[quote author="j.hall"]now you mentioned a 2 octave bell.....i don't want that. i understand how Q works. i want 1/3 octave bell with the center frequency at 50Hz.[/quote]
Since the bandwidth of a bell is usually defined as the width between the -3dB points, if you want a 1/3 octave bell, there would be 1/6th of an octave above the center freq. and 1/6th octave below the center frequency (to the two respective -3dB points). -Simplifying it a bit and assuming that the slope has settled into a reasonably steady section of the curve, if the signal drops by 3dB in a sixth of an octave, you'd be looking at a slope of six-times-three dB per octave... in other words 18dB/8ve. (in practice, owing to the flat section at the center frequency and the shallow slopes either side, the true response slope will in fact be rather steeper, more like 24dB/8ve, but let's go with 18 for now...)
This is a long way from the current slope of (assuming 12dB max boost and 2 octaves before the curve returns to flat) 6dB/8ve.
I have a few sphere EQs here and looking at the circuit I have to say you're probably not going to be able to do it. -that circuit is performing according to the nature of how that design operates. You have to used a much more resonant circuit in order to get a sharper slope. It's going to sound vastly different (but then.. you want a vastly different slope and thus a vastly different EQ, so nothing conflicting there).
I'd say don't waste your time changing the sphere. If you want it to work like a single band of a 27-band graphic EQ (for example) or even better, a single band of a parametric EQ, you're going to find it a lot simpler, cheaper and more serviceable to just build a seperate circuit to do just that, and leave the Sphere alone, not to mention available as an alternate LF EQ for the times when such a narrow bandwidth isn't what you want... (which is usually more often than not: The beauty of the Pultec for example, is that it's broad, gentle LF curves boost 'warmth' and low end without bringing attention to any single 'ringy' point.
If you're looking for a 'ringy' filter to add more 'note' to a kick drum (for example... one of the few occasions when it's more common to boost the LF at a narrow 'Q' setting in conventional EQ use) then sometimes it's even more helpful to have a parametric EQ, since the amount of 'ring' -as well as the musical note on which it is centered- can be adjusted.
The idea of adding a bell switch is a good and very useful one, and CJs suggestion will work, but if you want super-tight, narrow boost, then the spere is entirely the wrong topology to try and modify in my experience.
If you like the way the sphere sounds now, keep it as-is, and build a specialised tool for the specialised job. :grin:
Keith