Input buffer for ROG Ruby amp

[jhherren]

I don't have any Jfet transistors handy. Can I use a TL072 as mentioned on http://www.muzique.com/lab/buffers.htm and on the datasheet http://www.ti.com/lit/ds/symlink/tl072.pdf (single supply unity gain amplifier)?

[tonyharker]

TL071 would be better unless you're making a stereo.

[GibsonGM]

Yes, you sure can use a '72, but you'll be 'wasting' an opamp section. You should terminate that properly if you decide to use only one section, but there's no harm in doing so.   It's just not 'elegant', and as tony says, the '71 is a nicer part to use if finding one is easy for you.   Electronically, no issue... I'd have no qualm at all about doing that if I didn't have a single opamp available.

All you need to do is connect the unused section's output to its " - " input, and take its " + " input to Vref.    The other section can then be used how you wish (ironically, also in a buffer configuration just like that!)

BTW, welcome to the forum   

[PRR]

The JFET is part of the distortion.

Maybe too much so. Input overload varies wildly across different JFETs (even of the same type number).

The opamp will work fine though "clean". The only distortion will be when you smack the output LM386 to the rails. Which is of course how most guitar amps distorted before this trend for master volumes and preamp distortion.

[jhherren]

Appreciate the advice! Didn't know about needing to terminate the unused side. I don't mind underutilizing a chip while I wait for little pink baggies to arrive.

And thanks for the welcome! I've only made a couple pedals at this point but it's shaping up to be a nice hobby.

[GibsonGM]

Appreciate the advice! Didn't know about needing to terminate the unused side. I don't mind underutilizing a chip while I wait for little pink baggies to arrive.

And thanks for the welcome! I've only made a couple pedals at this point but it's shaping up to be a nice hobby.

Sure, that's what I figured. I'd use a dual for a one-off, something I'm playing with on breadboard, but wouldn't plan around doing that, lol.  It's clunky.   If you were a pedal seller, people would look inside and make fun of you for doing it, tho not harmful.  And as paul says, the buffer will change the character of the Ruby...but worth trying.   Keeping in mind that TL071 and 72 can't be swapped into an IC socket on a made PCB, of course as they have different pinouts... 

Here, you'll want to use the method under the smiley face:

[antonis]

You may also use the unused part of op-amp as a pre-gain stage (just 'cause it is there..) to "simulate" the distortion part of the JFET buffer..
(as Paul said..)

[GibsonGM]

You may also use the unused part of op-amp as a pre-gain stage (just 'cause it is there..) to "simulate" the distortion part of the JFET buffer..
(as Paul said..)

Or use a mosfet boost.   

[antonis]

Or use a mosfet boost.   

How peculiar for your Highnenss to say so, Sir...!! 

BTW, it should be an interesting mod, despite phase reversal..

[Steben]

Even a BJT stage can work well. As in Rangemaster style: You can bias it just over the base emitter knee voltage, to get the most of the assymetrical transient. Or as in a DS-1 without decoupling cap: the gain gets reduced AND the input impedance gets higher which can be handy.