A few questions about bacon bits

[9Lives]

Hi everyone, I looked at the schematic I downloaded a while back and just made a small vero layout because latley I'm very much obsessed with op amp gain stages. So intresting to me. Can some one explain a lil bit what a rail to rail power supply is exactly? I know it provides both + and -. But what does this do? What benefits does it have? Usually you want higher voltage than the peak of your pickup signal  for max head room. I can't rap my head around this but I absolutely love the way this booster works and sounds compared to the typical fet/transistor boost. How does +-9v give head room? Wave form I'm thinking. Would this circuit benefit from 12v power supply?

[mgwhit]

You're correct about headroom.  +/-9V is essentially the same as +18V in terms of head room that the wave form has before clipping -- it's still 18V -- you just don't have to superimpose the incoming signal on a reference voltage the way so many 9V pedals do.  This circuit could have used the charge pump to provide +18V, but then a +9V reference voltage would have been required on the op-amp inputs.

Check out this picture.  In the split-rail power supply you have positive and negative voltages determining the upper and lower bounds of the wave.  In a typical +9V pedal, the upper bounds is +9V, the lower bounds is 0V and the middle line is a +4.5V reference voltage, typically created by a voltage divider directly off the power supply and labeled VB.

Every electronics book I've ever read has shown split rail power supplies to op amps, so when I first started learning about op amps in 9V effects I had to figure out what was going on with the voltage divider and the VB reference voltage I saw in all of those classic pedals.  Of course, I was also looking at those same books and wondering where the heck I was going to get the negative voltage for the lower rail they showed in every circuit.

Hope that made some sense.

[madbean]

Just to add to mgwhit's comments. In your average op-amp overdrive you have a feedback setup where you attempt to maximize the gain of the opamp to create overdrive. Op-amp clipping tends to be harsh and square, so we add diodes either in the feedback path or clip the signal to ground afterward to soften the edges and give it a more natural sound.

In the case of the BB, the goal is not so much to generate overdrive in the circuit, but lots of clean boost to drive the front end of an amplifier and goose into its own natural (and superior) overdrive. The split rail set up gives us a larger amount to "swing" the guitar signal so that we get the boost without the harsh clipping of the op-amp. It actually does not have a lot of gain. Rather, it has just enough output to drive the amp without getting brittle or harsh.

It uses split rail rather than an 18v setup because that allows one to use ground as a reference voltage to supply the bias of the op-amp rather than having to use additional resistors to create a voltage divider off an 18v supply. Reducing the part count lets one make the PCB footprint smaller---critical for 1590A builld.

[nzCdog]

Interesting!  I like learnin new stuff

[9Lives]

this is valuable info yall. That just closed such a major gap for me. I was wondering when I first made this (since I cheated w/ the vero) I kept checking voltage and wondering why there was no 18v+ and why there was no bias. Clearly I was misunderstanding the point of bias to begin with. How about one more question. How come it still clips some what at 9v. I still can't get max setting out of the circuit w/o some clippage. I DO have very hot set up. But when I increased it to 12v it fried my tlc272 and then I read the specs.. (lil backwards huh.. Story of my life) I then switched to opa2134 and it sounds good and has less clipping. When I use a compressor in front of it I get max boost. Either way I love this project b/c it gave me some key principles yall helped me out with.

[mgwhit]

Are you sure it's the op-amp clipping and not the front end of your amp clipping from the boosted signal?

[9Lives]

good point... No I'm not sure.. Lol that could actually be the soarce of alot of my probs. This amp has given me head aches. Kustom quad65dfx. I've done som mods. But nothing to the clean channel. It definitely clips w the active pickups. How could I improve this?