Tech 21 Double Drive

[aziltz]

Has anyone build or studied this? Its not obvious to me what is generating the clipping.  The A/B stage obviously has clipping diodes in the feedback, but the A stage has none of the obvious distortion generating components.  Does the IC itself clip?  Also, it looks like the BIAS isn't at 4.5, its lower, why is that?

[MikeH]

Are those diodes connected to the Jfets at point 'A' supposed to be conected to anything?

[aziltz]

Are those diodes connected to the Jfets at point 'A' supposed to be conected to anything?

well yeah, thats how its switched (the jfets).  the entire switching circuit isn't shown.  i'm just concentrating on the two gain stages on the top row

[sevenisthenumber]

Is it cascading a into b?

[aziltz]

Is it cascading a into b?

A into A/B?  I thought maybe, but the A/B Drive Pot attenuates the input to the 2nd gain stage.  I'm just confused where the Drive A clipping occurs.

[Alman]

The 'A' part is just like the MXR Micro Amp/Distortion +  -- DOD overdrive 250 type of drive stage. When the drive A is increased the volume will exceed the opamp supply rails (biasing), I believe, and the opamp will distort or will boost the front end of the second stage which will clip it with the diodes and 220k in its feedback loop. The 220k in parallel with the diodes is there to create asymmetrical clipping. If this stage was the last going to an amp it would probably have lots of volume.

As far as the bias not being 4.5v at vb2 I am not too sure of. Hopefully someone can expand on that.

I hope I made some sense.

Al