The Eminence bass
I believe the average bass amp's bass tone knob boosts everything below a given frequency indiscriminately. It swamps the low-end with much that is unwanted, so something more refined is required, but a graphic eq is too frequency specific. 2
Instead, perhaps, I should compensate for a small speaker cabinet with a boost of low frequencies which are sympathetic to it's roll-off characteristic3 (speaker specific),
...but then roll-off the boost below the bass range to reduce speaker strain4 (instrument specific),
...and then reduce the sustain on my bass which is attributable to VLF acoustic feedback5 (venue specific).
1 headphone output. 6
2 a parametric eq could be used to test various ideas. 7
3 fixed Chebyshev ripple boost. 8
4 Chebyshev/Butterworth roll-off. 8
5 tunable bass notch filter. 9
6 achievable with a single op-amp.
7 but I object to the lack of information/graduation on the knobs.
8 complex maths which I have yet to understand better.
9 maybe also an adjustable Chebyshev ripple boost/roll-off.8
At least some of the above has to be on-board to keep piezo cables short: an onboard preamp runs on batteries, and battery life is largely dependent on the complexity of the circuit, or "how many op-amps?"
My current preamp has one op-amp, with negative feedback centred somewhere around 60Hz, and proven to have not quite enough gain for average use. If I re-locate the piezo onto the bass body, the output will be much lower, requiring a higher gain, preferably shared with a second stage. I should also include the anti-feedback notch-filter at around 795Hz, which adds another op-amp. The headphone output will require two additional op-amps for stereo output, though I may only use one earpiece; after that, options need serious consideration. Four op-amps and counting, so the rest go under a separate heading now.
A tunable bass notch filter requires a quad op-amp circuit, with a single knob adjustment. Somewhat elaborate for on-board the bass, but this is best for accessible tuning. A quad op-amp still counts as four op-amps though.
"Chebyshev" gets three mentions, which will probably end up as one op-amp. The adjustable one includes a bulky dual-potentiometer, and needs a second stage for best results. Two op-amps then.
I'm a bit squeamish now about having two knobs trying to do the same thing different ways, perhaps some modelling would help the decision.
Piezo to be mounted mid-body (how?), so the first op-amp should be nearby to keep the input cable short. The headphone socket could ideally be near my head in the top of the bass body, so this commends another op-amp (or two) to the bass-body. Suddenly everything appears to be heading inside the bass, so I'll have to work out where to put the battery cage, to give me the best access for surgery. Prototyping will proceed in an external box for now.
The dual-pot Cheby and the anti-sustain notch filter:
As it is currently only my untested assertion that it could be useful, I think I'd prefer to stick it in it's own box with it's own battery. Perhaps then a second knob for volume. It might even wait to see if indeed there is still a sustain problem once the other bits are in place.
To retain as much low frequency content from the piezo pickup, a short cable is the most important part, since it dictates that there has to be a preamp on-board the bass. Having established this, I now have to choose how much of a preamp belongs on-board, and what can be left out.
Version 1, 2 and 3 . . .
The single FET preamp ( just Google "J201 preamp" for further info), with small gain and poor bass response, followed by variants of same, fiddling with extra components, still left wanting. Not worthy of further comment.
The 5534 single op amp, as seen at Megalithia, using the feedback loop shown here, but also some RF filtering.
I've been using this for a year, while I figure out the V4 tailpiece. I can recommend it as a basic preamp, the battery lasts forever, and I could have left out the volume control. Components can be selected using these calculators.
The piezo on the bridge has a howl or two, with or without the preamp. It suffers feedback at about 795Hz, and a lesser squeal at 1590Hz. (Chromatic instrument tuners on mobile phones are cool!) I expect to notch the lower one out, as it causes feedback grief at gigs. Then see if the higher one is still a significant problem. Interesting number, 10000/4Π = 795.775Hz approximately. Why it should appear on my bridge I don't know, but it makes the components I need easier to write down.
From CEPD's notch filter page, a notch filter with Q = 8, or bandwidth of ( 795.775 / 8 ) Hz, looks like this.
Though such a filter can be much more elaborate, it is only to enable the fine tuning of f0 or notch depth, if components cannot be hand-picked for accuracy. This really belongs on the bass as part of the in-built sound shape. It will be interesting to see if the second piezo, once fitted, has the same feedback issue.
Before I get to build the full-featured preamp model, I have to get the essentials together and leave the rest to my amplifier. Read more...