There's very little written about Jackson Spectra basses that isn't simply repeated from the ad copy, and since I've committed to make one my main instrument, here's what I've learnt so far from playing it, analyzing its electronics, and fixing some problems.
I have the mid-range 5-string model (X Series) in electric blue (which is darker than the apparent baby-blue of the online pictures). I got it in 2021 for about $700 CAD plus taxes. It's a 2019 build, and the serial number starts with "ISJ", which I assume means "Indonesia, Samick, Jackson", given the similarity to Ibanez basses also built there. I suspect it's a complete sub-contract for FMIC, as writing to Jackson requesting technical info got me a reply from Fender, who had no service manuals or other information at all.
The specs are as-advertised, with a few missing bits:
The build quality is a mixed bag, and varies from individual bass to bass, given the few I've tried in-store. Generally, the "furniture" (neck, body, frets, finish) is very good, and the hardware (tuners, bridge, pickups, electronics) can vary in quality.
I bought the Jackson Spectra because it seemed like a versatile instrument with some extra features usually only found on higher-end models: a 35" scale to help the B string timbre, compound radius (12" to 16") to enable a lower action, and a graphite-reinforced neck. I also bet on the neck-through construction requiring a higher quality control (else the whole instrument is junk), and while I don't know if that's generally true, in my case it worked out: the neck and fretwork are essentially perfect.
The stock electronics are mostly well built: good solder joints, shielded wire as much as possible, everything well-grounded except for the control cavity which has a poor, thin coat of carbon shielding paint (300 Ohms/sq) which also does not make contact with the metallized cavity cover.
The pickups are conventional series neck and bridge humbuckers, with different pole spacing to follow the strings, and a bit more windings on the bridge pickup. Coils wire pairs are green/red and white/black, with red/white as the coil tap, and black usually tied to the bare shield wire, all run through a shielded 4-conductor cable. Each pickup has its coils in reverse order to the other, so when you short the coil taps to ground, the remaining active coils are the outermost, forming a humbucking Jazz bass configuration. The Spectra sounds very clear in this mode.
Each pickup is wired as a series humbucker and has its coil tap go to one half of a DPDT switch to short out one coil to convert the pickup to single-coil, which sounds much better (not muddy), but isn't humbucking except when blended 50/50 with the other. The switched pickups then go to a blend pot (set of MN250K) with center detent.
The blended signal then goes into the pre-amp, which is completely potted with only the gain trimpot showing, which measured at 0db to +11db. There are no markings. Bass/Mid/Treble EQ pots are B100K with center detent. The EQ is flat at noon (no difference with passive timbre) and does conventional bass/treble shelving, and a mid boost/cut, at an unknown frequency point (I estimated around 1kHz, confirmed below).
Both the pre-amp output and input then go to one half of a DPDT push-pull switch attached to the volume pot (B500K). By default, the volume pot sees the pre-amp output. Pulling the switch sends the pre-amp input instead to the volume pot, which is the direct passive signal out of the blend pot. The pre-amp is not switched off in passive mode and there's a reason for that: switching on the pre-amp would bring the bias voltage at the output capacitor from 0 to 4.5V or so, and that charging of the output capacitor would make quite a bang into your amp. Normally you never see that charging since you plug into the bass, which turns on the pre-amp, before you turn on the amp. The active/passive switch still pops loudly when used, unless you turn the volume down first.
Note the complete absence of any tone cap in the signal path. This will become very important later.
The bridge will not work as-is with large tapered B strings (like a Kalium 148, or a D'Addario 145T): The saddle height screws are too short to raise the action of a tapered B string, so the B string saddle need 3mm shims under both screws. There is just enough adjustment range for a tapered E string.
The shims make the saddle intonation screw stick out so much it will touch the tapered B string when plucked. The screw must be set back with wire (or a bushing) around the thread, at the head. (This tucks into the bridge, so it's invisible.)
The slight bulge at the end of a non-tapered B string larger than about .130 will not fit through the bridge. I had to drill it out to 11/64". The bridge is NOT made of brass (only plated), but of some unknown very soft silver metal (zinc?), so drilling was very easy and neat.
I re-shielded the control cavity with Solo Music Gear's carbon conductive paint as it's very cheap, well-reviewed, water-based, nearly odorless, and has good spread and adhesion. I brushed on one coat and let cure overnight. That brought the sheet resistance from 300 Ohms/sq to 30 Ohms/sq. Once the electronics were re-installed, their additional grounding wires brought that down to 15 Ohms/sq.
I rewired the pickups for more timbral options and consistent humbucking. I replaced the blend knob with a special DPDT on-on-on switch, and connected the pickups as follows:
Then, the three switch positions create the following pickup configurations, all humbucking:
Each pickup configuration has nearly the same resonnant frequency, peaking, and impedance, which means they sound the same, except for their position along the strings.
The only downside to all these changes is that now I'm left with a "producer switch": the original DPDT coil-split switch is now unused and I'm not sure what to do with it, if anything. I eventually removed it and put a black 5/8" panel plug instead.
Having these pickups as parallel humbuckers puts their resonant frequency peak very high: They sounded beautifully clear, but also full of nasty metallic string and fret noise, all over 5 kHz to 10 kHz according the the spectrum analyzer. I analyzed the pickups and pre-amp with a Bode Analyzer on a Red Pitaya scope. It's not the best tool for the job, as it's meant for RF, but it's enough to find the resonant frequency and any boost/cut. The test signal was fed to an unshielded 100 mH inductor magnetically stuck to one of the pickup poles.
Right away, the cause of the metallic noise was evident: when wired as parallel humbuckers these pickups resonate sharply with a +21dB peak at 7 kHz, creating a large treble boost from about 3.8 kHz to 9 kHz (the +3dB points) There is no loading capacitor anywhere, even inside the potted pre-amp, as determined by the resonnant frequency not appreciably changing when the pickups were solo or in parallel.
Some measurements and simulation showed the parallel humbuckers to have 3.1K resistance, 780 mH inductance, and about 650 pF of parasitic capacitance. That's a fairly low-impedance pickup. Adding a 1.1 nF capacitor in parallel to the pickup selector switch output dropped their resonant frequency (f0) to about 4.6 kHz, above which there is little to no useful tonal signal. Then adding 15k of parallel resistance reduced the 4.6 kHz resonnance to -3dB (a Q factor of 0.707), giving a flat second-order filter response. Any peaking can be added with EQ later on. The only cost is a -1.64 dB signal loss, more than made up by the pre-amp after.
Some simple tests on the pre-amp bass/mid/treble EQ shows it to be a plain-vanilla Baxandall-like bass/treble shelving centered at close to 1KHz, and the mid boost/cut is also centered at about 1KHz. Max boost/cut in all cases is +/- 12dB. The pre-amp shelving isn't restrained, so the Bass shelf affect everything below 1 kHz, including infrasonic noise, and the Treble shelf affects everything above 1kHz, including fret noise, string noise, and external interference all the way up to 30 kHz, where I stopped measuring.
At near unity gain, the base level output of the parallel pickups, and also of pre-amp (set flat), is about 100mVpp. A perfectly normal instrument-level signal. I chose to set the pre-amp gain to maximum, giving about 300mVpp output, to minimize overall system noise. If it's too much, I can use the Volume pot.
To reduce infrasonics, I placed a 3.3nF capacitor in series between the pickup selector switch and the pre-amp input. With the 1 MOhm input impedance of the pre-amp (derived from measurements), this creates a passive 1st order HPF with the -3dB point at about 48 Hz. This filter attenuates infrasonic junk from the strings and the fundamentals of the B and E strings slightly, which very few speakers can even reproduce. A 2nd-order filter would have been nice, but that's active electronics I don't have room to put in there.
To really remove everything above 5KHz, at the output of the pre-amp I placed a passive RLC 2nd order LPF with f0 at 4.4 kHz and a Q of 1. This filter composes with the flat 2nd order pickup response (Q = 0.707) at 4.6 kHz, creating a 4th order (-24 dB/oct) filter with a Q of 0.707 (flat response) and a -3 dB point at 4.4 kHz.
This low-pass filter is composed of 1.1 kOhms in series, composed of the 1KOhm output impedance of the preamp and a 100R resistor, feeding a humbucking 43 mH inductor with a winding resistance of 105R, followed by a 30nF cap to ground.
The humbucking inductor is composed of two 100 mH inductors in parallel in a particular way: Imagine two inductors side-by-side, with the same polarity and winding direction. An external interfering magnetic field generates the same signal at both sets of pins. Flip one inductor upside down, rotate it 180deg along its axis, and place it below the first, in-line in the magnetic field. Now the pins of the inductors show opposite polarity signals. Wire them up like that, and now the interfering voltage cancels out at the pins and the interference exists only as a tiny circulating current in the inductors. There will be a small reduction in the parallel inductance due to magnetic interaction between the inductors. You now have an inductor which rejects external interference.
This filter removes even more metallic string noise, fret click, fret buzz, and some EMI sources (LED lamps) I noticed at 6KHz (worsened by the initial untuned resonance of the pickups), and makes a headphone amp sound more like a bass amp with a cab simply by removing the distracting noises that don't get reproduced as loudly on a bass cab, but will be mercilessly reproduced by full-range studio headphones. It should have little effect on a real bass amp since their speakers usually reach up to 4-5 kHz (tweeters notwithstanding).
One final note: The output LPF can be affected by the capacitance of the cable to the amp (lowering the cut-off frequency), and by the amp's input impedance (lowering the Q), but a cable's typical capacitance is in the range of a few hundred pF to maybe 2nF (for a loooong cable), much smaller than the parallel 30nF filter capacitance, and even a low input impedance amp input sits at a few hundred kOhms, dwarfing the 1.1K output resistance. The overall effect is minimal.
Here is the CircuitJS1 simulation file.
(More detailed bass guitar setup notes)
I re-strung with long scale D'Addario Chromes, 40-55-75-95-132 for balanced tension with the B string (about 40lbs per string), which just fit on the 35" scale (the silks reach the tuner side of the nut). The neck relief went down to a little less than 1/64" (about 0.010") at the 7th fret without any fret buzz. The truss rod is barely doing any work here. (I know from a past string set that the neck can handle 250lbs of tension well.) The action at the 17th fret goes from 3/64" at G to 6/64" at B, again without any fret buzz. There is no clank unless I dig in. Intonation adjustment was minimal and uniform, with about 1/4" total difference from the G to the B bridge saddles. With a precise strobe tuner to get the intonation just right (especially needed for the heavy B string), all strings stay in tune with eachother up to about the 15th fret, and the G/D/A strings play well further up the neck.