Not so basic buffer - newb question

[ashcat_lt]

I've got the idea to build a variable impedance guitar buffer.  The basic circuit comes from over here, the second one down (with the bias voltage circuit).  I figured I could replace R1 with a 500K static and a 10M pot to get a nice wide range of loads.  But is it really that easy, or am I missing something?

Also, is 9V enough to get good headroom out of this circuit, or should I go for more?  18V would be nice and easy.  What (if anything) would I need to change in the circuit to adapt it to 18V?

Thanks for any and all input, feedback, and advice.

[slacker]

I've got the idea to build a variable impedance guitar buffer.  The basic circuit comes from over here, the second one down (with the bias voltage circuit).  I figured I could replace R1 with a 500K static and a 10M pot to get a nice wide range of loads.  But is it really that easy, or am I missing something?

yeah that will work, depending on what you're going to use it for you might not notice much difference between 500K and 10M so you might want to make your resistor smaller.

Also, is 9V enough to get good headroom out of this circuit, or should I go for more?  18V would be nice and easy.  What (if anything) would I need to change in the circuit to adapt it to 18V?

Again it depends on what sort of signal you're feeding it. If you're plugging a guitar straight into it then 9 volts should give plenty of headroom.

[NoFi]

Sorry i might do some bit of hijacking but I don't quite understand the purpose of the 10uF on the output of those buffers.
I've built one using a 4.7 uF cap and it seems to work well blocking the DC, am i missing something ?

[slacker]

You don't want the buffer to change the sound so you use a big capacitor so all the low frequencies get through. The value isn't particularly important.

[gez]

I figured I could replace R1 with a 500K static and a 10M pot to get a nice wide range of loads.  But is it really that easy, or am I missing something?

With that much input resistance you probably won't notice too much difference.  Perhaps a little high end loss if gate capacitance kicks in with the 10M post at max?  If this were me, I'd go for deliberate loading and use a 1M pot plus stop resistor from gate to ground.  A relatively small value stop resistor - 10k? - would require a large value cap to pass all frequencies, but the benefit of JFETs is that the input cap can be omitted...though that's not always wise if you don't know what's going to drive the input.

Also, is 9V enough to get good headroom out of this circuit, or should I go for more?  18V would be nice and easy.  What (if anything) would I need to change in the circuit to adapt it to 18V?

The circuit is only a follower and output is slightly less than unity with a FET wired up a such.  As a rule of thumb, the voltage at the source will give you an idea of available output swing.  Whatever it is, double it and that will give you the max peak-to-peak signal before negative swings are clipped.  If it's not enough, use a divider to pull the gate voltage up a little, or you can use an extra resistor which connects between ground and the lower end of the gate + source resistor to shift the source upwards...though that bootstraps things too (which you might not want).  Increasing supply is a waste of time, 9V should be fine.

[NoFi]

I thought 0.1 uF was enough for not cutting the bass frequencies with the adequate resistor, so i'm a bit surprised because every buffer schem i saw has 10uF caps on the output.
The mxr booster also has, the adjusticator too.
Anyway RG answered that question in another thread about the Mutron :

Well, first off  you have to recognize that only the person who originally chose that value really knows why it's there. I'm only making guesses based on my experience with both circuits and other circuit designers' mental processes, what little they've explained to me over the years. I personally might have done it differently.

As to a smaller capacitor stopping DC, sure, it will. Let's make it 1pF. What's wrong with that? Stops DC just fine.

The problem is, of course, that a 1pF cap will only pass the audio spectrum into a 7.9 GIGAohm resistor. The smaller the cap is, the more a following input impedance must be.  A 1nF cap will pass the audio spectrum into a 7.9 megohm resistor A 100nF cap will pass the audio spectrum into a 790K resistor. 1uF is good into 79K, 10uF is good into 7.9K. And this gets us to the real question - what's the input impedance of the following equipment?

We have no clue. Could be a 1M guitar amp input. Could be a 1K Fuzz Face input. So a designer drops in whatever they think is good enough for whatever they think the equipment will encounter in most cases, and whatever the bean counters will let them use. Sometimes they flip in something from way out in left field for fun.

And that's as close to why as we can get without knowing exactly what follows the circuit.

I have my answer thanks and sorry for the hijack.