Resistor values in buffer circuit.

[Buffalo Tom]

Input impendance? Is it 500K here because of R1 and R3 in parallel. So for 1M input Z these resistors should be 2M?

Is R4 doing something good or not needed?

How about value for the volume pot? If I use 100K instead of 10K what would the difference be?

[ElectricDruid]

Input impendance? Is it 500K here because of R1 and R3 in parallel.

Roughly, yes. In reality, both of those resistors are also in parallel with the input impedance of the op-amp, but we can assume that is too high to worry about and ignore it.

So for 1M input Z these resistors should be 2M?

Yes.

Is R4 doing something good or not needed?

I don't see why it is needed. There needs to be *some* resistor to ground after the cap to drain any charge on the cap and prevent pops when switching the circuit in and out, but R5 and the volume control can do that job, so R4 is unnecessary.

How about value for the volume pot? If I use 100K instead of 10K what would the difference be?

It'd affect the output impedance when the pot is lower (at low volume you could have 9K in series with the op-amp output, say), but practically it won't make much odds unless you're feeding something with a really low impedance. It'd also lower the cutoff of the highpass filter formed by C2/R5+Volume, but given that 22uF sets that cutoff stupid-low already, it makes no odds there either.

Now my turn for a question: What are R6 and R7 for?

[Buffalo Tom]

Now my turn for a question: What are R6 and R7 for?

Thank you for explaining. On most buffer schematics Ive seen its usually some series resistance in 50-500ohm range on the output. Using two resistors on each side of the switch was something I picked up from the output section on a Cornish pedal.. I also think the switching was more quiet this way.. but maybe that was just imagination.

Cornish pedal output section

[antonis]

Is R4 doing something good or not needed?

It's usually set there to reduce output distortion at small signal voltages..
(and contributes to out HPF corner frequency..)

About R5/R6 :
https://electronics.stackexchange.com/questions/156441/how-does-adding-an-output-resistor-isolate-an-op-amp-from-a-capacitative-load

[PRR]

....usually some series resistance in 50-500ohm range on the output. ...

On the output of the amplifier. You already have that at R5.

How often does a "buffer" need a Volume control? I can think of marginal cases but I'd be sure of myself before buying pot and drilling hole.

[ElectricDruid]

Using two resistors on each side of the switch was something I picked up from the output section on a Cornish pedal.. I also think the switching was more quiet this way.. but maybe that was just imagination.

Pete Cornish uses a very idiosyncratic switching method on his pedalboards, since he doesn't like true bypass (and he has some decent reasons why not for the situations his work gets used in - http://www.petecornish.co.uk/case_against_true_bypass.html). Instead, he keeps the inputs of all pedals connected at all times, and then switches *only the output* between the clean signal and the effect signal. No-one else has done it like this since the late 70's, I don't think. It used to be fairly common because it only requires a SPDT switch. Since this being effective requires that the pedals all have a high input impedance and are driven by a low impedance source, Cornish sticks buffers *everywhere*, often between every pedal on the board. Lucky for him his buffer design has a good reputation (it's a bootstrapped BJT, nothing particularly magical).

I suppose what I'm saying is that his weird use of two sets of output resistors probably stems from his unusual switching arrangement and doesn't necessarily apply in other situations.

[antonis]

it's a bootstrapped BJT, nothing particularly magical.

Says the guy who tends to copy-paste any badly disigned BJT buffer..

P.S.
You know I'm kidding Tom as well as you know I'm absolutely right..

[ElectricDruid]

it's a bootstrapped BJT, nothing particularly magical.

Says the guy who tends to copy-paste any badly disigned BJT buffer..

P.S.
You know I'm kidding Tom as well as you know I'm absolutely right..

LOL! Fair cop!

In my defence, I haven't used a BJT buffer in years. There's one on the Flangelicious, but I think that's the only time.