Buffer Impedance

[ziotasgiorgos]

Hello I am currently building my pedalboard and I was wondering if there is any way to be sure if the pete cornish buffer I want to make does indeed have 1mohm input impedance. Also If anyone knows what is the output impedance supposed to be?

[GibsonGM]

It's not hard to measure, but you'll need some kind of AC signal, a 1meg pot and decent DMM (at minimum)

[Vivek]

If you post a schematic

We can help by calculating the impedance

Or simulating on SPICE

[antonis]

https://www.diystompboxes.com/smfforum/index.php?topic=124166.0



Just add R2 (10M) in parallel with Zin below ..

[ziotasgiorgos]

Antoni I don't understand any of this mate. I just make pedals like a monkey. Monkey sees, monkey does. :/

[antonis]

Ρε συ Γιωργο, post your particular Cornish buffer schematic and we'll tell you about its input impedance..
(and output one, of course..)

edit: No welcome, guys..?? 

[ziotasgiorgos]

Ρε συ Γιωργο, post your particular Cornish buffer schematic and we'll tell you about its input impedance..
(and output one, of course..)

edit: No welcome, guys..?? 

The first one is supposed to be the schematic the vero is build on. I obviously build the vero version. It has some differences though.
Thanks in advance btw

[antonis]

Your layout is more close to the schematic posted above..
(with the addition of 100R Collector resistor.. you may short it or let it be there..)

After a rough estimation, input impedance is about 1M (strongly dependent on particular BC549 h_FE) and output one is about 40R..

P.S.
I should strongly recommend BC549C..
(or BC549B, at least..)

[Vivek]

I know how to say "Welcome to the group" in Greek

My Greek friend taught me

https://youtu.be/Yb5rKv8MTeA

[GibsonGM]

well, the in/out Z is known and given, right?  So, we either accept that it's true, or obtain it empirically as I hinted at above....and Welcome

[ziotasgiorgos]

Your layout is more close to the schematic posted above..
(with the addition of 100R Collector resistor.. you may short it or let it be there..)

After a rough estimation, input impedance is about 1M (strongly dependent on particular BC549 h_FE) and output one is about 40R..

P.S.
I should strongly recommend BC549C..
(or BC549B, at least..)

Thanks re Antoni. I think I will stick with this buffer then. Everybody seems to recommend it and I really can't hear any audible diff.
Thanks again m8!

[ziotasgiorgos]

well, the in/out Z is known and given, right?  So, we either accept that it's true, or obtain it empirically as I hinted at above....and Welcome

Thank you for your time and welcome
I am going to read up on how you do that. Google is my friend

[antonis]

Of course, you can implement any (more simple) op-amp FET input buffer with practically no audible difference..

A dual op-amp could serve as an IN & OUT buffer for your pedalboard..

[GibsonGM]

well, the in/out Z is known and given, right?  So, we either accept that it's true, or obtain it empirically as I hinted at above....and Welcome

Thank you for your time and welcome
I am going to read up on how you do that. Google is my friend

Yes, not hard to find.  You input an AC signal, and add resistance at the DUT input until you get 1/2 the voltage that you put in...so DUT impedance is the same as the pot setting (each share 1/2 the voltage...). See bottom link.

https://www.diystompboxes.com/smfforum/index.php?topic=87001.0

HERE is the rig you need to do it:
https://craiganderton.org/how-to-check-input-impedance/