Metropoulos Forum

Hi there, I was wondering what you have to do to modify the impedance of a guitar signal that had just come in through the input of an amp. Thanks

I'm not completely sure what you mean by this. You are modigying the impedance constantly inside the amplifier. Basically it comes into the amp at very high impedance (500k and above approx depending on the pickups) and comes out of the amp at very low impedance to feed the speakers (ie: 4,8,16 ohms etc).

The tubes, resistors, capacitors and transformers all change the impedance of the signal inside the amp.

Rob

Here is the thing, Teese once told me that he had a knob on a custom wah he made for an asian artist in which the knob basically changed the impedance of signal, and from what I remember this is a cool thing to have if you have several different types of guitars. So I was wondering how something like this works. Thanks

Your guitar pickup is primarily resistive and inductive, your volume pot is resistive and your tone control is resistive and capacitive, so you've got a nice RLC filter going on in your guitar which is also influenced by the input impedance (yet more R) of the amp or first pedal.

You can tweak the effective resonant frequency of the pickup by varying the input impedance, which is usually the 1M (or thereabouts) to ground in parallel with the input impedance of the first triode.

Seymour Duncan uses this in the SD-1 Pickup Booster pedal to allow fattening up of single coils, RunOffGroove have a similar thing going on in their Omega Boost pedal, and others do the opposite and simulate the output of a guitar to allow a fuzz face to be used after a buffered pedal (e.g. the Psionic Audio Fuzzy Logic).

None of this is massively relevant to building a better Marshall though. Stick a 1M resistor across the input jack and you're done.