FAB3346 is an unbalanced to balanced line driver module using a THAT1646 PDIP8 chip.

The THAT 1646 is a new generation of monolithic audio differential line driver offering improved performance over conventional cross-coupled  designs. Based on a high-performance, fully differential opamp and laser-trimmed thin-film resistors, the THAT 1646 exhibits low noise and distortion, high slew rate, and wide output swing. The THAT1646 is stable when driving difficult loads, and have short-circuit protected outputs.

The THAT 1646 chip is pin-compatible with the TI DRV134 and DRV135, as well as the Analog Devices SSM2142.

The only external components needed are 0.1uf bypass capacitors for the power supply V+ and V- lines. There is a 250mV of common-mode DC offset at the output if the outputs are directly coupled. Capacitors C2 and C3 AC-couple the common-mode feedback loop, thus reducing this common-mode offset. C2 and C3 are non-polarized electrolytic capacitors.

Though the THAT1646 chips have internal output protection, we can further improve it’s robustness by adding diodes D1 to D4. (For example: If you accidentally plug the output of the THAT1646 to your mixing board/preamp’s input and applied 48Volts phantom power, you’ll be sending this 48Volts directly to the THAT1646 — for this reason, we need to guard against this accidental condition.) The pair of diodes (for each differential line) shunts this discharge current to the power supply lines, thus protecting the output of our THAT1646 chip.

In the FAB3346 Module, signal IN- is tied to GND.  Only the IN+ signal goes to the input pin of the THAT1646 chip. The balanced output of the THAT1646 chip then goes to the OUT+ and OUT- lines of the FAB Module.

The output of the FAB3346 Module can be connected directly to our FAB3010 XLR Output Module. Since you’re converting an unbalanced signal to balanced signal, there is a relative +6dB increase in output across the differential lines (with respect to the input signal).

Optional: You may also insert an Output Transformer between the FAB3346 and FAB3010 module, providing additional galvanic isolation and transformer “iron-flavor” to your audio signal. Normally, one uses a 1:1 ratio for the output transformer (600:600 ohm impedance). You’re free to experiment and try out a 1:2 ratio for another +6dB increase in output signal.