Input impedence of effects in a multi-effect board...

[guitylerham]

Hello all,

I'm knee deep in designing my analog multi-effect pedal. Here is my chain:

                                                              >Tuner
Guitar>DynaComp>OpAmp (or Jfet) splitter>TS-5>Tube Sound Fuzz>Distortion+>Stage3 Boost>
                                                               
                                                                                                                                                    >novelty VU meter
some chorus>EA Tremelo>DMM Clone>DigiVerb (using Belton Digi-log module)>OpAmp (or Jfet) splitter>board output to amp

My question: Does it make sense for me to modify the input impedences to every pedal after the splitter/buffer to a low impedence to reduce the likelihood of noise? I am still learning about analog electronics but I know that general you'd want a high input impedence for the initial guitar to see (hence leaving the dynacomp alone) but after the buffer, which will be always on, can I lower all other input impedences? I'm trying to go all out and make the most silent pedalboard I can. I plan to use an internal power supply consisting of either a single transformer or multiple ones with the correct voltages.

I still need to see if I absolutely need the 10-12 isolated power supplies or if I can daisy chain a few together. I know the DMM will require its own transformer or tap to get the higher voltage. What do you guys think so far?

[amptramp]

The noise level is set by the parallel combination of output impedance of the previous stage in parallel with the input impedance of the stage under consideration.  I prefer to use a high input impedance and a low output impedance so that if I want to change the order in the effects chain, I don't have to worry about overloading.  I also prefer to use large coupling capacitors at the input and output to permit the low frequency noise to be limited by the parallel combination of resistances.  If you have a high turnover, say 80 Hz at the input to an effect, high impedance will provide a lot of noise below that frequency, with the noise being reduced only once the frequency is high enough that the impedance of the coupling capacitor can be ignored and the output and input resistances end up in parallel.  A lower turnover will result in lower noise at low frequencies at the expense of larger coupling capacitors.  If you want rolloffs and turnovers in the frequency response, put that in the middle of the pedal where it has buffered inputs and outputs so there is no change in effect regardless of the impedances of other pedals in the chain.

If you have effects with positive and negative supplies, you can ignore the power supply to some extent - but none of yours appear to.  So plan on separate regulated linear power supplies for each unit unless you find a fortuitous combination that does not need it.  The issue is that the ground side of the output is also connected to the power supply on one side (usually negative unless you have a PNP fuzz).  Any changes in current drain will modulate the output signal by moving the ground voltage around.  The grounding through the signal cable should not be trusted since the voltage across the ground is added directly to the signal.  Separate supplies prevent current drain from modulating the ground, since no power supply current flows through it.