help would be appreciated

[zyhpheigh]

Hi all, i could use a bit of help with a couple of questions i have.  i have looked all over but cant really find the answers im looking for.  ive been making pedals on and off for a little while and each one that ive made i have followed a schematic to the letter. now im to the point to where i really want to know what im doing without going to electronics school or learning a bunch of formulas and stuff that will muddy the water.  first is on buffers, im working on a buffer,booster, compressor, and noise gate pedal, each of the schematics im looking at modifying have input buffers, do i need all of the buffers before the stages or will the first buffer stage be fine?  second im looking at the mosfet boost over at AMZ i want a boost pedal but i would love to be able to switch between a treble boost, a mid boost, and a full signal boost as cleanly as i can.  in biasing a jfet the input cap and the biasing resistor create a high pass filter, where would that be in the schematic?  http://www.muzique.com/schem/mosfet.htm here is the link for the schematic.  AMZ also have the rc filter calculators so if i can just figure out where the filter is in the schematic i can modify it to the frequency response i want.  thanks guys, any help is appreciated

[R.G.]

...now im to the point to where i really want to know what im doing without going to electronics school or learning a bunch of formulas and stuff that will muddy the water. 

Aye, there's the rub. Those forumulas and stuff ARE the water. It's not possible to really know what you're doing without knowing some of that.

first is on buffers, im working on a buffer,booster, compressor, and noise gate pedal, each of the schematics im looking at modifying have input buffers, do i need all of the buffers before the stages or will the first buffer stage be fine?

It depends. In general, those individual pedal circuits were designed to work with a guitar at the front. The output of each section is not a guitar, as it has a different impedance. Generally you can leave them out.  For why, you gotta learn formulas.

second im looking at the mosfet boost over at AMZ i want a boost pedal but i would love to be able to switch between a treble boost, a mid boost, and a full signal boost as cleanly as i can.  in biasing a jfet the input cap and the biasing resistor create a high pass filter, where would that be in the schematic? 

C1/R3

AMZ also have the rc filter calculators so if i can just figure out where the filter is in the schematic i can modify it to the frequency response i want. 

It takes almost as long to type it into a calculator as to do it in your head. The significant frequency of every single-R, single-C is 1/(2*pi*R*C). Worse yet, using a calculator means you'll probably never go back to learn what the calculator is doing for you.

Did anyone ever watch "The Hungarian Phrase Book"?

[brett]

Hi
those are some great aims and questions.
The simplest question and answer first: The rc filter ("one pole") for a jfet booster is created by the capacitor leading to the gate, and the resistor (the "bias" resistor) from gate to ground. If you do the maths - cutoff frequency = 1/(2.Pi.R.C), you'll usually get a number in the hundreds of kilohertz. Often a 0.1uF cap is used when a 0.01uF would do.
I use a simple story to help me think about how an RC filter works. Imagine a signal going in though the capacitor and getting to the junction with the resistor. At what frequency would half of the signal (3dB) be lost through the resistor? It's when the 'resistance' through the capacitor is the same as the resistance through the resistor. But depending on the frequency of signal, the signal "sees" a different amount of "resistance" in the capacitor. The frequency at which the impedance of the C equals the impedance of the R is 2.Pi.R.C (which you can work out from the formula for the impedance of C, and knowing the value of R).

Concerning buffers - you're right that the most important buffer is the first one. It's about having a lower output impedance than the input impedance of the next device.  The output impedance of a guitar pickup is high, so a buffer (high input impedance) is handy after it. But the output impedance of pedals is low compared with their imput impedance, so pedals can follow pedals with few issues. Exceptions to all of this are the few pedals with high output impedance, and pedals like the fuzz face with low input impedance, which gives them a "sag then bloom" tone.

Hope that helps a bit.
cheers

[Kipper4]

I love this forum.
I hope to understand circuits one day to the poimt of designing my own.
This is gold right here. I cant pretend i understood everything you put but i think i got the gist.

[zyhpheigh]

So if r3 (10m ohms) and c1 (.001uF) form the rc filter, if I wanted to change both of those would changing r3 affect the bias of the transistor.  10m ohms is pretty high input impedance, so If I lowered it would that affect how much gain I can get out of the transistor?

[Kesh]

Hi
those are some great aims and questions.
The simplest question and answer first: The rc filter ("one pole") for a jfet booster is created by the capacitor leading to the gate, and the resistor (the "bias" resistor) from gate to ground. If you do the maths - cutoff frequency = 1/(2.Pi.R.C), you'll usually get a number in the hundreds of kilohertz. Often a 0.1uF cap is used when a 0.01uF would do.
I use a simple story to help me think about how an RC filter works. Imagine a signal going in though the capacitor and getting to the junction with the resistor. At what frequency would half of the signal (3dB) be lost through the resistor? It's when the 'resistance' through the capacitor is the same as the resistance through the resistor. But depending on the frequency of signal, the signal "sees" a different amount of "resistance" in the capacitor. The frequency at which the impedance of the C equals the impedance of the R is 2.Pi.R.C (which you can work out from the formula for the impedance of C, and knowing the value of R).

a high pass filter in the 100s of kHz is definitely not a good thing for audio

and half the signal, when we're talking volts, is -6dB. -3db is 1/sqrt(2). approximately.

[therecordingart]

You need to learn the math. Without that you won't really know what is going on in a circuit. I'm about 6 months away from finishing up a two year EE degree, and the whole damn thing is math and triangles.