Ben79 wrote:It doesn't seem to matter how many times I try, I can't get my head round the real world meaning of impedance.

Impedance is reflected load or nominal source resistance. Getting the two to match up is not nearly as important as it was 60 years ago.

Take these TL07n opamps for example. A stage has an output capability rated at 20ma, and they're powered by 30 volts. Plug those *maximum* ratings into Ohms Law and it will return 1500 ohms as being the absolute minimum resistance acceptable.

Resistance is a DC measurement, the AC equivalent is impedance. In many ways they are interchangeable, unless you're talking moving coils, inductive or reactive loads - in those cases the reflected impedance will shift depending on signal frequency, and a "nominal" figure is offered.

I have reverb tanks that reflect a 20 ohm DC resistance on input, which is a coil that drives the spring. AC impedance MAY be much higher at typical frequencies, but as the frequency drops it will get nearer to a DC state and the reflected impedance will itself shift toward that 20 ohms... which in the case of a TL07n would likely cause component failure.

So: impedance is used when dealing with AC signal because often a static resistance won't cut it. It's not incredibly important inside fuzz circuits and so forth because those are largely resistive loads and impedance will only factor in when calculating curves (like EQ, etc) that are frequency related. From a frequency perspective, impedance is a curve and resistance is a straight line.

It becomes critical when transmission of power is at play, and as small as it is, a spring reverb is just a little power amp pushing a spring.