#x27;s a coil too.  This is by design :)<p>So you can usually just measure the Ohms from input to ground to determine the input impedance of an audio device, since they are just plain resistors in there and give the same ohms to your meter as they do to your signal.  You&#x27;re measuring resistance but thats virtually the same as impedance Ohms when it&#x27;s not some kind of an inductor.<p>On a line input to a preamp (rather than a guitar input) the resistor to ground will often be 10Kohm since your incoming line signal will not have such high impedance behind it, very litte signal is lost to ground.  But plug a guitar directly into that one and your signal is badly compromised.<p>De-power the source of your line signal, and the output impedance can be checked with an ohmmeter too.  Solid-state or tube since tube line preamps usually do not have transformer outputs.<p>Now capacitors have impedance too, while being very <i>high</i> in DC resistance, but that is used more so in the deeper parts of the audio circuit or power supply, so don&#x27;t get me started on that.