Buffers and pulldowns.

[m_charles]

Hi,

My question is this:

say you had something like a tonebender or big muff etc. You decide to add a jfet op-amp (tlo71, 72 etc.) buffer at the input of the circuit. Would you still need the input pulldown for pop prevention at the buffer's input, or does the buffer make the pull-down obsolete?

Bonus question! What's the difference between a jfet, like a J201 and a JFET op-amp, like a tlo71?

Thank you so much in advance!

chuck
I'm so much learning....

[jakehop]

Hey Chuck, let me try to answer these (I'm a "student" myself):

The pull down resistors are there normally to prevent any floating caps from popping when true bypassed (e.g. when one leg is lifted). Some are in the same place for ground reference, and in some way to set the input impedance of the ciruitry. If your buffer has no cap at the input, a pull down resistor would not be needed. However, the input would not be DC-isolated, so that probably wouldn't be the case for the circuits floating around here.

An operational amplifier, is a small circuit itself - IC, integrated circuit. When we talk about FET opamps, the idea is that the first active stage in the IC itself is a FET transistor, as opposed to a BJT transistor. Think of the IC's as circuits enclosed in a little black box.

Hope this helps, and more importantly that I'm not making this stuff up.

Kind regards, Jake

[brett]

Hi
good answers Jake.

Usually, we want to keep things simple.  So unless a circuit already has an op-amp chip in it, we prefer to use a JFET for the buffer because we only need 5 new parts - an input capacitor, a gate-grounding resistor, the JFET, a drain resistor, and a source resistor (and an output capacitor, but this have been there as the input cap, so we are only re-naming it).  JFETs often give pleasant distortion if driven hard, which is a bonus.

An IC buffer is more complex, but can be used if there is a "spare" op-amp in a multi op-amp chip (e.g. dual or quad), or if fancy filtering is wanted.
cheers

[m_charles]

THANKS for your help guys.

ok, heres one more:

If you build a buffer onto the input of an effect, would you put a pulldown in between the out of the buffer and the in of the effect?

also,

when you have a polarized out-cap on the buffer out, does that keep the out cap of the buffer from running series, and pulling down the value of the buffer out-cap and the effect in-cap?
Say, 1uf polarized -out from buff, to .47uf effect-in-cap= .31uf?

sorry for all the questions....

chuck

[brett]

Hi

If you build a buffer onto the input of an effect, would you put a pulldown in between the out of the buffer and the in of the effect?

If they are always "tied" together, with no switch in between, I doubt whether you'd ever need a pull-down resistor.  This is because the impedance to ground of the output stage of buffers and other effects is quite low (typically 10 or 20 kohms - check the size of the drain resistor or collector resistor), and therefore any charge on the "left" side of the "linking" cap is dissipated quickly.

Re: output cap and input cap

Just have one cap, and make it the usual input cap for the effect.  This becausze the cap "fits" the input impedance of the effect.  e.g. whopping great caps for low impedance (2.2uF for the 20k fuzzface) and tiny caps for JFETs and ICs (often 0.01 to 0.1uF)