1. Project Overview
This post documents a complete Bluetooth audio upgrade for the Collins 312B-3 speaker cabinet, adding dual power modes: external 24 V DC adapter and an internal rechargeable Li-Ion battery pack rated for a minimum five-day autonomous runtime. The design preserves the 312B-3’s vintage S-Line aesthetic while delivering modern Bluetooth 5.0 audio with aptX HD fidelity.
The 312B-3 is a compact speaker-only cabinet housing a 5×7 inch elliptical 4 Ω driver, matched to the Collins 32S-1 transmitter, 75S-3 receiver, and S-Line accessories. Its external dimensions are 10″ W × 7.75″ H × 8″ D — notably shallower than the 516F-2. All electronics are self-contained within the existing chassis space. A 24 V DC adapter is mounted externally (wall-wart style) with a barrel jack on the rear panel; the shallow depth rules out an internal mains adapter. No permanent alterations to the original cabinet structure are required.
2. System Architecture
2.1 The 4 Ω Speaker Constraint — No Bridge Mode
The 312B-3’s 5×7″ elliptical driver is rated at 4 Ω. This is a critical design constraint: the TPA3116D2 amplifier in the KAB-250v4 supports a minimum load of 4 Ω per channel in stereo mode, but in BTL (bridge/mono) mode each channel sees half the speaker impedance. Bridging into 4 Ω presents a 2 Ω load per channel — well below spec and a guaranteed failure mode involving overheating and shutdown.
The correct configuration: use the KAB-FC harness to sum the Bluetooth stereo audio to mono internally, then drive the 4 Ω speaker from the LEFT channel output only. This gives 50 W peak capability into 4 Ω — far more than adequate for a room speaker at comfortable listening levels.
2.2 Signal & Power Flow
Power management is handled by the two paralleled LBB-5Sv2 battery boards, which arbitrate between the external DC adapter and battery seamlessly:
- DC adapter connected: 24 V external adapter → DC barrel jack → LBB-5Sv2 boards → KAB-250v4 amp → 4 Ω speaker. Batteries charge while playing.
- Battery only: LBB-5Sv2 boards (5× 18650 each) → KAB-250v4 amp → 4 Ω speaker. Adapter absent; battery output sustains operation automatically.
- Transition: Plugging or unplugging the DC adapter is hot-switchable with no audio interruption. The LBB-5Sv2 manages source priority internally.
2.3 Functional Block Diagram
3. Battery Runtime Analysis
3.1 Energy Budget — Five-Day Target
Running in single-channel stereo mode (not bridged), the KAB-250v4 draws slightly less quiescent current than in BTL mode. The five-day runtime target is achievable with two LBB-5Sv2 boards and ten Samsung 40T cells.
| Mode | Draw (21V rail) | Daily Hours | Daily Wh |
|---|---|---|---|
| BT Standby (idle, paired) | ~60 mA (≈1.3 W) | 16 h | 20.3 Wh |
| Active listening (moderate, 1W acoustic) | ~170 mA (≈3.6 W) | 4 h | 14.3 Wh |
| Sleep / powered off | ~5 mA (≈0.1 W) | 4 h | 0.4 Wh |
| Daily total | 24 h | 35.0 Wh |
3.2 Cell Selection for 5-Day Runtime
35.0 Wh/day × 5 days = 175 Wh required. The LBB-5Sv2 holds five 18650 cells in series for a 21 V nominal output. Usable capacity (80% depth of discharge) per board:
| Cell Option | Capacity | 1-board usable (×5 cells, 80%) | 2-board usable | Runtime (2h listen/day) |
|---|---|---|---|---|
| Samsung 30Q / LG HG2 | 3000 mAh | 50.4 Wh | 100.8 Wh | ~3.8 days |
| Samsung 40T / Molicel P45B | 4000 mAh | 67.2 Wh | 134.4 Wh | ≈5.3 days |
| Molicel P50B / Lishen 26650 | 5000 mAh | 84.0 Wh | 168.0 Wh | ~6.6 days |
4. Bill of Materials & Sourcing
All components are available from Amazon US or Parts Express (parts-express.com). The IEC C14 inlet used in 8 Ω builds is omitted here — the 312B-3’s shallow chassis requires an external DC adapter only.
| Ref | Component | Source / Part # | Est. USD | Qty |
|---|---|---|---|---|
| A1 | Dayton Audio KAB-250v4 BT 5.0 aptX HD, TPA3116 Class D 2×50W stereo · single channel into 4Ω 4–8Ω, 12–24 VDC, 7-day BT standby |
Amazon ASIN B09T3SXB97 Parts Express #325-505 |
$34.95 | 1 |
| B1 | Dayton Audio LBB-5Sv2 5× 18650, 21V out @ 4.8A max Charges from 5–24 VDC input Over/under-charge protection |
Amazon ASIN B09T3WKVZP Parts Express #325-502 |
$31.95 | 2 |
| B2 | Samsung 40T 18650 4000 mAh High-drain Li-Ion flat-top Lishen 18650 3000 mAh acceptable |
Amazon: search “Samsung 40T 18650” Illumn.com, 18650batterystore.com |
$7.50 ea / $75 total | 10 |
| PS1 | 24V 2A DC Switching Adapter 2.1×5.5mm barrel plug, centre + UL-listed · external wall-wart only (internal mounting not feasible in 312B-3) |
Amazon: search “24V 2A DC adapter 2.1mm” Mean Well GS25B24-P1J (premium) |
$12–18 | 1 |
| PS2 | Panel-mount DC barrel jack 2.1×5.5mm female chassis mount Fits 12mm round hole in rear panel With 2A inline blade fuse holder |
Amazon: “2.1mm DC jack panel mount” Digi-Key CP-002A-ND or equiv. |
$4–6 | 1 |
| D1–D2 | 1N5822 Schottky Diode 3A 40V Parallel battery board OR-ing Low forward drop (0.35V typ) |
Amazon: Chanzon 1N5822 25-pack AliExpress: 1N5822 schottky |
$6 (pack) | 2 |
| W1 | Dayton Audio KAB-FC Function Cable Kit Pre-terminated harnesses for KAB boards Volume knob, AUX input, BT reset button Use mono-sum jumper for single-speaker use |
Amazon ASIN B01N9RST4K Parts Express #325-121 |
$9.95 | 1 |
| V1 | Dayton Audio KAB-VC Volume Control Panel-mount rotary volume pot Plug-and-play with KAB-FC harness |
Parts Express #325-119 | $4.95 | 1 |
| L1 | Dayton Audio LBB-5CL LED Status Kit 4-LED charge level indicator Power and charging status LEDs |
Parts Express #325-520 | $6.95 | 1 |
| Estimated Total | ~$185–200 | |||
5. Wiring Guide
5.1 DC Power Input
- Drill a 12mm round hole in the rear panel for the chassis-mount DC barrel jack. Position it centrally or to one side to clear the LBB-5Sv2 board footprint inside.
- Fit a 2A inline blade fuse in the positive lead between the barrel jack and the LBB-5Sv2 input connector.
- Run a short lead (20 AWG, red/black) from the barrel jack to the 2.1×5.5mm input jack on LBB board #1. A splitter Y-cable then feeds both boards simultaneously from the same barrel input, or wire sequentially: jack → B1 in, B1 passthrough → B2 in.
5.2 Battery Board Connections (OR-ing)
- Install 10× Samsung 40T cells (5 per board) observing polarity markings.
- 21V output positive from each LBB board → 1N5822 anode. Both 1N5822 cathodes → common 21V bus wire.
- 21V common bus → KAB-250v4 battery power input (Molex Mini-Fit Jr. harness, red lead). All GND leads tie to a single chassis point.
5.3 Speaker Output — Single Channel 4 Ω
- Use the KAB-FC mono-sum jumper (consult the KAB-250v4 manual) to sum the BT stereo signal to mono at the input stage.
- Connect the 312B-3 speaker to LEFT channel (+) and LEFT channel (−) output terminals only.
- Leave the RIGHT channel output terminals unconnected. Do not attach a dummy load — the TPA3116 is stable into an open circuit on the unloaded channel.
- Speaker wire: 20 AWG, keep run short. Do not connect speaker ground to chassis.
5.4 Panel-Mount Controls
- Volume knob (KAB-VC) → rear panel, 6mm D-shaft hole. Plug into KAB-FC harness J3 connector.
- Bluetooth reset button → small rear panel hole (6mm). Press once to enter pairing mode, hold 3s to clear pairing.
- Battery status LEDs (LBB-5CL) → 5-LED cluster flush-mounted on rear panel. Plug into J9 on each LBB-5Sv2 board.
- Power switch: SPST mini rocker or toggle on rear panel — interrupts the 24V DC feed from barrel jack.
6. Installation Notes — 312B-3 Chassis
The 312B-3 provides approximately 9.5″ W × 7.25″ H × 7.25″ D of usable interior space after accounting for panel thickness and the rear speaker baffle. The shallower depth compared to the 516F-2 requires careful component placement.
6.1 Layout Strategy
- Front zone (nearest speaker): KAB-250v4 amp board (3.6″ × 2.7″), centred on the chassis floor with 10mm nylon M3 standoffs. Minimises speaker wire length.
- Battery zone (centre/rear): Two LBB-5Sv2 boards placed side by side across the width (each 4.8″ × 3.6″ — combined 9.6″ across the 9.5″ interior). Boards may need to be offset slightly or their nylon standoff tabs trimmed by 1–2mm to clear the side walls. Alternatively orient boards 90° so each is 3.6″ wide × 4.8″ deep — combined 7.2″ width × 4.8″ depth, which fits comfortably.
- Rear panel: DC barrel jack, power switch, volume knob, BT reset button, LED cluster. All controls should be positioned to avoid conflicting with the LBB board footprint behind the panel.
6.2 Depth Budget
| Component | Depth (front→rear) | Cumulative | Margin to rear wall |
|---|---|---|---|
| KAB-250v4 amp (front zone) | 2.7″ | 2.7″ | 4.55″ remaining |
| Gap + wiring clearance | 0.4″ | 3.1″ | 4.15″ remaining |
| LBB-5Sv2 boards (90° orient.) | 4.8″ | 7.9″ | −0.65″ — needs 90° orient. or stagger |
| LBB-5Sv2 boards (standard orient.) | 3.6″ | 6.7″ | 0.55″ clearance ✓ |
- Bluetooth antenna: the 312B-3 cabinet is all-metal. Fit an external BT antenna (Parts Express #325-128) via SMA pigtail through a 6mm rear panel hole.
- Ventilation: no forced cooling needed. Class D at 10W average dissipates ~1W.
7. Charging & Runtime Summary
| Mode / Source | Time / Duration | Notes |
|---|---|---|
| 24V 2A adapter (external) | ≈2.5 h per LBB board / 5 h total | Fast charge — recommended |
| 12V 2A (car / field power) | ≈6 h per board | Acceptable if 24V unavailable |
| 5V USB 2A | ≈28 h per board | Emergency only; very slow |
| Battery runtime (2 h listen/day) | ≈5.3 days | 35 Wh/day; 134 Wh usable (10× Samsung 40T) |
| Battery runtime (4 h listen/day) | ≈3.8 days | Same pack, higher daily draw |
| BT Standby only (no audio) | ≈8–9 days | Matches Dayton 7-day claim with 2 boards |
8. Safety Notes
- Do not exceed 4.8A output per LBB-5Sv2 board. Two boards in parallel support up to 9.6A — well above any realistic demand from the KAB-250v4 at normal listening levels.
- The 2A inline fuse on the DC barrel jack input protects the external adapter feed.
- Source 18650 cells from reputable suppliers only (Samsung, LG, Molicel, Lishen). Avoid unbranded cells — capacity and protection ratings are frequently falsified.
- Secure all wiring away from sharp chassis edges. Use split loom or PTFE sleeving on exposed DC rail wiring inside the cabinet.
9. Internal Chassis Layout — Top-Down View
The diagram below shows a top-down plan view of the 312B-3 interior with the lid removed (approximate scale 34 px/in). The cabinet is 8″ deep — 3.5″ shallower than the 516F-2 — which drives the two-zone layout: amplifier board forward, both battery boards side-by-side across the width in the battery zone, rear panel controls flush-mounted at the back wall.
Colour Key
| Legend | |
|---|---|
| KAB-250v4 Bluetooth amplifier board | |
| LBB-5Sv2 battery board (×2) | |
| 18650 Li-Ion cells (×10 total) | |
| DC barrel jack / panel controls | |
| D1, D2 — 1N5822 Schottky OR-ing diodes | |
| Power flow (amber dashed arrow) | |
10. Ferrite RFI Suppression
10.1 Threat Model
The 312B-3 conversion introduces three switch-mode noise sources into a shack that almost certainly also contains a Collins 75S-3, 32S-1, or other S-Line equipment — receivers with front-end sensitivity spanning 500 kHz to 30 MHz. All three noise emitters share a common mechanism: fast-switching transistors generating broadband conducted and radiated interference across that entire range.
| Noise Source | Mechanism | Primary Emission Frequencies | Risk |
|---|---|---|---|
| 24V DC switching adapter | Flyback or buck converter, 65–130 kHz switching. Conducted noise on DC output lead and radiated from adapter body. | Harmonics at 65 kHz × N to 30+ MHz. Worst on 40m, 80m, broadcast AM. | HIGH |
| TPA3116 Class D amp (KAB-250v4) | PWM at ~300–400 kHz. Speaker leads radiate if unfiltered. | 300–400 kHz fundamental and harmonics to 30 MHz. | HIGH |
| LBB-5Sv2 charge controller | Synchronous buck charger, 200–500 kHz. Conducted on 21V rail. | 200–500 kHz and harmonics on DC rail. | MEDIUM |
| Bluetooth 2.4 GHz module | FHSS spread-spectrum. Fundamental far above HF. Sub-harmonics possible if antenna lead unfiltered. | Fundamental 2.4 GHz (no HF concern). Sub-harmonic leakage rare. | LOW |
10.2 Ferrite Mix Selection
Mix selection is critical — the wrong material provides negligible suppression in the HF range. All mixes below are available from Fair-Rite (manufacturer) and Amidon Associates (distributor). Do not substitute generic unlabelled ferrite cores.
| Mix | µ (initial) | Peak suppression range | Use in this build |
|---|---|---|---|
| Mix 31 (Fair-Rite #31) — MnZn | ~1500 | 1–300 MHz; peak ~30 MHz. Best all-round HF common-mode suppression. | DC lead, 21V rail, speaker leads — primary HF treatment |
| Mix 43 (Fair-Rite #43) — NiZn | ~850 | 10 MHz – 1 GHz; peak ~200–400 MHz. | BT antenna pigtail, control harness wiring |
| Mix 77 (Fair-Rite #77) — MnZn | ~2000 | 0.1–50 MHz; peak 1–10 MHz. Best for low-frequency SMPS harmonics. | Optional second pass on 24V DC lead for enhanced 160m/80m suppression |
10.3 Suppression Points — Location Map
Seven ferrite suppression points cover all major noise paths in the 312B-3 build.
| Point | Location | Core | Winding | Target impedance |
|---|---|---|---|---|
| F1 | DC barrel jack cable — on the external adapter DC lead, as close to the cabinet rear panel as practical. | Fair-Rite 0431177081 clamp-on (Mix 31, 1.1″ ID) | 3 passes of the cable through core (both wires together). | >200 Ω @ 10 MHz common-mode |
| F2 | DC internal lead — from barrel jack to LBB-5Sv2 input. Wind both + and − wires together (bifilar). | FT140-31 toroid (1.40″ OD, Mix 31) | 7 bifilar turns of both wires together. | >500 Ω @ 5–30 MHz common-mode |
| F3 (optional) | DC internal lead — second choke for 160m/80m enhancement, in series with F2. | FT114-77 toroid (1.14″ OD, Mix 77) | 5 bifilar turns. | >300 Ω @ 1–5 MHz common-mode |
| F4 | 21V DC rail — from LBB-5Sv2 OR-ing junction to KAB-250v4 power input (Molex Mini-Fit harness). | FT114-31 toroid (1.14″ OD, Mix 31) | 5 bifilar turns of both harness wires. | >300 Ω @ 5–30 MHz common-mode |
| F5 | Speaker leads — from KAB-250v4 LEFT channel output to 312B-3 4Ω driver. Wind both leads together. | FT140-31 toroid (1.40″ OD, Mix 31) | 3 bifilar turns. Mount close to amp board, not speaker terminal. | >200 Ω @ 1–30 MHz; <1 Ω @ audio (20 Hz–20 kHz) |
| F6 | Bluetooth external antenna pigtail coax — SMA pigtail from board to rear panel antenna. | Fair-Rite 2643002402 clamp-on (Mix 43, 0.6″) | 1 pass, 25mm from chassis exit. | Suppress sub-harmonic leakage; minimal effect at 2.4 GHz fundamental |
| F7 | KAB-FC control harness — volume, BT button leads. Snap-on at board exit to suppress PWM pickup on control wiring. | Fair-Rite 2643102002 snap-on (Mix 43, 10mm) | Snap around bundled harness wires at KAB board exit. | Suppress TPA3116 PWM harmonics radiated by control wiring |
10.4 Winding Instructions
All toroids use a bifilar common-mode choke winding: both conductors of a pair wound together so differential-mode current (the wanted signal or DC) sees near-zero inductance while common-mode current (RF noise) sees a high impedance.
- Bifilar technique: Twist the two conductors loosely (1–2 twists/cm), then wind both through the core together. Each pass through the hole = one turn.
- Wire gauge: 20 AWG minimum for DC rail wires (F2–F4) to carry up to 3A without heating. Speaker leads (F5) 20–18 AWG. Control wires (F7) 24–26 AWG.
- Insulation: PTFE-sleeved or PVC-insulated hook-up wire. Never bare copper on live DC rails.
- Core gap: Leave a small gap between first and last turn at the core entry point to prevent voltage breakdown.
- Securing: Cable-tie finished toroids to nylon standoffs, or use a dab of RTV silicone to prevent vibration chafing.
10.5 Ferrite Bill of Materials
All parts available from Mouser Electronics, Digi-Key, or Amidon Associates (amidon-inductive.com). Fair-Rite part numbers listed; Amidon equivalents in parentheses.
| Ref | Part / Description | Fair-Rite P/N | Amidon | Est. USD | Qty |
|---|---|---|---|---|---|
| F1 | Clamp-on, Mix 31, 1.1″ core ID | 0431177081 | BSB-31-1105 | $3.50 | 1 |
| F2 | FT140-31 toroid, Mix 31, 1.40″ OD | 5977003801 | FT-140-31 | $4.50 | 1 |
| F3 (opt.) | FT114-77 toroid, Mix 77, 1.14″ OD | 5977007701 | FT-114-77 | $3.75 | 1 |
| F4 | FT114-31 toroid, Mix 31, 1.14″ OD | 5977003101 | FT-114-31 | $3.25 | 1 |
| F5 | FT140-31 toroid, Mix 31, 1.40″ OD | 5977003801 | FT-140-31 | $4.50 | 1 |
| F6 | Clamp-on, Mix 43, 0.6″ core ID (coax) | 2643002402 | BSB-43-0604 | $2.75 | 1 |
| F7 | Snap-on, Mix 43, 10mm opening (ribbon) | 2643102002 | BSB-43-1001 | $2.50 | 1 |
| Wire | 20 AWG hook-up wire, red/black pair, 1m each | — | — | $5.00 | 1m ea |
| Ferrite Sub-Total | ~$30 | ||||
10.6 Verification
With the 312B-3 Bluetooth system powered and playing audio, tune across 3.5–14 MHz on a connected receiver or transceiver with the BFO on. Note the noise floor.
- After F1 and F2 (DC lead treatment): the dominant SMPS noise should drop 15–25 dB on the S-meter.
- After F4 and F5 (21V rail and speaker leads): residual TPA3116 PWM hash should disappear.
- If 160m/80m noise persists after all chokes, add the optional F3 (Mix 77, FT114-77) in series with F2.
- For waterfall verification, a before/after waterfall screenshot across 0–30 MHz on any connected receiver or transceiver provides a definitive record of suppression achieved.