Op-Amp buffer - Mod question

[jammybstard]

I've built the splitter Blend:


I'd like to Mod the buffer send stage so I can switch in some gain.

This is what I was thinking:



Does that work?

[Gurner]

Yes, that'll work....just bear in mind if your opamp is single supply, then you'll need a DC blocking cap in your path to ground ...which might make for some interesting thuds or clicks when  you flick the switch.

[jammybstard]

Cheers Gurn.
It is yes!
What's a typical value for that cap?

[Gurner]

That would depend on the resistor vaules you're using, but normally about 4.7uf will get you in the ballpark.

[jammybstard]

I'm guessing 1/(2Pi.R2.C) comes into this somehow?

[Gurner]

I'm guessing 1/(2Pi.R2.C) comes into this somehow?

You'd be right ....   http://people.ece.cornell.edu/land/courses/ece4760/ideas/singlesupply.pdf figure 1,     BW2

[ashcat_lt]

Or connect R2 to your Vref instead of ground.

If you go with R2 and the cap, you can just leave that connected and simply short across R1 to turn off the gain.  This defeats the voltage division which causes the gain and avoids possible popping from switching the cap.

[jammybstard]

Or connect R2 to your Vref instead of ground.

If you go with R2 and the cap, you can just leave that connected and simply short across R1 to turn off the gain.  This defeats the voltage division which causes the gain and avoids possible popping from switching the cap.

Good point!

[Thomeeque]

Left 1M (+ input bias) must go to Vref as well of course.. T.

[familyortiz]

Keep your eye out for any oscillations in the case the switch is open. For example, I've seen buffers which oscillate with 10k feedback. The situation is usually fixed by lowering the value, but of course will lower your available gain.

[jorri]

just wanted to reopen this to add that some gain and attenuation mods are on this site: http://seanm.ca/stomp/bblender.html#gain and ask whether they would work ok for the OPs schematic.

How would i go about adding them to the return stage of the wet signal? i see there is an extra resistor in there to do with phase switching.

My intention is to have pads and boosts to run at rack level just from the wet send to the return, or in this case with the boost on u2a to run a pedal efficiently in a line-level effects loop.
there's more about that here:http://www.uneeda-audio.com/pads/ but it seems best to stick to the b. blender values as source impedance is what matters.

[Gurner]

Just an observation about that schematic (http://seanm.ca/stomp/bblender.html#gain )...

If 1% or better tolerance resistors are used, then I reckon OPa, OPc don't really need output caps.....& then OPb wouldn't need the 0.1u input cap nor the 1M resistor....OPb will be biased by OPa & OPc ....any (minimal) DC voltage differential at OPa & OPc outputs will be catered for by the blend pot.

[jorri]

anybody?

can i just put a 10k pot in parallel with the 220k thats between pins 6+7 on the top right op-amp. with a 1k resistor to Vref for a gain control?
why is the 220k resistor there and not on the b.blender? is it to do with the phase switching?

[Gurner]

anybody?

can i just put a 10k pot in parallel with the 220k thats between pins 6+7 on the top right op-amp. with a 1k resistor to Vref for a gain control?
why is the 220k resistor there and not on the b.blender? is it to do with the phase switching?

Ok, you were confusing me (your links point to the bblender, but you're talking about the schem at the top of this page)

No, you can't put the pot where you want (to control gain), well not without whacky results should you flick the polarity switch. All those 220k resistors are needed to ensure the same output level when switching the signal polarity.

If you don't want the polarity switching, then just lose the 220k resistor connected to pin 5,  you're left with a fixed inverting opamp setup, you could then have a variable gain arrangement by using what ever potential divider setup you prefer from pin7 feeding back to pin 6 (couple with the another resistor to Vref......or another alternative....a DC blocking cap to ground)

[jorri]

thanks- i gave the b.blender links because it mentions the gain mod...

I thought something like this would be the case, so i can save time now and just stick a simple boost in- or convert that 2sk117 stage into a boost somehow.