Jackson Spectra: details, electronics, and some fixes/upgrades

[taketwo]

There's very little written about Jackson Spectra basses that isn't repeated
from the ad copy, so since I've been working to make one my main bass for 2
years, here's what I've learnt so far.

Details

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 (they retail for $1000 CAD now). 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
for technical info got me a reply from *Fender*, who had no information at all.

The specs are as-advertised, with a few missing bits:

- The truss rod is dual-acting with a 4mm (metric) Allen hex nut.
- The blend knob is indeed reversed from what you'd expect: CCW blends to neck.
- The bridge will not work with large or tapered B strings. (See below)

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) is very cheap.

 

[taketwo]

Mechanical Fixes/Upgrades

On mine, I've had to fix the following hardware:

• The nut was soft plastic that bound to the strings: a GraphTech PT-1445-00 nut is a drop-in replacement (just a bit of sanding to fit and adjust string height at first fret).
• The Jackson sealed tuners hold steady, but the gears bind, making tuning a chore. Tightening the screws and a couple sprays of WD-40 Dry Lube (PTFE based) down the center and sides of the posts fixed that.
• The nuts holding the knobs and switch were loose.
• The volume pot was crackling, sometimes without even touching it: blowing it out with compressed air got rid of some crud inside causing that problem.

Bridge Limitations

The bridge will not work as-is with large and/or 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 you 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 .125 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.

 

[taketwo]

Electronics

The electronics are mostly well built: good solder joints, shielded wire as
much as possible, everything well-grounded. Except for the bridge ground, which
has a 12 Ohms resistance, which is odd but still works. The pickups are
grounded internally, but unevenly: I measure anywhere from 60 to 200 Ohms
across any given pair of poles, or to the bare shield wire. The control cavity
has a thin, poor coat of carbon shielding paint (300 Ohms/sq) which does not
make contact with the metallized cavity cover.

The pickups are conventional neck and bridge humbuckers, with different pole
spacing to follow the strings. 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 of the other, so when you short the coil taps to ground, the
remaining active coils are the outermost, forming a humbucking J-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 (pair of
MN250K) with center detent.

The blended signal then goes into the pre-amp, which is completely potted with
only the gain trimpot showing. There are no markings. Bass/Mid/Treble EQ pots
are B100K with center detent. The EQ is flat at noon (no difference with
passive tone) and does conventional bass/treble shelving, and a mid boost/cut,
at an unknown frequency point (I estimate around 1kHz).

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*: the other half of the DPDT
switch, which could have been used to switch the power to the pre-amp, is left
unused. So this active/passive switch is really only meant as a failsafe for a
dead battery. The active/passive switch pops loudly when used, unless you turn
the volume down first.

Note the complete absence of any tone cap in the signal path.

 

[taketwo]

Electronic Fixes/Upgrades

The Spectra has a boomy, muddy sound unless it's in J-Bass mode, and
single-coil mode hums too much in any other configuration. You can cut the bass
on the EQ, but it does not make the sound clearer.

To reduce noise, I re-shielded the control cavity. Based on Bruce Johnson's
notes here on TB, I chose 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.

To eliminate hum, I rewired the coil-split switch as a series/parallel coil
selector for the neck pickup only, mimicking the same option on my 1992 Fender
Precision Plus. I hardwired the bridge pickup into parallel humbucking mode.

To reduce the muddy tone, I replaced the blend knob, which was likely loading
down the pickups too much, with a mini toggle DP3T on-on-on switch wired up to
give the usual neck/parallel/bridge pickup selection, also an idea copied from
my Precision Plus.

To further reduce the muddy tone, I built-in a crude on-board passive HPF. I
added a series 20nF capacitor between the pickup selector switch and the
pre-amp input, and another series 20nF capacitor between the active/passive
switch and the volume pot. This cuts the lowest sub-sonic rumble to the
pre-amp input, and filters the pre-amp output further, and still works the same
if switched to passive mode.

I assume here that the pre-amp has an input impedance of 1MOhm, and that the
pre-amp output will be plugged into an amp with a 1MOhm impedance also, which
in parallel with the 500kOhm of the volume pot gives 333kOhm. Plug these values
into a 2nd Order CR HPF Design Tool (
(Sample) 2nd order CR Low-pass Filter Design Tool - Result - ) and you get a curve with the
following approximate attenuation:

• 10 Hz: -15dB
• 20 Hz: -9dB
• 30 Hz: -6dB
• 40 Hz: -4dB
• 50 Hz: -3dB
• 60 Hz: -2.5dB
• 70 Hz: -2dB
• 80 Hz: -1.5dB
• 90 Hz: -1.3dB
• 100 Hz: -1dB

It's not a nice 2nd order curve, but it's better than nothing, it's very simple
to implement, and the 50 Hz -3dB point approximates a lot of what I see here on
TB as a useful setting for outboard HPF.

To improve headroom, I adjusted the pre-amp gain trimpot to unity (by ear),
which so happens to be when turned fully CW. Having the output hotter
made using a pedal more difficult, and would cause quite a volume drop should I
need to switch to passive mode due to a dead battery. Also, it limited the
headroom of the pre-amp EQ: I could not use the full boost range without
clipping inside the pre-amp, which sounds *ugly*.

 

[taketwo]

Final Setup and Results

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", without any buzz. The truss rod is
barely doing any work here. The action goes from 3/64" at G to 6/64" at B.
There is no clank unless I dig in. Intonation adjustment was minimal and
uniform, with about 1/4" total difference from G to B.

To improve headroom and clarity even more, I raised the pickups as much as they
would go, then fixed the warbling by playing high up the neck on both the
treble and bass sides and then lowering the same side of the neck pickup very
slightly as needed. No adjustment was needed to the bridge pickup.

And that's it. The muddy tone is gone. I have simple and flexible tone options
in active and passive mode, without any hum. The B string is clear. It's
pleasant to the ear and hands.

 

[TallestGuy]

Awesome break down. I have a spectra pro series that I plan to redo the electronics for. Definitely taking notes regarding the nut and shielding.

 

[Lo-E]

Those are pretty well thought-out mods. Enjoy your newly improved bass!

 

[taketwo]

Awesome break down. I have a spectra pro series that I plan to redo the electronics for. Definitely taking notes regarding the nut and shielding.

Thanks! Let us know what you encounter, and how the Pro series varies from the X Series.

 

[taketwo]

Those are pretty well thought-out mods. Enjoy your newly improved bass!

Thanks. I spent a lot of time planning, and talked myself down from much more elaborate changes like a Tonestyler-type rotary knob and making my own pre-amp. I even tried to make a hi-fi pickup from scratch (didn't quite work out). In the end, the right approach was: what can I improve given what's already there? Then the simple little changes became obvious one by one, and stacked up quite nicely.

 

[Malafaia]

I am not into Jacksons, but THIS is a review. Thanks!

 

[taketwo]

A lesson learned the hard way: don't turn the pre-amp gain all the way up, leave it at unity.

I was thinking of using higher output to improve gain staging (less noise, more overdrive if needed), but it had no such improvement and it made the bass a clanky, buzzy, unplayable mess.

High gain forces you to play softer to get a similar volume, which makes for good tone and good speed.
However, playing softer does not reduce any normal fret clank or buzz by the same proportion, so you've just made it louder relative to the plucked notes. It sounds like metallic trash.

So this implies there is a range of good plucking force: plucking too lightly means relatively louder fret noise, and plucking too heavily leads to absolutely louder fret noise (clank, buzz), thus needing a higher action/relief to compensate, which means slower, more difficult fretting, etc...

So that seems to give a way to figure out how hard you should play on a given bass.

 

[taketwo]

And of course, *where* you pluck affects this trade-off: Setup Action, Plucking, and Clank

 

[Mechayoshi]

As a Jackson fan I loved reading these observations. This was a bolt on model? I was curious about the higher end ones.

Edit: Refreshed myself with the specs. Nvm. I saw that is was neck thru. It also appears the pro series has Nordstrand electronics.

 

[taketwo]

Electronic Fixes/Upgrades (Part 2)

I rewired the neck pickup as parallel only, as having a series/parallel switch for the neck pickup is not useful here. When in series the neck pickup is still boomy, lessens the natural mid-scoop when placed in parallel with the bridge pickup, and has a high enough impedance that it picks up a bit of noise.

I replaced the two 20nF capacitors with a better pair of filters:

I placed a 3.3nF capacitor in series between the pickup selector switch and the pre-amp input. With the (assumed) 1MOhm input impedance, this creates a 1st order HPF with the -3dB point at about 48 Hz. This filter attenuates sub-sonic junk and the fundamentals of the B and E strings slightly, which very few speakers can even reproduce, and *might* balance the output of the B and E strings, which would be quite a bit louder due to their heavy gauges from using a balanced tension set.

At the output of the pre-amp, I placed a 2nd order LPF with a -3dB point at about 2.5KHz and a Q of about 0.715. This filter is composed of 2kOhms in series feeding a 100mH inductor, followed by a 40nF cap to ground. This filter removes a lot of metallic string noise, fret click, fret buzz, and some EMI sources I noticed at 6KHz (LED lamps). This filter 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 studio headphones. It should have little effect on a real bass amp since their speakers usually reach up to 4-5 kHz (tweeters notwithstanding).

Common mixing practice (and for users of the Broughton LPF/HPF pedals) seems to be to EQ to remove everything above 5KHz, and I confirmed this by listening to solo bass performances through a 31-band EQ: even with bright slap/pop technique, you can cut out everything past 5 kHz and not alter the sound significantly. You can even go down to about 3 kHz and not change the basic tone (i.e.: it will sound the same in a mix). Though, the cut must have a smooth roll-off (e.g.: -12dB/octave) because a sharp brick-wall at 3KHz gives a bass a telephone-like hollow sound. This might be an EQ artifact.

I went for 2.5 kHz instead of 5 kHz for three reasons: I *really* wanted that metallic noise gone, it matches the theory that the 6th harmonic (last consonant one) of the highest fretted note (G4, ~400 Hz) is 2400 Hz, and observing the raw bass output showed that all the strings, at any fret, don't generate much tonal sound past 2.5 kHz anyway.

For this bass model, having the pickups in parallel puts their resonant frequency peak *very* high: I would observe fret and string noise all over 5 kHz to 10 kHz. This means I can expect the filtered range between 48 Hz and 2.5 KHz to be pretty flat, without the usual bump at 2-3 kHz that pickups are commonly designed to have. This leaves a blank slate for later EQ.

I tested this setup, and it all sounds good. The B and E strings are slightly quieter, but now I can bump the bass shelf EQ to warm things up without getting muddy. The fret/string noise is virtually gone, and the string tone doesn't sound muffled or dull. I can also raise the treble shelf EQ to brighten the sound without making the string and fret noise much worse.

The only downside is that now I'm left with a "producer switch": the DPDT switch, which was the series/parallel switch, is now unused and I'm not sure what to do with it, if anything.

 

[taketwo]

Electronic Fixes/Upgrades (Part 3)

Although the last modification (adding the output LPF and HPF) worked, I still wasn't confident about the result, so I gave in and 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 simply stuck to one of the pickup poles by its own magnetism.

Right away, the cause of the metallic noise was evident: when wired as parallel humbuckers, these pickups resonate, quite sharply, at around 6 kHz! There is no tonal signal in that region, only clang and other impulse noises. There is no loading capacitor anywhere, even inside the potted preamp.

Some quick experimentation showed that placing a 4.4nF load capacitor in parallel with each pickup would bring their resonance back down to around 2.4 to 2.8 kHz (depending which pickup, and if they were both in parallel like a J bass), which is much more in line with conventional pickup+tone cap configurations, and happens to place the resonance peak in the area of highest sensitivity to the human ear (re: Fletcher-Munson Curve), and attenuates everything past that point at -12dB/8ve, where little to no tonal signal exists.

I couldn't tell how high the resonance peak was now (not the right equipment), but it seemed flat, so I altered the output LPF to give the expected behaviour: bypassing one of the 1K resistors brings up the Q of the filter to 1.58, which should give a +4.4dB boost at around 2.2 kHz. A resonance peak of +4dB matches many existing commercial pickups, so should sound "normal".

Some crude 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. Some more tests shows that I'm better off leaving the gain trimpot to very slightly above 1 to compensate for the insertion loss of the output LPF, so I can add as much B/M/T boost as I want without unpleasant distortion. Base level output of the pickups, and also of pre-amp (set flat), is about 100mVpp. Perfectly normal instrument-level signal.

Adding these passive filters really fixes things. Before, any bass boost was muddy since the shelf went way past down the B and E fundamentals, and any treble boost highlighted string and fret noise. Now, boosting the bass is heavy and warm, without farting or muddiness. Boosting the treble makes the sound bright and the string attack very present, and does bring back a bit of string/fret noise, but nothing unpleasant.

Given the total pickup+LPF attenuation of -24dB/8ve at about 2.5 kHz, I can see pretty much nothing on the spectrum analyzer past 2.5 kHz, and only some initial attack noise up to 5 kHz if I try hard, even with the treble EQ maxed-out. It's marvelous. It's the best of both worlds: simple shelving control with the neat boundaries of filter-based EQ.

There is one downside: the HPF and LPF filters don't play well with eachother when connected directly by switching to passive mode. The total output drops to almost nothing and isn't usable. Having the pre-amp in the middle to isolate them is a must.

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), and even a low input impedance amp input sits at a few hundred kOhms, dwarfing the 1K output impedance here. You might notice an effect, but it's not likely in most setups.

Here's the final (I hope) schematic of the modified electronics (minus the 500K volume pot before the output jack, which I forgot to include here):

 

[dcoyle]

Thanks for taking the time to post this. It's very interesting (and impressive) to see how far you have developed the passive/active/passive signal chain.

You've already committed to a battery on board, what is your thinking on the idea of a final opamp unity gain buffer, like a OPA1642, after the 500K pot, to eliminate the cord/amp frontend variable? I wonder if it might have the sonic effect of tightening the bass a wee bit further?

Have you found the inductor vulnerable to being a noise antenna?

 

[taketwo]

Thanks for taking the time to post this. It's very interesting (and impressive) to see how far you have developed the passive/active/passive signal chain.

You've already committed to a battery on board, what is your thinking on the idea of a final opamp unity gain buffer, like a OPA1642, after the 500K pot, to eliminate the cord/amp frontend variable? I wonder if it might have the sonic effect of tightening the bass a wee bit further?

Have you found the inductor vulnerable to being a noise antenna?

Thanks. The key insight is to treat an amp as an impedance isolator, regardless of EQ or other functions, so that splits up the overall problem into multiple simple problems with (mostly) known input/output impedances and gets double-duty from some pieces, like the HPF also diminishes the ultra-low frequency portion of the broad-spectrum impulse of the initial note attack, so the amp might not waste dynamic range on that or distort. It's not a certainty, but it goes down the right road.

My point with the effect of the cable/amp is that it should be insignificant in anything but the most extreme cases with unusual equipment, which for any other bass would have already required a clean boost pedal or other front-end anyway to mitigate. It also meant that I could design without having to think about those influences.

But on the other hand, when you write up things like this late at night, you miss details, like the 500K volume pot (which gives 333K in parallel with the usual 1M amp input impedance), or the fact the inductor has about 210 Ohms resistance, which is a 20% difference from 1K alone! So the Q values I wrote are lower in actuality, but again, precise values won't get you anywhere different, so long as the ears say it sounds good. These are also all ordinary, cheap parts. No 1% tolerances here.

Another thing I could have done is use that unused set of poles on the push/pull active/passive switch to simply bypass the HPF when in passive mode, eliminating the capacitive voltage divider with the 40nF in the LPF and restoring the passive output.

And there are some limits to this passive/active hybrid design: I would have loved a 2nd order HPF, but you can't build one passively for 48 Hz without massive honking expensive inductors. You can't just use a small inductor and a huge capacitance, as the Q factor will be so low the filter will be useless. Ditto for 2nd order passive RC networks. So this is a place where adding a unity buffer (I like the OPA145/141 parts) to split it into two would enable a 2nd order RC HPF at least.

But I do like that this hybrid idea would work equally well with a fancy name-brand preamp, or with a cheap no-name one like I have, turning a shelving EQ preamp into something more.

Yes, the inductor is unshielded, in a shielded control cavity, and so it does pick up some magnetic 60 Hz noise, but it's tolerable and much less than from a single-coil pickup, which has on the order of 10x higher inductance (nevermind the cross-section!). However, I was testing in my lab/office, so it wasn't just 60 Hz, but a truckload of higher harmonics from the misc. gear around me. That's a downside of this particular inductor: it's meant for switching power supplies, so it will have low impedance to HF noise. Next version will use a shielded inductor.

 

[taketwo]

A while in, and it's still working well for me. I can choose more good tones, both bright and dark, from the preamp. I'm playing more freely now since I don't have to fret like I'm walking on eggshells to reduce string/fret noise. The controlled sub-low end from the HPF means I'm not driving effects into clipping whenever I raise the bass shelf control, and the atonal high noise removed by the tuned pickups and LPF means any distortion isn't driven by that noise, and so doesn't sound shrill and buzzy, and I can control it's colour somewhat via the B/M/T preamp controls. I just tried a bunch of the built-in overdriven amp models on my Fender Mustang Micro, and explored extreme settings on my BDI21 (SansAmp clone), and other than having to avoid clipping the output, it all sounds good now. Amazing.

 

[Nightsprinter]

I got one of these and one of the saddle height adjustment grub screws stripped out almost immediately when I was adjusting it *without* string tension. Now I gotta figure out what to do about a stuck screw and maybe beg jackson for a replacement. Any tips? I got it from someone who isn't an authorized dealer soooo yeah.

 

[Nightsprinter]

As expected, they won't just send me a simple saddle that probably costs them 20 cents from China. They want me to take it to an authorized fender tech. For a stripped grub screw!!! Big yikes, Fender.