Balanced Line Driver

The signal path is short and the design relies on two purpose-built ICs. Reading from input to output: a coupling cap blocks DC and feeds a level-trim pot, the wiper drives the balanced line driver IC (U1) directly, and the IC's two complementary outputs pass through DC-blocking bipolar electrolytics to the XLR jacks. A small clamp-diode network protects the driver from any phantom power that may be present on the receiving end.

The supporting circuitry is the power section: a charge-pump IC (IC1) inverts the +9V rail to produce −9V, giving U1 the ±9V split supply it needs. A series diode on the input and a catch diode on the −9V output give reverse-polarity protection.

Stage-by-stage signal path

Input stage — The signal enters at IN, passes through C8 (10µ bipolar) for DC blocking, and lands on the high (E) end of the ADJ trim pot. The trim wiper feeds U1's VIN pin. C8 is bipolar because the wiper sits near 0V but can swing in either direction, so a polarised electrolytic would see reverse bias on negative half-cycles.

Driver IC (U1) — The DRV134PA / THAT 1646 is a self-contained differential line driver. A single-ended input at VIN produces two complementary outputs at +VO and −VO, each delivering half of the differential signal. The IC has internal sense pins (+SENSE, −SENSE) that close the feedback loop after the output coupling caps — this is what gives the design its very low output impedance and its ability to drive long cables without sounding dull.

Output coupling — C4 and C5 (10µ bipolar) sit between each output pin and the corresponding sense pin, blocking any DC offset before it reaches the XLR jack. Because the sense loop closes after these caps, the driver actively compensates for them — so they don't introduce frequency response error the way a passive coupling cap would.

Phantom-power clamp — Each XLR output pin is bracketed by two diodes — one to +9V and one to −9V. If a console accidentally sends 48V phantom to the line driver, these diodes forward-bias and clamp the input to roughly +9.6V or −9.6V, dumping the surge current into the rails instead of the driver IC. This is essential — the DRV134/THAT1646 outputs are not natively phantom-power tolerant.

Power section — D1 is the reverse-polarity protection diode on the +9V input; C1 (100µ) provides bulk decoupling. IC1 (ICL7660S) is wired in its standard voltage-inverter configuration with C2 (10µ) as the flying capacitor between pins 2 and 4, and C3 (10µ) as the output reservoir on pin 5. D2 protects against −9V being driven positive by an external source. C6 and C7 (100n) are local high-frequency bypasses at U1's supply pins.

This circuit has no filters, no discrete gain stages, and no clipping path — all of the audio work happens inside U1. So the relevant analysis is just the operating envelope of the driver IC itself. Both supported parts are pin-compatible drop-in replacements; the differences below are subtle and audible only in extreme situations.

DRV134PA (Burr-Brown / Texas Instruments)

THAT 1646 (THAT Corporation)

The board has no resistors — the differential gain network is fully inside the driver IC. The only adjustable element is the ADJ input-level trim pot.

The board is small and the parts count is low — a careful builder can finish this in well under an hour. Populate by part height, lowest first, so each step keeps the board flat against the workbench.

Wire your XLR jack to the board following the standard pin assignment for balanced audio. The XLR1 and XLR2 pads on the board are the two complementary outputs — XLR1 carries the in-phase (hot) signal and XLR2 carries the inverted (cold) signal.

Use a shielded twisted pair from the board pads to the XLR jack if the run is more than a few centimetres. Inside a small enclosure, plain hookup wire is fine — the differential output rejects local interference very well.

Whether you build this as a stand-alone unit or drop the board inside an existing pedal or amp enclosure is entirely a matter of preference — the four mounting holes accept M3 standoffs, so retrofitting is straightforward.

Setting the input trim

With the pedal connected to a mixer or interface that has a clip indicator, play your loudest expected source signal. Start with the ADJ pot fully counter-clockwise (minimum) and slowly rotate clockwise until the receiving input is reading a comfortable level — typically with peaks around −6 dBFS for digital inputs, or with the channel meter LED just below clip on an analogue console. If you can't reach a useful level even with the trim fully clockwise, the source is too quiet — check earlier gain stages in your chain.

Use cases

Pedalboard DI — Place the line driver as the last block in your effects chain, feeding straight to the front-of-house mixer. This sends a clean, balanced signal over the long cable run to the desk while you also keep your local amp feed.

Studio recording — Connect the XLR output directly to a recording interface's line input. Because the output is balanced and the IC has very low output impedance, you avoid cable-loading high-end roll-off even with consumer-length XLR runs.

Amp simulator buffer — Pair this with a cabinet simulator (analog or IR-based) to send a mic-replacement signal directly to the desk in addition to your stage amp.

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