Cornish Buffer Help

[mickeybellinello]

Hi to all,

I'm going to built a looper for completing my pedalboard and I got the idea to put a buffer on it.

Making some research on internet I discovered the Cornish Buffer which has a lot of positive feedback.

I searched for schematic ecc but I found big differences about components value.

Taking this schematic:

http://4.bp.blogspot.com/-CxlGXAjoT1Y/Ty2FtTFpWRI/AAAAAAAAA3o/DBq3PPr4ddE/s1600/cornish_buffer.png

and making a comparison with the relative perfboard layout you can see big differences:

http://4.bp.blogspot.com/-fMYuy0RSqu8/T0Iv5T4z0RI/AAAAAAAAA-U/rB4BC5uLekw/s1600/CornishBuffer.png

So, do you have any verified schematic or layout for this circuit?

Thanks

[spoontex]

Hi,

take a look on this and compare:


https://www.dropbox.com/s/miogjh7i0t42j30/cygnus_documentation.pdf?dl=1

It's from Aion Cygnus.

[antonis]

Also..
http://effectslayouts.blogspot.com/2016/03/cornish-buffer.html

BTW, if you trace:
http://4.bp.blogspot.com/-fMYuy0RSqu8/T0Iv5T4z0RI/AAAAAAAAA-U/rB4BC5uLekw/s1600/CornishBuffer.png

you'll result into:


which is what you posted:
http://4.bp.blogspot.com/-CxlGXAjoT1Y/Ty2FtTFpWRI/AAAAAAAAA3o/DBq3PPr4ddE/s1600/cornish_buffer.png

with addition of D1 (PS reverse polarity protection diode), R1 & C1 (PS LPF) and R10 (Out stability issues..)

P.S.
>I discovered the Cornish Buffer which has a lot of positive feedback<
More politically correct should be "Raising input impedance by bootstrapping voltage divider bias resistors"

[mickeybellinello]

Thanks to all for reply...

I see even here some different schematic.

For example c4 in the picture posted is 4u7 and in cignus diagram is 22uf. Why That?
Same thing for r8 and r9 (in the posted picture r9 and r10) 150r and 50k against 51r and 1M. In this case in the first schematic I posted r8 (r9 in the picture posted) is 51R and there is any resistor connected to ground.

What kind of changes that different values mean?

Thanks.

[antonis]

I see even here some different schematic.

For example c4 in the picture posted is 4u7 and in cignus diagram is 22uf. Why That?
Same thing for r8 and r9 (in the posted picture r9 and r10) 150r and 50k against 51r and 1M. In this case in the first schematic I posted r8 (r9 in the picture posted) is 51R and there is any resistor connected to ground.

What kind of changes that different values mean?

It's all up to particular designer point of view..
About caps value: It's everywhere the same..
(4μ7 for bootstrap and 22μF for output caps..)
Bigger boostrap cap makes more effective boostrapping for low frequencies where bigger output cap lowers high-pass filter corner frequency..
51R resistor is set to 150R together with 50k shunt one just because..
(it's more good design practice than the one initially posted by you - which I presume it's "cut" from a bigger circuit..)

[garcho]

Sometimes values change RC filters. Follow the current --> capacitor then resistor to ground = high-pass filter. resistor then cap to ground = low pass filter. Often these lauded circuits get over analyzed in respect to component values. Changing the 150R to 51R will do very little, probably nothing. Usually, designers who know what they're doing add something like that to give the semiconductor a little protection from unknown circumstances, like jamming your guitar cable into a light socket or something. Or a faulty cable sending your output to ground, more realistically. Often, 100K is the value of the resistor to ground at the signal's output. I'm sure there are forces at work I'm ignoring when I say this, but basically, it's just to drain the final output coupling cap so it doesn't pop during switching. Considering the final coupling cap is 22u, 51K will drain it quicker. Maybe R8 is there to roll off some muddy low end? Not sure why R8, R9, and R10 are in that configuration. Either breadboard, or pick some values, solder it up, and leave yourself working room on the board to de-solder and re-solder values, listen by ear and take notes. Or socket those resistors and caps and then solder them after you've determined what sounds best.

You should seriously consider using a very basic op amp buffer, not this. Op amp buffers will preserve your pickups tone, prevent high frequency loss, sound transparent, are extremely simple, have less components which means most likely less hiss/noise, and can provide enough current with a low-impedance output to deliver true guitar tone to whatever pedals you're using. Anyone who tells you op amps "sound" a certain way doesn't really understand audio electronics. You can always find an example of a magician making an argument about one thing or another but you've never heard a Cornish device that hasn't gone through a studio or FOH board. What's inside of a Neve? A million dollar SSL broadcast console? A bunch of NE5532s. A buffer shouldn't color the sound or it's not a buffer, it's a preamp. Why would you want to color ALL the sound going to every pedal? I am definitely not questioning Pete Cornish's specific concerns with his specific devices for his specific customers, but don't over think this. No one has ever heard a record or seen a concert and said to themselves "Damn, too bad that guitar player isn't using Cornish's emitter follower buffer instead of that crappy op amp buffer, sounds awful!" And any sonic value to using the Cornish buffer will be gone once you play through a few pedals.

[mickeybellinello]

Sometimes values change RC filters. Follow the current --> capacitor then resistor to ground = high-pass filter. resistor then cap to ground = low pass filter. Often these lauded circuits get over analyzed in respect to component values. Changing the 150R to 51R will do very little, probably nothing. Usually, designers who know what they're doing add something like that to give the semiconductor a little protection from unknown circumstances, like jamming your guitar cable into a light socket or something. Or a faulty cable sending your output to ground, more realistically. Often, 100K is the value of the resistor to ground at the signal's output. I'm sure there are forces at work I'm ignoring when I say this, but basically, it's just to drain the final output coupling cap so it doesn't pop during switching. Considering the final coupling cap is 22u, 51K will drain it quicker. Maybe R8 is there to roll off some muddy low end? Not sure why R8, R9, and R10 are in that configuration. Either breadboard, or pick some values, solder it up, and leave yourself working room on the board to de-solder and re-solder values, listen by ear and take notes. Or socket those resistors and caps and then solder them after you've determined what sounds best.

You should seriously consider using a very basic op amp buffer, not this. Op amp buffers will preserve your pickups tone, prevent high frequency loss, sound transparent, are extremely simple, have less components which means most likely less hiss/noise, and can provide enough current with a low-impedance output to deliver true guitar tone to whatever pedals you're using. Anyone who tells you op amps "sound" a certain way doesn't really understand audio electronics. You can always find an example of a magician making an argument about one thing or another but you've never heard a Cornish device that hasn't gone through a studio or FOH board. What's inside of a Neve? A million dollar SSL broadcast console? A bunch of NE5532s. A buffer shouldn't color the sound or it's not a buffer, it's a preamp. Why would you want to color ALL the sound going to every pedal? I am definitely not questioning Pete Cornish's specific concerns with his specific devices for his specific customers, but don't over think this. No one has ever heard a record or seen a concert and said to themselves "Damn, too bad that guitar player isn't using Cornish's emitter follower buffer instead of that crappy op amp buffer, sounds awful!" And any sonic value to using the Cornish buffer will be gone once you play through a few pedals.

So finally you suggest to me to make an opamp buffer like the one in the Klon Centaur for example?

[garcho]

Yes, that would be the way to go, in my opinion.

How will you do switching? One thing to consider is how to drain the input coupling capacitor when disengaged so it doesn't pop when you do engage it. E.g., you switch over, and "pop!", the capacitor discharges its stored current into your speaker. Maybe you're familiar with that, but just wanted to point it out because the Klon example doesn't have a resistor that will discharge the input cap. Try something like this, and consider R3 and R6 to be optional:



sorry to hijack the thread

[antonis]

IMHO, Gary's buffer might exhibit a couple of issues..

Voltage divider resistors should be of much lower value and their junction should be connected to non-inverting input via a high value resistor (1M say..)

Except from noise issues with such a high value resistors, there also is Vref stability issue due to lack of regulator capacitor..

By making R2 & R3 10k, R4 1M and connecting C1 right leg directly to non-inverting input, things should be much better..
(INPUT HPF corner frequency should not alter..)
(Well done..!!)

edit: (on revised schematic) R6 value should be in the range of 470R - 1K, just to limit op-amp output current in case of circuit's OUTPUT connected on veeeeeery low impedance load..
(low values of R6, 47R - 100R say, are mainly set for "isolation" purpose, in case of mixing signals (op-amps in parallel..)

[garcho]

I totally agree with antonis, i sketched that up too quickly. Let me amend it.

EDIT: I changed the schematic. Surely things can be different but good enough place to start.

[mickeybellinello]

Thanks for schematic and infos...so the last schematic is a good built?

[antonis]

Good is less than better which in turn is less than best..

But, yes..!! You can proceed to it as a plain transparent buffer..
( if you think 1.6Hz IN & 16Hz OUT HPF corner frequencies are OK for your demands..)

[marcelomd]

Opamp inputs look reversed. Or did I miss something?

[antonis]

 

You didn't miss anything...!!
(we missed the obvious..)

P.S.
Nice oscillator, Gary..

[garcho]

ARRRRGGGG! sorry, was a rough day.

[marcelomd]

Happens to all of us =)

By the way. I've seen a lot of circuits include a low valued cap to ground somewhere after R3 to filter high frequency noise. 33-100pf.

[garcho]

The schematic is fixed. was having EagleCAD problems since I updated. For some reason I couldn't remove nets, or find my old libraries. Anyway I truly am sorry for flubbing something so simple and making it confusing.
Yeah, input and output coupling cap values could be altered. 100N is very standard around here, as is TL072. I haven't seen "a lot" of circuits with the HF bleeder cap at the input buffer stage. I'll keep my eyes open. The point of this whole embarrassing exercise was that a simple op amp buffer goes a long, long way. Simple was the point, until I messed that up. 

Maybe edit out the incorrect schematic, just to make this thread clearer? My mistake is textually embedded here for eternity, might as well make the graphics right.

[garcho]

.

[garcho]

R6 value should be in the range of 470R - 1K

Assuming max output of 20mA, and TL072 200 Ohms output, a 100 Ohms resistor would make maximum 30mA, which is maybe 10mA too much, am I slaughtering it again? 470 Ohms is ~15mA, 5 lower than max?

[antonis]

R6 value should be in the range of 470R - 1K

Assuming max output of 20mA, and TL072 200 Ohms output, a 100 Ohms resistor would make maximum 30mA, which is maybe 10mA too much, am I slaughtering it again? 470 Ohms is ~15mA, 5 lower than max?

I've the bad habbit to always consider op-amps output impedance almost zero..

So, for full +/- 4.5V swing, output current should be limited to slightly less than 10mA for 470R load..
(of course, it could be double but many people don't feel comfortable designing an op-amp working at its maximun current rating..)