Pedal impedance and signal loss
"Impedance
matching" of pedals is not as straightforward as it might appear at first
glance.
For the record, "matched impedance" is a
wonderful thing because it gives maximum power transfer. This is
important when you are transferring substantial quantities of power around,
like in a power-amp to speaker connection.
In pedals, however, the thing we are most interested
in is signal transfer. Here are a couple of examples that look at
impedance vs. signal transfer, and show why "matched" impedance is
not what we seek for pedal connections.
Assume a guitar is plugged into a high impedance
pedal input (pedal #1) - the pickups won't be loaded down and the guitar will
perform as required (there is a lot of literature on pickup loading and the
effect on tone - I won't go into it here).
Now imagine a second pedal connected to the output
of the first pedal (pedal #2).
Situation 1: Assume that pedal #1 has a low
impedance output and that pedal #2 has a high impedance input. In any circuit
like this the maximum voltage is developed across the highest impedance in the
circuit. If pedal #2 has an input impedance 100 times bigger than the output
impedance of Pedal #1, then 100/101 of the signal will be applied to the input
of pedal #2 (and 1/101 of the signal will drop across the output of pedal #1).
Situation 2: Assume that pedal #1 has a high
impedance output and pedal #2 has a high impedance input (the output of pedal
#1 is "matched" to input of pedal #2). Again, the maximum voltage is
developed across the highest impedance in the circuit, but if both pedals are
the same impedance, then half of the signal will be applied to the input of
pedal #2 (and half of the signal will drop across the output of pedal #1)...
So what does this mean?
In situation 1 the input voltage to pedal #2 is
100/101 (or close to 100%) of the output of pedal #1, and 100% = no signal loss
with a low-to-high impedance connection.
In situation 2 the input voltage to pedal #2 is 50%
of the output of pedal #1, and 50% = 3dB signal loss with a "matched"
impedance connection.
Conclusions
A "perfect" pedal would have infinitely
high input impedance (in reality Meg-Ohms) and zero output impedance (in
reality a few hundreds of Ohms).
The Boss FA-1 is a pretty good example of this...