Does the input and output buffer dertermine what the circuit is for?

[nguitar12]

Let say I want to design a pedal for vocal mic. I just need to adjust the input and output buffer to suit the impedance of the mic. Is it correct? If so how to I calculate it?

[Kipper4]

Amz is your friend there Google amz impedance.

[merlinb]

Let say I want to design a pedal for vocal mic. I just need to adjust the input and output buffer to suit the impedance of the mic. Is it correct? If so how to I calculate it?

In general, a naked buffer will have a very high input impedance and very low output impedance.
You can always make the input impedance lower than what the naked buffer provides, by connecting a resistor between input and ground, thereby deliberately 'loading down' the mic with whatever it wants to see.
You can always make the output impedance higher than what the naked buffer provides, by adding a resistor in series with the output, although there is rarely any need to do this.

[nguitar12]

Read the "AMZ Impedance"

In general, the ideal situation is to have a low output impedance connected to a high input impedance in order to pass the best fidelity signal across the audio frequency band.

So why don't make every pedal a "almost infinitely high impedance" by connecting a almost infinitely high resistor between input and ground?

Also how the circuit identify the resistor between a pull down resistor that prevent popping and the resistor that decide the input impedance?

[merlinb]

So why don't make every pedal a "almost infinitely high impedance" by connecting a almost infinitely high resistor between input and ground?

Inevitably, you always need a resistor to provide a stable bias voltage to the opamp / transistor or whatever. There's a limit to how big you can make that resistor and still have stable and reliable bias. So, that resistor sets the upper limit of the input impedance. Sure, you can make it smaller if you don't need the highest possible figure, but then, you'll need a bigger coupling capacitor instead. It's a compromise.

Also how the circuit identify the resistor between a pull down resistor that prevent popping and the resistor that decide the input impedance?

Those resistors (and any others that happen to be there) are doing different jobs and serving different purposes, but they all load down the source. It's up to the designer to juggle these various resistors to get the overall total (parallel) resistance that he wants to load the source with. One resistor may serve the dual-purpose of preventing pops and dominating the input impedance.

[induction]

So why don't make every pedal a "almost infinitely high impedance" by connecting a almost infinitely high resistor between input and ground?

Keep in mind that any additional resistor between input and ground will be in parallel with the input impedance of the rest of the circuit. If that input impedance is low, then adding a high value resistor in parallel will not increase the input impedance, it will decrease it just a little. The higher the parallel resistor value, the less the reduction in input impedance. A 'truly' infinite parallel resistor would be no resistor at all, which is what you started with.

[nguitar12]

How do I measure the output impedance of my guitar pick up?

[hymenoptera]

It's easy to measure the DC resistance of the guitar pickup. I've used a multimeter set to ohms, with leads clipped on to the T and S terminals of a brand new unused 1/4" TS plug. Volume all the way "up". Be sure to consider pickup selector and wiring configuration! It ain't perfect, but it'll give you a rough idea.

I think what you really want to know though, is what would your target input impedance be? 100k to 1Meg input Z is typical for guitar use.

[antonis]

It's easy to measure the DC resistance of the guitar pickup.

To get a little further, it's also easy to find cap's value (sometimes by just unscrewing the back plate..) and calculate it's capasitive resistance at some frequencies (say 82Hz to 1k1Hz for foundamental tones), combine it with DC resistance and get an idea of guitar's output impedance width..