THAT Receiver – Rev. 1.0

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 Receiver addresses this. The intent of the THAT Receiver circuit is two-fold. The circuit can be used to retrofit a differential input to your amplifiers and preamplifiers. If used with the THAT Driver, the THAT Receiver can also be used as the input section of a fully differential preamplifier.

The THAT Receiver is available in a stereo version as well as a mono version. This page shows the performance of the stereo circuit. The performance of the mono version is identical. Aside from only containing one receiver channel, the mono version has been optimized for size, hence, does not feature on-board voltage regulation.

The THAT Receiver features:

  • High-performance, stereo, differential receiver circuit with vanishingly low 0.000051 % THD.
  • EM/RF interference input filter 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.
  • Neutrik Combo input connector supporting the use of TRS as well as XLR connectors.
  • Default gain: 0.0 dB (1×). May be lowered to -3.0 dB or -6.0 dB by receiver IC selection.
  • ESD protection on input allows for hot-plugging of cables.
  • Integrated low-noise voltage regulators (stereo version).
  • 76 × 69 mm PCB size (stereo). 61 × 45 mm PCB size (mono). 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.

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The fully assembled circuits are shown below. The cost of all the components for the stereo version is just shy of $32 (Mouser, November 2015). The mono version will set you back about $15 in parts.

THAT_Receivers

Specifications

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

Parameter Value Notes
Gain 0.0 dB (1.0×)
Output Noise Floor -158 dBV 1 kHz
Residual Mains Hum -140 dBV
THD 0.000051 % Vout = 2 V RMS
THD+N 0.00070 % 1 kHz, 20 kHz BW
THD+N ≤ 0.00077 % DC – 5 kHz, 60 kHz BW
THD+N ≤ 0.0012 % 20 kHz, 60 kHz BW
Common-Mode Rejection Ratio -90 dB  60 Hz
Common-Mode Rejection Ratio -85 dB  20 kHz
IMD: SMPTE, 60 Hz + 7 kHz, 4:1 -98 dB
IMD: 18 kHz + 19 kHz, 1:1 -112 dB
Gain Flatness 0.02 dB
Bandwidth 3.4 MHz -3 dB
Power Supply Voltage ±18 V – ±40 V
Power Supply Current 12 mA

Measurements

The plot below shows the THD+N versus frequency at +18.5 dBu. Note that the THD+N is essentially flat versus frequency. This is likely the explanation for the very natural sound of this differential receiver circuit.

THAT RX R1.0_ THD+N vs Frequency @ +18.5 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 2.5 dBu are caused by the range switching of the APx525.

THAT RX R1.0_ THD+N vs Input Level

The THD is low enough that a precision oscillator is needed for an accurate measurement. 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 25-30 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 RX R1.0_ Harmonic Spectrum @ Vout = 2.0 V RMS, Precision Oscillator

The distortion is predominantly second order distortion and measures -126 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 -98 dB (0.00126 %). The test was performed at an output voltage of +18.5 dBu.

THAT RX_ IMD SMPTE (60 Hz + 7 kHz @ 4_1) @ +18.5 dBu

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 -112 dB (0.00025 %). The test was performed at an output voltage of +18.5 dBu.

THAT RX_ IMD DFD (18 kHz + 19 kHz @ 1_1) @ +18.5 dBu

The Common-Mode Rejection Ratio (CMRR) of the THAT Receiver is shown below. THAT Corp. specifies the CMRR of the THAT1200 to be 90 dB at 1 kHz. As my measurement below shows, some samples perform quite a bit better than the typical specification.

THAT RX R1.0_ Common-Mode Rejection Ratio vs Frequency

The noise floor of the THAT Receiver is shown below. Note the incredibly low -158 dBV noise floor. Also note the residual mains hum of -140 dBV.

THAT RX R1.0_ Noise Floor

The amplitude response is shown below (note the scaling on the vertical axis). The gain flatness is guaranteed by THAT Corp. to be within ±0.02 dB.

THAT RX R1.0_ Amplitude Response

The -3 dB bandwidth was measured using a function generator and oscilloscope. The bandwidth of the THAT Receiver is DC – 3.4 MHz (-3 dB).

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