Mid scoop mod for BSIABII

[grs149]

This idea was partially inspired by Mark Hammer's "Stupidly Wonderful Tone Control" idea of a few days back, and partially inspired by my quest for a mid-scooped tone for my recent BSIABII build.

With the addition of two caps to the existing 2-pole lowpass filter after the tone control, you can add a very dramatic (albeit a bit broad) mid-scoop effect to the BSIABII. Best of all, the caps can be switched in and out of circuit without making any mods (i.e. cutting wires/PC traces) to the exising circuit, so you can easily go back to the stock tone (which, by the way, I think is amazing).

This mod basically "hijacks" the 2-pole lowpass filter and turns it into a bridged-T notch filter. I am not currenly able to post a schematic, so we'll have to do this by referring to part numbers in the original BSIABII schematic. The first new capacitor "Ca" bypasses the two 10k resistors (R17 and R18), and the second new capacitor "Cb" shunts to ground, in parallel with C13. In a true bridged-T filter, C14 would not be present but this IS supposed to be a simple mod, so we must make compromises....

Addmittedly, this mod takes a pretty big chunk out of the sound, and it eats up some gain, but my build of the BSIABII has plenty of gain to spare. I would be interested in finding a way to narrow the bandwidth of the filter a bit, but I don't yet fully understand the workings of the bridged-T circuit - and I want to keep things SIMPLE.

This mod is definitely worth a try. For experimentation, I did it with clip leads, without even plugging in my soldering iron. As a starting point, the following values for Ca and Cb produced good results (to my ears):

Ca=5n6, Cb=22n (Fc=1600 Hz)

Ca=2n, Cb=10n (Fc=3200Hz)

Further developments on this idea are welcome, and if anyone can point me toward a cheap/decent/simple schematic drawing tool, I would be greatful!

Greg Smith

[Rutherberg]

Is the schematic right like that ?
Does this modification reduce the high ends ?
I seek more low ends for my BSIAB, what do I have to modify for that ?
Thanks

Rutherberg