Sparkle Boost input impedance question

[oliphaunt]

I have been messing around with a circuit based on the Sparkle Boost, and I am wondering about the input impedance. Can anyone tell me what the input impedance is and how to calculate it to potentially change it?

[Minion]

It looks like 1m

[PRR]

The input of a happy naked JFET is "infinite" for our purposes.

If the "0.022u" cap is passing audio (this scheme will pass everything above 8Hz), then we may consider it a "short" for practical purposes.

Then we have just 2.2Meg in parallel with 1Meg. About 688K. High-enough for any stage-amp purpose except maybe Piezos.

> potentially change it?

If you want a lower impedance, you may just reduce the "2.2Meg".

The highest practical impedance with unselected JFETs may be "2.2Meg" changed to 10Meg and "1Meg" changed to 5Meg, about 3.3Meg effective impedance.

Why does it matter? 500K-1Meg is a happy value for most inputs. 50K-100K has some magic for swamping resonances, just put 100K or 68K across what's already there.

[Derringer]

I have been messing around with a circuit based on the Sparkle Boost, and I am wondering about the input impedance. Can anyone tell me what the input impedance is and how to calculate it to potentially change it?

first you need to recognize, as PRR stated, that the input impedance is the parallel combination of the 2.2 meg and 1 meg resistors.

To calculate, you would plug the values into the equation:

__R1*R2__
   R1 + R2

[oliphaunt]

Great information, thanks!

[CynicalMan]

If you were planning to change the impedance, you'd want to change the input cap so that its value multiplied by the value of what's now the 1Meg resistor stays the same.