can some1 explain topology of boss od3 to me??

[swt]

Hi guys. Just trying to understand how the distortion is generated in a boss od3 and boss blues driver. In the od3 there's a pair of diode clippers in a feedback mode, but in a strange ( at least to me :?: ) differential or something, fet configuration. I'm cooking with different stages of distortions and wanna give this one a try, but also wanna understand the way it works. Thanks a lot for your wisdom!

[WGTP]

There was a discussion some time back about that distortion and it may have been on the old forum, I don't remember.  I think there was a discrete op amp in there somewhere.  Might look back in time.  

[Transmogrifox]

Yeah, the FET diff pair and the followin npn BJT gain stage form a discrete component operational amplifier, more or less.  Notice the familiar looking feedback network composed of the pot, 22k R, 47pF cap, and the 1.5k R(or is it 15k?) in series with the .015 uF cap.

After the first "op amp" stage (which does likely clip some), they put it through a typical 3-band eq circuit that is set basically hard wired to what one would consider a desired sound.

After this, the signal is clipped by the double diode head-toe pair (though mildly since there is a little gain loss through the eq circuit), then fed into the second "op amp" stage.

This stage, of course, introduces some clipping, which is then filtered by the following stage and the tone stack.  

Finally, there is some limiting introduced by the diode pair across the + and - terminals of the opamp, which i assume is a soft limiting.  Notice it is dependent on the volume level.

That gyrator and cap look like they're set to boost the mids, probably a power-amp feed-back resonance simulator.

It looks like all of the stages contribute some limiting/clipping.  I would say all of the stages introduce hard clipping.  The distortion you hear first is probably from the diode pair.  As you crank up the gain, you hear more hard clipping from the discrete op amp stages limiting, though I believe the second discrete stage is likely clipping at any gain setting since it's being driven by the first stage.  

I'm assuming that JFETs are used for the purpose of high input impedance as opposed to their noted "tube-like" tone.  In this feedback network, you would never hear the non-linear effect of the JFET since the output is already limited to one of the rails before the FET stages actually get significantly nonlinear.  

The key to this pedal's sound I would think is the use of multiple clipping stages.  You could probably make an equivalent circuit by replacing the discrete op amps with regular op amps. Just use an op amp that limits more gracefully like a TSH22 or an OP275.

[Gilles C]

Check this page to understand the reason they used that design.

http://www.rolandus.com/products/details.asp?catid=3&subcatid=13&prodid=OD-3

If you compare the sound of the OD-3 to the other Boss pedals that use op-amps, or the the other Boss pedals that use discrete circuits, you will be able to note the change in sound.

For example, they used the same configuration in the DS-2 pedal, with not exactly the same way...

http://www.mif.pg.gda.pl/homepages/tom/files/DS-2.gif

What I like with Boss is that have so many designs that you can compare how they sound all in the same company.

Ok, I admit, I love Boss pedals  

[swt]

Thank you all guys for such a detailed info...