THAT Driver — Rev. 1.01

The advantages of differential signalling are well-established in the literature and differential connections are standard in professional audio. However, adoption has been slow in home hifi products. The THAT Driver addresses this. The intent of the THAT Driver circuit is two-fold. The circuit can be used to retrofit a differential output to your single-ended source. If fitted with a volume control, it is also ideally suited as a preamplifier for amplifiers with differential inputs, such as the Modulus-86, Parallel-86, DG300B, etc.

The THAT Driver features:

  • High-performance, stereo, differential driver circuit with vanishingly low 0.000021 % THD.
  • EM/RF interference filters on both input and output ensure stellar performance even in the presence of RF signals, such as cell phone signals, WiFi, Bluetooth, etc.
  • Minimizes the hum and buzz associated with ground loops by removing the ground loops from the signal path.
  • Default gain: 6.0 dB (2×). May be increased by resistor options.
  • 100 kΩ input impedance. Changeable by resistor option.
  • ESD protection on output allows for hot-plugging of cables.
  • Integrated low-noise voltage regulators.
  • 76 × 76 mm PCB size. PCB optimized for the best signal integrity possible.

Circuit boards are available for sale. Payment is handled via Paypal. You can pay with any of the major credit cards and do not need an account with Paypal to complete the purchase. The complete Design Documentation, including a full set of schematics, circuit description, assembly guide, and bill-of-materials will be provided to paying customers.


A fully assembled board is shown below. The cost of all the components is just shy of $40 (Mouser, August 2015).THAT Driver Rev. 1.0 Assembled Board



The full set of specifications for the THAT Driver Rev. 1.0 are tabulated below.

Parameter Value Notes
Gain 6.0 dB
Output Noise Floor -152 dBV 1 kHz
Residual Mains Hum -138 dBV
THD 0.000021 % Vout = 2 V RMS
THD+N 0.00032 % 1 kHz, 20 kHz BW
THD+N ≤ 0.0007 % DC – 5 kHz, 60 kHz BW
THD+N ≤ 0.0010 % 20 kHz, 60 kHz BW
IMD: SMPTE, 60 Hz + 7 kHz, 4:1 -104 dB
IMD: 18 kHz + 19 kHz, 1:1 -114 dB
Gain Flatness 0.02 dB
Bandwidth 5.7 Hz – 320 kHz -3 dB
Power Supply Voltage ±18 V – ±40 V
Power Supply Current 25 mA


The plot below shows the THD+N versus frequency at +18.7 dBu. Note that the THD+N is essentially flat versus frequency. This is likely the explanation for the very natural sound of this differential driver circuit.

THAT Driver R1.0_ THD+N vs Frequency @ Vout = +18.7 dBu, 60 kHz BW

The THD+N vs input level graph is shown below. This measurement was taken at 1 kHz. The jumps in the THD+N curve above 12 dBu are caused by the range switching of the APx525.

THAT Driver R1.0_ THD+N vs Output Level @ 1 kHz, 22 kHz BW

The THD is low enough that a precision oscillator is needed. The built-in oscillator of the Audio Precision APx 525 is just not good enough. The precision oscillator does introduce some spurs at multiples of 30 Hz, so beware of that when interpreting the THD plot below. The THD was measured at an output voltage of 2.0 V RMS.

THAT Driver R1.0_ Harmonic Spectrum @ 1 kHz, Vout = 2.0 V RMS, Precision Oscillator

The distortion is predominantly second order distortion and measures -133.5 dB (ref. 1 kHz).

The intermodulation distortion is vanishingly low as well. The common 60 Hz + 7 kHz (4:1 ratio) SMPTE IMD is shown below. The total SMPTE IMD is -104 dB (0.00063 %). The test was performed at an output voltage of +18.5 dBu.

THAT Driver R1.0 IMD (SMPTE)

Perhaps a better indicator of good sound quality and certainly a more demanding test is the 18 kHz + 19 kHz (1:1 ratio). The result of this measurement is shown below. The IMD measures -114 dB (0.00020 %). The test was performed at an output voltage of +18.5 dBu.

THAT Driver R1.0 IMD (18k + 19k)

The noise floor of the THAT Driver is shown below. Note the incredibly low -152 dBV noise floor. Also note the residual mains hum of -138 dBV.

THAT Driver R1.0_ Noise Floor

The amplitude response is shown below (note the scaling on the vertical axis). The gain is down 0.02 dB at 20 kHz (ref. 1 kHz).

THAT Driver R1.0_ Amplitude Response

The -3 dB bandwidth was measured using a function generator and oscilloscope. The bandwidth of the THAT Driver is 5.7 Hz – 320 kHz (-3 dB).