GGG jfet buffer questions

[manson]

I've built the GeneralGuitarGadgets jfet buffer, and it works fine. I have a few questions though to help understand this little circuit:

I read at AMZ that the impedance of this circuit is R1 paralleled by R3 (500k). Does the first 2m2 resistor (anti popping?) have any effect on the impedance as well?
Second, the output capacitor is a 10uF electrolytic. Is is critical to use a polarized cap here? And why the value of 10uF?

Thanks
Peter


Buffer schematic link:
http://www.generalguitargadgets.com/pdf/ggg_buffer_jfet_sc.pdf

[RonaldB]

Yes the value of 2m2 has an effect on the impendance of the buffer.

The formula for this is R=(2200 * 500) / (2200+500) = 407.4 So that's your new impendance.

The value of 10uf is to let al frequences through, this can also be 1uf and there is no need for it to be polarised.

Ronald

[petemoore]

I read at AMZ that the impedance of this circuit is R1 paralleled by R3 (500k). Does the first 2m2 resistor (anti popping?) have any effect on the impedance as well?
 Try different impedances out to see, anytime a resistance to ground is introduced, it is a pathway for signal to ground...ie an alternate destination than input.
 In the overall equation, 2m2 [big] is an 'inconsequential' value [doesn't lower impedance by much], anything over say 1meg is probably inconsequential to signal making it to the input.
    Second, the output capacitor is a 10uF electrolytic. Is is critical to use a polarized cap here? And why the value of 10uF?
 Non-polarized capacitors have no polarization to observe.
 10uf won't attenuate LF's, smaller values might...put whatever value you want there.

[manson]

Thanks for answering!

Ok, so I can also use a 1.0uf MKT type cap in place of the 10uf without problems, also letting all guitar frequencies through?

And to get a higher input impedance, I could use a 3m3 for the anti popping resistor, and 2x 2m2 for R1 and R3. This would calculate to: (3300 x 1100) / 4400 = 825.0
Using 3x 2m2's would lead to 733.33 ... Or is this overkill? I read that an 1M input impedance is considered the optimum...

[manson]

By the way, what would happen if I put another resistor, say 33k between the pulldown and the input cap (r4/c1 in the schematic)?

[RonaldB]

I would surgest you should just leave the anti plop resitstor out. Then you have a perfect 1M impendance.

[manson]

True, but that might lead to, well, popping

And what if there was another resistor between the pulldown and the 2x 2m2's?