Jazz Bass Master Harness: Volume · Q · Blend + Series/Parallel + Phantom Buffer

A standard Jazz Bass has Volume / Volume / Tone. Two volume knobs is a 1960s compromise. You ride two controls to balance the pickups and lose level every time you back one off. The tone knob, meanwhile, only takes away.

Volume · Q · Blend. One knob for level, one to mix the pickups, and a third that does the opposite of a tone control — a passive resonant boost. There is no tone pot in this harness. A push-pull on the volume stacks the pickups in series for a fat, mid-forward voice and swaps the filter cap to match.

Flat when you leave it, climbing into a peak as you dial it down. Turn it and the bass walks from J toward P. Details in the Q-filter build.

Built for the Fralin Split Jazz. Works with the Aguilar 4J-HC and stock Fender pickups.

Every solder joint numbered.

What you get

Three controls. Two coil modes. The Q dial sits over all of it.

Push the volume and the pickups run parallel — the Q-filter sweeps a presence peak from flat up to an 800–1165 Hz growl. Pull it and they stack in series: a darker, thicker voice where the Q control becomes a treble rolloff instead of a boost. In series the blend can’t mix freely any more — it acts as a balance tilt, not a crossfade. That’s true of every series/parallel J, not a fault.

Parts

All passive except the buffer. No battery. Power arrives on the TRS ring as phantom: 48 V from a mixing desk, or 9 V off a small supply for jamming.

The buffer is its own small board — a compound 2N5457 → 2N3904 follower on a half-rail divider bias, not a single JFET. It builds and tests on its own bench; this guide treats it as a module with three pads: IN, GND, RING. The full board build, BOM, and meter gates live in the Q-filter build.

Tools

• Soldering iron with temperature control (340°C for wire, 380°C for pot casings)
• Multimeter (continuity mode + resistance mode)
• Wire strippers
• Flush cutters
• Small pliers or hemostats (for holding wire against hot pot casings)
• Screwdriver (for mounting pots to plate)
• Lighter or heat gun (for heat shrink)

Before you start

Three checks and two prep tasks before touching the plate.

Know your lugs

The guide references lugs by number.

Blend pot (MN250K, 6 lugs in two rows):

• Top row = Gang A: Lug 1 (input), Lug 2 (wiper), Lug 3
• Bottom row = Gang B: Lug 4, Lug 5 (wiper), Lug 6 (input)
• Wipers are always the middle lug of each row

Volume pot (CTS push-pull, 3 pot lugs + 6 switch lugs):

• Pot lugs: Lug 1 (CCW), Lug 2 (wiper), Lug 3 (CW)
• Switch lugs (two rows of 3): SW1–SW3 (Pole 1), SW4–SW6 (Pole 2)

Q pot (CTS 50K reverse-log, 3 lugs):

• Lug 1 (CCW), Lug 2 (wiper), Lug 3 (CW)
• Wired as a rheostat: Lug 1 ties to Lug 2

Verify the push-pull switch

The DPDT switch on the back of the volume pot has 6 lugs. Which pairs connect in each position varies between manufacturers. You must check yours.

1. Set multimeter to continuity mode
2. Push shaft down (default). Probe all 6 switch lugs to find connected pairs.
3. Pull shaft up. Probe again.
4. Map your results:

If your switch connects different pairs, adjust the wiring in Step 5 accordingly. The principle is the same: each pole has a center common and two throws. You need to know which is which.

Verify the blend pot rotation

1. Set multimeter to resistance (Ω)
2. Measure Gang A: Lug 1 to Lug 2 while turning the shaft
3. Full counterclockwise should give minimum resistance (full signal)
4. Do the same for Gang B. It should be the opposite (maximum resistance at full CCW)

Target: Turn left = neck only, center detent = 50/50, turn right = bridge only.

If it’s backwards, swap which pickup connects to which gang.

Pre-tin pot casings

The hardest part of Step 2 — do it now while the pots are loose.

1. Set iron to 380°C
2. Hold the pot in a vise or third hand
3. Touch the iron tip to the back of the pot casing, feed solder into the joint
4. Build a small puddle of solder, ~5 mm across
5. Repeat for all three pots

The first one might take a few attempts. The casing is a big heat sink. Hold the iron on for 3–4 seconds before the solder flows. Do not exceed 5 seconds. Prolonged heat can damage the pot’s resistive element.

Pre-assemble the filter caps

Each mode cap goes from one DPDT throw to the ground bus. Tin a short ground-bus tail onto one leg of the 6.8 nF cap and one leg of the 22 nF cap now, while you have room. They install in Step 5.

The build

Nine steps. Each numbered sub-step is one solder joint or one action.

Step 1: Mount pots on the plate

1.1 Drop all three pots through the plate holes. From neck to bridge: Blend, Volume, Q.

1.2 Finger-tighten the mounting nuts. Leave them slightly loose. You may need to rotate the pots to align lugs for clean wire runs.

1.3 Orient the lugs: pot lugs should face the same direction (toward one edge of the plate) so the wire runs between pots are short and tidy.

Step 2: Solder the ground bus

2.1 Cut a length of silver wire long enough to span all three pot casings with ~1 cm extra on each end.

2.2 Hold the wire against the pre-tinned spot on the Blend pot casing using pliers. Touch the iron to the existing solder puddle, let it reflow around the wire. Remove iron. Hold still for 2 seconds.

2.3 Repeat for the Volume pot casing. The wire now bridges blend and volume.

2.4 Repeat for the Q pot casing. The wire now spans all three.

2.5 Tug-test each joint. The wire should not move.

Check: Multimeter continuity between all three pot casings. Should read < 1 Ω.

This is the hardest step. The pot casings absorb a lot of heat. Use 380°C, hold the iron on the joint (not the wire), and let the solder flow to the wire. If it doesn’t stick: let it cool completely, scrape the spot with a blade, re-tin, try again.

Step 3: Wire the blend pot

Seven connections. Start with the grounds, then the signal path.

3.1 Short jumper from Gang A Lug 3 to ground bus ●

3.2 Short jumper from Gang B Lug 4 to ground bus ●

3.3 Solder signal wire to Gang A Lug 1, the neck hot lead. Leave 20 cm long, don’t cut yet. ●

3.4 Solder signal wire to Gang B Lug 6, the bridge hot lead. Leave 20 cm long. ●

3.5 Solder a wire from Gang A Lug 2 (wiper), route toward volume pot. ●

3.6 Solder a wire from Gang B Lug 5 (wiper), route toward volume pot. ●

3.7 Join the two wiper wires (3.5 + 3.6) together at Junction X. Twist, solder, heat shrink. Run a single wire from Junction X to Volume Pot Lug 3. ●

Check: Continuity from Gang A Lug 3 to ground bus. Same for Gang B Lug 4. No continuity between any signal wire and ground.

Step 4: Wire the volume pot

Three connections. The volume wiper is the signal node — the single point everything downstream hangs off.

4.1 Short jumper from Lug 1 (CCW) to ground bus ●

4.2 The wire from Junction X should already be on Lug 3 (CW) from step 3.7. If not, solder it now. ●

4.3 Solder a wire from Lug 2 (wiper). This is the signal node. It fans out to three places: the Q-filter (Step 6), the buffer input (Step 7), and the jack tip (Step 8). Leave enough length to reach all three. ●

500K is non-standard for bass. Most J-Bass harnesses use 250K. The higher value loads the pickup less, letting the resonant peak ring. The buffer downstream means the cable never adds its own loading on top — what the pickup sees is the pot and nothing else.

Step 5: Wire the DPDT switch

The push-pull does two jobs in one pull. Pole 1 reconfigures the pickups parallel↔series. Pole 2 swaps the Q-filter cap to match. Six lugs, two rows.

Pole 1 — series/parallel:

5.1 Jumper from SW1 to ground bus ●

5.2 Solder a wire to SW2 (Pole 1 common), the neck ground lead. Leave 20 cm long. ●

5.3 Solder a wire from SW3 that tees into the bridge hot lead (the same wire on Blend Gang B Lug 6). ●

What this does: Push (down) connects SW1↔SW2 — neck ground goes to the bus, pickups run parallel. Pull (up) connects SW2↔SW3 — neck ground goes to bridge hot, stacking the coils in series (~9.6 H).

Pole 2 — cap swap:

5.4 Solder the 6.8 nF cap from SW4 to the ground bus. ●

5.5 Solder the 22 nF cap from SW6 to the ground bus. ●

5.6 Leave SW5 (Pole 2 common) free — the Q pot wiper lands here in Step 6.3. ●

Push (parallel) connects SW5↔SW4, so the Q-filter sees the 6.8 nF cap — the boost. Pull (series) connects SW5↔SW6, swapping in the 22 nF — a darker treble rolloff for the thicker series voice.

Check: Resistance, push the shaft down, measure SW2 to ground bus: < 1 Ω. Pull up: open. That confirms Pole 1.

Step 6: Wire the Q-filter

A series resistor and a cap to ground. The pot is the resistor, wired as a rheostat; the cap is whichever one Pole 2 selected. As the resistance drops toward 2 kΩ, the resonant peak climbs.

6.1 Tie Q Lug 1 (CCW) to Q Lug 2 (wiper). A short jumper, right on the pot. This makes it a two-terminal variable resistor. ●

6.2 Solder a wire from the signal node (Volume Lug 2, step 4.3) to Q Lug 3 (input). ●

6.3 Solder a wire from Q Lug 2 (wiper) to DPDT SW5 (Pole 2 common, step 5.6). The filter now reaches ground through the selected cap. ●

One continuous sweep. Reverse-log taper. Full CCW is ~50 kΩ — the cap barely loads the signal, response flat. Roll toward full CW and the resistance falls; the cap starts to resonate against the pickup inductance, lifting a peak in the low presence band. Reverse-log spreads that action across the whole rotation instead of bunching it in the last few degrees.

Check: Measure Q Lug 3 to Q Lug 2 (wiper) and sweep the pot — it should travel roughly 50 kΩ to 2 kΩ with no dead spots (that’s the rheostat). Then confirm continuity from Lug 2 → SW5 → the cap’s ground leg; the cap blocks DC, so that path reads open — correct.

Step 7: The ring-hot buffer

The buffer hangs off the signal node in parallel with the jack tip. It converts the pickup’s high-impedance signal to low impedance at the source. The cable stops loading the pickup — the Q-filter resonates against the coil, not the capacitance on the other end of the wire.

It’s the compound follower board — built and tested on its own bench. The measured specs are in the Q-filter build.

  signal node ──●── jack TIP            (passive, always live)
                │
                └── IN ─[6M to half-rail bias]─ gate
                          2N5457
                            └─ source ── base 2N3904   (compound follower)
                                             │
                                        emitter ─[100µF]─ RING   (AC out)
                                             ▲
                  phantom on RING (~29V studio) ┘ feeds the 18V rail

The board has three pads: IN, GND, RING. Full board build, BOM, and the meter gates are in the Q-filter build.

7.1 Wire the signal node to the buffer IN pad. The buffer runs in parallel with the tip — it doesn’t interrupt the passive path. ●

7.2 Wire the buffer GND pad to the ground bus. ●

7.3 Mount the board inside the control cavity with foam tape. The RING pad connects to the jack in Step 8.

The tip is passive, always — hardwired to the signal node.

Step 8: Wire the output jack (TRS)

A Switchcraft 12B (TRS panel jack), not a mono jack.

8.1 Cut ~15–20 cm of shielded cable. Strip both ends.

8.2 At the plate end: solder inner conductor 1 to the signal node. This is the passive-direct path to the tip. ●

8.3 Solder inner conductor 2 to the buffer RING pad. This carries the buffered output and the phantom power, both on the ring. ●

8.4 Twist the braided shield tight, tin it as a bundle. Solder it to the ground bus. ●

8.5 At the jack end: solder conductor 1 to the Tip lug. ●

8.6 Solder conductor 2 to the Ring lug. ●

8.7 Solder the braided shield to the Sleeve lug (ground). ●

Tip = passive direct (always). Ring = buffered output + phantom power. Sleeve = ground.

Step 9: Final connections and check

These connections happen at installation time, when the harness goes into the bass.

9.1 Bridge ground wire → ground bus (leave long, trim at install)

9.2 Copper shielding → ground bus (one short jumper from control cavity shielding to nearest pot casing, one point only)

9.3 Connect pickup leads to the harness (see Pickup Wiring Reference below)

Full verification

Run through every check before you put the pickguard back on.

• Continuity: All ground points → jack sleeve, < 1 Ω
• No shorts: No continuity between any signal wire and ground
• DPDT Pole 1: SW2 to ground < 1 Ω pushed down, open pulled up
• Q sweep: With a cap selected, Q wiper to ground through the cap sweeps ~2 kΩ to 50 kΩ, no dead spots
• Blend rotation: Left = neck, center = both, right = bridge
• Volume sweep: Smooth, no crackle, full attenuation at 0
• Series mode: Pull switch, noticeably fatter, more mids
• No hum: Touch strings/bridge, hum should stop (hum-cancellers stay quiet either way)
• Phantom power: With a TRS cable to the desk, measure ring to sleeve at the jack — ~29 V studio (48 V phantom through the desk’s 6.8 kΩ feeds and the buffer’s 2.2 kΩ harvest resistor), ~7.7 V on a 9 V jam supply (after the 1N5817 drop and the 1 kΩ harvest). The buffer’s internal rail is 18 V / ~5.4 V — read that at the board’s RAIL test pad, not the jack ring
• Passive fallback: Unplug TRS, plug a TS cable — signal still present at the tip with no power

Complete wiring reference

The full diagram with every connection.

Hover over any wire to isolate it. Click to explore connections.

Pickup wiring reference

The harness works with any passive J-Bass pickup. The only difference is how many wires each pickup has and what color they are. Connect them at install time.

Fralin Split Jazz (2 wires per pickup)

The pickup in the bass. Vintage voice, hum-free, the most rolled-off top of the three — which is exactly what leaves room for the Q-filter to lift.

Verify colors against your actual pickups.

Stock Fender (2 wires per pickup)

Aguilar 4J-HC (3 wires per pickup)

Verify colours against your actual pickups — both 4J-HC units should be identical (white = hot, black = ground).

Four-conductor pickups (DiMarzio Area J / Ultra Jazz)

Four conductors plus bare. Default to series wiring: Red = hot to the blend input, Green = ground (neck → SW2, bridge → bus), Black + White soldered together and insulated, Bare = shield drain to the bus at the pot end only. DiMarzio uses the same color code across its 4-conductor J pickups, but confirm against the sheet in the box.

If something’s wrong

No sound at all:

• Trace the signal path: blend wiper (Lug 2 of either gang) → Volume Lug 3 → wiper → jack tip.
• Check the jack tip is on the signal node, not the ring.

Sound from only one pickup:

• Check that both blend pot inputs have continuity to their respective pickup hot leads.
• Verify blend pot rotation direction.

Hum that stops when you touch strings/bridge:

• Normal for the stock single coils soloed. Hum-cancellers should stay quiet.
• If hum is excessive: check ground bus continuity, check the bridge ground reaches the bus.

Hum that does NOT stop when you touch strings:

• Ground loop. Shielding must connect to ground at only ONE point.
• Check for stray ground connections (a wire touching shielding, a solder blob bridging lugs).

Series mode sounds the same as parallel:

• DPDT Pole 1 wired wrong. Re-verify which lugs connect in push vs pull.
• Check SW3 is actually connected to bridge hot.

Q-filter does nothing:

• Confirm Q Lug 1 is tied to Lug 2 (rheostat) and the wiper reaches SW5.
• Confirm at least one mode cap reaches ground from its DPDT throw. With the shaft down, SW4’s 6.8 nF should be in circuit.

Q-filter works in parallel but not series (or vice versa):

• One of the two caps isn’t reaching ground. Check SW4 (6.8 nF) and SW6 (22 nF) both land on the bus.

No buffered output but the passive tip works:

• The ring isn’t getting power. Check phantom at the jack (ring to sleeve), the buffer RING pad to jack ring, and the buffer GND to bus.

Series mode with blend centered is louder than either pickup solo:

• Normal. Series wiring sums the pickup voltages. The topology working as designed.