Differential Preamp 8×2 – Rev. 1.0

2019/03/24: The Differential Preamplifier 8×2 has been discontinued. Future all-SMD builds will be machine-assembled as I, thereby, can provide a better and more consistent product for a lower price.

The Differential Preamplifier 8×2 is a preamplifier with four differential stereo inputs and one stereo differential output. The inputs are selected by Kemet relays with gold plated silver contacts designed for use with low-level signals. The volume control is implemented by an Alps “Blue Velvet” (RK271-series) conductive polymer potentiometer. The Differential Preamplifier 8×2 is intended as a high-end audio subsystem and has been designed to deliver world class performance using readily available parts.

The advantages of differential signalling are well-established in the literature and differential connections are standard in professional audio. By using differential (aka balanced) signalling, any ground loop is effectively removed from the signal path, resulting in a dramatic (~90 dB) reduction in mains hum. Despite the advantages, adoption of differential signalling has been slow in home hifi products. Hence, the Differential Preamplifier 8×2 does make it possible to build the any of the inputs to accept a single-ended (unbalanced) source. It is also possible to connect a single-ended source to the differential inputs by using a pseudo-differential cable.

The Differential Preamp 8×2 features:

  • High-performance 4-input stereo differential preamplifier with vanishingly low 0.000070 % THD and a signal-to-noise ratio of 112 dB.
  • Alps “Blue Velvet” volume potentiometer.
  • Kemet relays with gold plated silver contacts optimized for low-level signal routing.
  • EM/RF interference filters on both inputs and outputs ensure stellar performance even in the presence of RF signals, such as cell phone signals, WiFi, Bluetooth, etc.
  • ESD protection on both inputs and outputs allowing for hot-plugging of cables.
  • Minimizes the hum and buzz associated with ground loops by removing the ground loops from the signal path.
  • Default gain: 5.55 dB (1.90×). May be increased by resistor options.
  • On-board international power supply (85 – 264 VAC @ 47 – 440 Hz).
  • Board dimensions: 10.8 × 3.50 inches (approx. 275 × 89 mm).

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 (including a project link set up at Mouser Electronics) will be provided to paying customers.

A fully assembled Differential Preamplifier 8×2 is shown below along with the companion input selector board (Selector x4). The BOM cost of the Differential Preamp 8×2 is approximately $165. The Selector BOM is about $16. In addition to the parts included on the BOM, you will need an IEC AC mains entry module, a power switch, and a 2 A slow-blow 5×20 mm fuse. Suggested Mouser part numbers for these items are provided in the design documentation. The two boards shown below are available separately or as a board set.

Differential Preamplifier 8x2, Fully Assembled


The full set of specifications for the Differential Preamplifier 8×2 Rev. 1.0 are tabulated below.

Input Impedance, Differential48.0 kΩ
Gain5.55 dB (1.90×)Max. volume
Signal-to-Noise Ratio (SNR)112 dBMax. volume, Vout = +20 dBu
Signal-to-Noise Ratio (SNR)103 dBWorst case gain (-0.45 dB), Vout = +20 dBu
Output Noise + Hum18.5 µVA-weighted
Output Noise + Hum21.0 µVUnweighted, BW = 20 kHz
Residual Mains Hum-125 / -125 dBVInput 4 / 1, min. volume
Residual Mains Hum-127 / -115 dBVInput 4 / 1, max. volume
THD0.000070 %1 kHz, Vout = 2 V RMS
THD0.000126 %1 kHz, Vout = +16.0 dBu
THD+N0.00047 %1 kHz, +20 dBu, 20 kHz BW
THD+N≤ 0.00093 %< 5 kHz, 60 kHz BW, +20 dBu
THD+N≤ 0.00135 %20 kHz, 60 kHz BW, +20 dBu
IMD: SMPTE, 60 Hz + 7 kHz, 4:1-100 dBVout = +20 dBu
IMD: 18 kHz + 19 kHz, 1:1-113 dBVout = +20 dBu
L/R Channel Separation> 110 dB
Attenuation of Inactive Input> 110 dB
Common-Mode Rejection Ratio90 dB60 Hz
Common-Mode Rejection Ratio85 dB20 kHz
Gain Flatness0.02 dB
Bandwidth700 kHz-3 dB, Vout = 2 V RMS
Power Supply Voltage85 – 264 VAC47 – 440 Hz
Power Consumption7.5 WTypical, idle, incl. Selector x4 @ 120 VAC


The THD+N vs Frequency of the Differential Preamp 8×2 is shown below. The measurement was taken at an output level of +20 dBu with the volume control at max volume. The high output level was necessary to get the harmonics to emerge from the noise floor. The observant reader will notice that the THD+N curve is essentially flat vs frequency. This is likely part of the reason for the open, neutral, and transparent sound of this preamp.

DIFF PRE 8x2 R1.0_ THD+N vs Frequency @ Vout = +20 dBu (7.74 V RMS) BW = 60 kHz

The THD+N vs output level is shown below. Note that this measurement was taken at 1 kHz using 20 kHz measurement bandwidth, hence, the lowest THD+N is even lower than that shown above.

DIFF PRE 8x2 R1.0_ THD+N vs Output Level @ 1 kHz, BW = 20 kHz

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. The harmonic distortion is mainly second order with a tiny bit of third order barely visible above the noise floor. The total THD is 0.000070 % (-123 dB).

DIFF PRE 8x2 R1.0_ THD @ Max Volume, Vout = 2.0 V RMS

Measuring the THD at a higher output level reveals that the distortion is indeed predominantly low-order (2nd and 3rd order) with a bit of the 5th and 6th harmonic showing as well. The measurement was taken at an output level of +16 dBu (4.89 V RMS). The voltage was limited by the output capabilities of the precision oscillator. The total THD measures a vanishingly low 0.000126 % (-118 dB).

DIFF PRE 8x2 R1.0_ THD @ Max Volume, Vout = +16 dBu (4.9 V RMS)

The intermodulation distortion is extremely low as well. The 60 Hz + 7 kHz (4:1 ratio) SMPTE IMD is shown below. The total SMPTE IMD is -100 dB (0.0010 %). The test was performed at an output voltage of +20 dBu.

DIFF PRE 8x2 R1.0_ IMD SMPTE (60 Hz + 7 kHz; 4_1) @ Vout = +20 dBu (7.74 V RMS)

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 -113 dB (0.00022 %). The test was performed at an output voltage of +20 dBu.

DIFF PRE 8x2 R1.0_ IMD DFD (18 kHz + 19 kHz; 1_1) @ Vout = +20 dBu (7.74 V RMS)

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.

DIFF PRE 8x2 R1.0_ Common-Mode Rejection Ratio vs Frequency

The residual mains hum is incredibly low at -127 dBV (446 nV) for the worst case mains harmonic. The measurement for Input 4 is shown below.

DIFF PRE 8x2 R1.0_ Channel 4, Residual Mains Hum, Max Volume

Input 1 is located right by the power supply modules, hence, shows slightly more mains hum as shown below. The measurement was done with the volume turned all the way up. The mains hum does reduce to -125 dBV when the volume is turned down.

DIFF PRE 8x2 R1.0_ Channel 1, Residual Mains Hum, Max Volume

To bring this hum level into perspective, it is worth noting that about 55-60 dB (500-1000×) of gain would be needed to bring this hum up to a barely audible SPL in an efficient speaker.

The channel separation between the left and right channels is shown for Input 1 and Input 4 in the plot below. The channel separation pushes the measurement limits of the Audio Precision APx525.

DIFF PRE 8x2 R1.0_ L_R Channel Separation vs Frequency

The frequency response of the Differential Preamplifier 8×2 is shown below. The gain is straight as an arrow and comes in at 5.55 dB. THAT Corporation specifies the gain variation of the THAT1200 and THAT1646 to be ±0.02 dB, and clearly, the ICs meet this spec.

DIFF PRE 8x2 R1.0_ Gain

The bandwidth of the preamp is limited by an internal RF filter to 700 kHz (-3 dB).