tv,
I was naively hoping the "humbar" to be the nulling technique as you described on previous page (with unity signal being fed into the "casing") ...
It's not really a "nulling" technique. By driving the shield from a low impedance node, you get the best junk reduction you can from a high impedance series-connected humbucking pickup. The reason the drive is in phase with the input is so it doesn't load the humbucker coils with more capacitance. The signal bounces up and down with the desired input signal, so the effective capacitance is roughly zero. Probably one of the luminaries around here can explain it better (probably in terms of Miller capacitance or some such). Since the pickup shield is attached to a low impedance node, it works a tad better at keeping the EMI\RFI out and shunting it away from your preamp.
In that one particular bass I have older bart stacked j's. They are pretty sensitive for CRT and similar noise.
I doubt you can improve on that very much. CRT noise is pretty intense if you are close to one. Unless you replace the barolinis. Stacked pickups reject hum OK but they can't do much to reject that kind of noise source. Maybe Bartolini makes a side by side "hotrail"-type design.
I wouldn't order a special smt filter do-dad. Just run some magnet wire through a ferrite bead a few times, maybe do the differential trick. Put it in series with the input and hang some capacitors on both sides. It will work as well as and probably better.
It's just not going to get much better with CRT noise and high impedance.
@ Peter,
Thanks for posting the article.
The op amps in the real deal Alembic I worked on were those old-timey bipolar op amps with the adjustable bias terminal - I forgot the number. TI thought it was a great idea for a while. Get this, they were in
flat-pack packages - remember those? (Sorry, tv, no 4558 mojo.) LOL. The whole instrument was pretty darn quiet for those days by virtue of the JFET inputs.
The magazine schematic looks about right for the tone control except the Alembic took the feedback from the op amp output directly to the 6800pf cap, not through the output capacitor. Plus of course the previous mentioned snafu tv corrected in his follow-up diagram.