PPCB 3PDT Board With Output Boost

[Ginsly]

Well, this looks pretty useful!

It has a trimmer which sets the level of boost, but I can see making that an external pot in certain situations.

Since it's IC-based, I assume there's no way to kinda mess with the bias of this gain stage..?

 

[lowpitch]

You could mess with the resistors in the voltage divider that establish vref but why would you? Maximum headroom and stability will be achieved by biasing to vref.

 

[Ginsly]

You could mess with the resistors in the voltage divider that establish vref but why would you? Maximum headroom and stability will be achieved by biasing to vref.

Oh, absolutely - this would be if I didn't want a straight-up clean boost but wanted to mis-bias the added gain stage for effect. For instance, there's a fuzz called the Baron Samedi which tacks a transistor-based boost at the end of a Bosstone, and by changing that third transistor's bias you can get some very interesting sounds.

I'm not certain this can be done with an IC, but would love to know!

 

[Robert]

If you want a fuzz on a 3PDT board we can make that happen....

 

[lowpitch]

Oh, absolutely - this would be if I didn't want a straight-up clean boost but wanted to mis-bias the added gain stage for effect. For instance, there's a fuzz called the Baron Samedi which tacks a transistor-based boost at the end of a Bosstone, and by changing that third transistor's bias you can get some very interesting sounds.

I'm not certain this can be done with an IC, but would love to know!

Well, vref will usually be 1/2 vcc. There's no schematic, but I'm guessing the two 1M resistors form the voltage divider that sets vref to 1/2 vcc. Change one of the resistors to another value and the reference voltage will be offset from the usual 1/2 vcc. This would allow the op amp to swing asymmetrically and also clip asymmetrically. This may or may not sound good.

 

[Ginsly]

If you want a fuzz on a 3PDT board we can make that happen....

That would rule the school, of course! I recently made a stripboard Boss T-Bone and it's the size of a postage stamp, so... it can go anywhere!

I guess what I mentioned essentially exists at PPCB as the Muffin Crumb Amplifier Stage, but not as a 3PDT board. I was thinking something like this but using a BJT instead of a JFET. Still, 99% of people just want a clean boost on a fs board and wouldn't mess around with the crap I'm talking about. This new board looks great, and I noticed the In/Out Jack pads on the "wings" too - nice touch!

Well, vref will usually be 1/2 vcc. There's no schematic, but I'm guessing the two 1M resistors form the voltage divider that sets vref to 1/2 vcc. Change one of the resistors to another value and the reference voltage will be offset from the usual 1/2 vcc. This would allow the op amp to swing asymmetrically and also clip asymmetrically. This may or may not sound good.

Yep I was looking at those 1M resistors... hmm. Just don't know how that kinda thing works with ICs.

 

[lowpitch]

Yep I was looking at those 1M resistors... hmm. Just don't know how that kinda thing works with ICs.

When you drive an op amp beyond its output swing range, 0V to 9V for example, the output signal will start to clip. More gain = more clipping. Less supply voltage = earlier clipping. You bias to 1/2 VCC, 4.5V in this case, to ensure the output can swing symmetrically around this midpoint because it will maximize the available range and thus clean headroom. When you intentionally misbias the op amp, the output will swing asymmetrically, meaning one side of the output signal will hit the power rails sooner than the other and thus also clip sooner. This will create asymmetrical distortion.
You should be aware, op amp clipping is kind of an aquired taste and doesn't always sound very nice unless the circuit is really built around it. That's not going to change much by misbiasing the op amp either.