A bass with no tone control. The third knob is a boost. Turn it and the Fralin’s resonant peak climbs out of the mix — +6.9dB at 1165Hz — and the bass moves from a scooped Jazz voice toward a P-bass bark without touching a switch.
The tone knob is a subtraction
A tone pot shunts the top end to ground through a cap. Best case, it takes something away. There is no setting where it gives you more than the pickup already made.
The Fralin makes the least top of the three pickups in this build — 4.8H of inductance, vintage, mid-forward, rolled off above the resonant peak. On a passive jazz bass that reads as dark. Roll the tone down and it gets darker.
What the Q-filter is
A series resistor and a cap to ground. The resistor is a 50kΩ reverse-log pot. The cap is 6.8nF.
That is the whole circuit.
At 50kΩ — knob flat — the network is invisible. The pickup’s natural curve passes untouched. Drop the resistance and the cap starts to resonate against the pickup’s inductance. The peak emerges, sharpens, and climbs in frequency as the Q tightens.
On the Fralin, parallel, blend centred:
+6.9dB at 1165Hz.
Solo one pickup and the loading shifts:
+4.9dB at 808Hz.
Blend position lands between the two: the blend’s 250kΩ load shifts as you turn it, nudging the peak.
Left: what a tone pot does. Subtraction, every position below 10.
Right: what the Q-filter does. The curves fan upward. The flat one is the knob backed off; the tall one is the boost dialled in. Same pickup, same cable.
The sweep
The rainbow is the 50kΩ reverse-log pot from top to bottom of its travel. As the resistance falls, the peak doesn’t just grow — it moves. A loose Q sits lower and wider; a tight Q sits higher and sharper. The reverse-log taper puts the whole transition in the back half of the rotation, where the ear can place it.
The same 6.8nF cap works on the other pickups. On the Aguilar it lands +7.4dB at 1314Hz.
For a brighter peak, 4.7nF pushes the whole thing up the spectrum. That’s the only other cap worth stocking for this position.
“Between P and J” is a knob position
A Jazz bass is scooped through 1–2.5kHz. A Precision barks at 600Hz–1kHz. Players spend money and whole second instruments chasing the ground in between.
The Q knob covers it as a continuum. The peak isn’t parked at one frequency — it climbs as the Q tightens, from 808Hz with a pickup soloed to 1165Hz at blend centre, sliding the resonance through the exact band that separates the two basses. Loosen it back toward flat and the Fralin’s scooped, glassy top above the 12th fret returns. You’re not switching between P and J; you’re driving the resonance across the gap between them, one degree of rotation at a time.
Why a boost there does anything
A bass puts about 99% of its raw energy below 500Hz. The ear doesn’t listen there.
The boost lives at 800–1200Hz — above where the bass makes its energy, square on where the ear is paying attention.
That gap is also why it travels. Boosting 800–1200Hz strengthens the upper harmonics the ear uses to reconstruct a fundamental it can’t actually hear — the missing-fundamental effect. The bass reads as bass on a phone, a laptop, an earbud, none of which can move air at 40Hz. It’s MaxxBass, done in analog, at the source, with one cap.
The push-pull
The Volume pot pulls up. One move swaps two things at once: the coils go parallel → series, and the filter cap goes 6.8nF → 22nF.
| State | Coils | Cap | Sound |
|---|---|---|---|
| Down | Parallel | 6.8nF | Boost on. The resonant peak, +6.9dB at 1165Hz centred. |
| Up | Series | 22nF | Darker, fatter. No boost — the cap rolls the top instead. |
Parallel is the working mode: the Q-filter does its job. Series is the fat setting — more output, lower voice, the cap behaving like a gentle treble roll because at that inductance and capacitance there is no peak to lift. A two-position character switch hidden under the volume knob.
The signal chain
Fralin Split Jazz (parallel / series via push-pull)
-> Blend (Bourns 250K MN)
-> Volume (CTS 500K log, DPDT push-pull)
-> Q-filter (50K rev-log + switched cap to ground)
-> Ring-hot buffer (2N5457 -> 2N3904)
-> TRS out: tip = passive direct ring = buffered + phantom power
No junction box. No active preamp. The output jack is a TRS: a plain TS cable in the tip pulls the pure passive bass straight off the pickups; a TRS cable feeds the ring, which carries the buffered signal and the phantom power that runs it — 18V in the studio, 9V for jamming. The cable is the switch.
The buffer earns its place
The buffer is required. Without it the cable capacitance loads the pickup directly and the Q-filter is fighting the cable instead of the pickup.
It’s what makes the phantom “condenser-mic” idea work at all.
It does not brighten a held note. The win is that the cable no longer has a say in the tone, not top-end.
The compound 2N5457 → 2N3904 follower sits on a half-rail divider bias. Sim-verified and built: 0.40% THD at 1.5Vpk, impedance balance inside 0.6%, the passive path within 0.27dB of bypass.
Why this instead of an active preamp
Active filter preamps — Alembic, Wal, ACG/John East, Lusithand — do this with op-amps. They boost and cut, sweep the frequency continuously, drive any cable. They cost $150–300 and need a battery in the body.
This is a subset, done passively. One reverse-log pot and one cap to ground. It can’t cut and it can’t sweep the centre frequency on its own — the cap sets the band, the pot sets the intensity. What it can do is take a vintage, rolled-off Fralin and give it a +6.9dB resonant lift exactly where the ear is listening, with no battery and no IC in the signal path.
On a sustained note, the three pickups in this build sit within about 3dB of each other. The pickup is not where the transformation lives. The harness is.
Parts
| Part | Spec | Cost estimate |
|---|---|---|
| Lindy Fralin Split Jazz (pair) | vintage-wind, hum-cancelling, 4.8H | ~$200 |
| Volume pot | CTS 500K log, DPDT push-pull, solid shaft | ~$12 |
| Blend pot | Bourns 250K MN, centre detent | ~$5 |
| Q pot | CTS 50K reverse-log | ~$8 |
| Parallel cap | 6.8nF WIMA film | ~$0.50 |
| Series cap | 22nF WIMA film | ~$0.50 |
| Bright alternate cap | 4.7nF WIMA film | ~$0.50 |
| Buffer | 2N5457 → 2N3904 compound follower, half-rail divider bias | ~$5 |
| Output jack | Switchcraft TRS — tip passive, ring buffered + power | ~$8 |
Total electronics (excluding pickups): around $40. Full build steps live in the Volume · Q · Blend harness guide.
Companion to the Volume · Q · Blend harness and the pickup shootout. The harness post has the wiring; this one is the filter that takes the tone pot’s place.