So it's been a while since i've posted here, so i thought i'd actually post something worthwhile! here is the first episode of Apocalypse Audio's CIRCUIT SPLINTERS. i hope to post little circuits that can be helpful when designing your own stompboxes- like gain stages, tone controls, filters, clipping arrangements, etc. i'll try to keep the things i post relatively new, because we all know places like geofex and amz have this kind of shit locked down, and i really don't want to post a bunch of circuits that you can find elsewhere on the more venerable stompbox sites on the net. most of the stages i'm going to present are things that i've culled from the various books and websites that i have found, and occasionally something that i have come up with or modified to my own liking.
the first circuit i am going to present is the high impedance input JFET/BJT buffer circuit below.
this is a rather common cookbook circuit that you can find in books and on the net, yet personally i have yet to see it in stompbox circles, so i thought i would post it. it uses an npn JFET DC coupled with a pnp BJT . the FET allows a a much higher input impedance than a BJT normally would. R1 could realistically be anything from 1M to 10M, this will only really affect the low end coupled with C1(which i have left open for you to decide the value). the BJT is "inverted" so that a pnp transistor can be used with a negative ground circuit, for what i presume is for it's lower noise specs. i haven't measured the output current of this configuration versus a single FET buffer, but that could be something i'll do in the future and report back. the real reason i am posting this buffer is because it can easily be turned into an amplifier stage, which can subsequently be used for a sweet booster/overdrive. all that and more in the next installment of CIRCUIT SPLINTERS!
bonus question: anyone want to tell me where to place the resistor in order to turn this buffer into an amplifier?
here is some info from RG Keen on the circuit-