Universal Buffer
$149
Current inventory: Out of stock, but available on backorder.
More is expected to arrive by Jan. 20, 2020.

The Universal Buffer is expected to arrive by mid-January, 2020. It will be available as a fully assembled module. I will not offer an unpopulated PCB option as I can deliver better performance at a lower price by having the boards assembled professionally.

The Universal Buffer is a two-channel (stereo) buffer, which can be configured to provide any of the following functions:

  • Single-ended (unbalanced, RCA) to differential (balanced, XLR) conversion.
  • Differential (balanced, XLR) to single-ended (unbalanced, RCA) conversion.
  • Straight-through buffer (single-ended to single-ended or differential to differential).
  • Signal inverter.
  • Can be configured with gain by adding two resistors.

By adding a volume control and input selector, multiple Universal Buffer boards can be configured as a full-featured preamplifier with support for both balanced and unbalanced sources and loads.

The Universal Buffer circuit board is designed and manufactured in Canada.

Canadian flag

The full set of specifications for the Universal Buffer are tabulated below.

Parameter Value Notes
Number of Channels 2
Power Supply Voltage ±2.5 – ±17 V
Quiescent Power Supply Current

50 mA

65 mA

Nominal

Maximum

THD

< 0.00001 %

(-140 dB)

2.0 V RMS
THD+N

< 0.00025 %

(< -112 dB)

2.0 V RMS
THD+N

< 0.00025 %

(< -112 dB)

+27.5 dBu
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 -107 dB 2.0 V RMS
IMD: DFD 18 kHz + 19 kHz @ 1:1 -119 dB 2.0 V RMS
IMD: DFD 917 Hz + 5.5 kHz @ 1:1 -109 dB 200 mV RMS
Multi-Tone IMD Residual < -148 dB Ref.: 2.0 V RMS AP 32-tone, 2.0 V RMS
Gain 0 dB (1.0×) Can be increased by resistor option.
Gain Flatness ±0.1 dB 20 Hz - 20 kHz
Bandwidth DC – 280 kHz 100 mV RMS, -3 dB
Slew Rate 2.4 V/µs 100 kΩ || 220 pF load
Total Integrated Noise and Residual Mains Hum 1.7 µV RMS 20 Hz - 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 2.1 µV RMS 20 Hz - 20 kHz, Unweighted
Dynamic Range (AES17) 137 dB 1 kHz
Channel Separation > 125 dB 20 Hz - 20 kHz
Common-Mode Rejection Ratio > 65 dB 20 Hz - 20 kHz
Input Impedance 48 kΩ
Output Impedance < 0.1 Ω
PCB footprint 60 × 65 mm
Fully assembled dimensions

60 × 65 × 15 mm

30 g

(W × D × H)

 

All parameters are measured at a power supply voltage of ±15 V, 100 kΩ load impedance, and differential input/output, unless otherwise noted.

Sound Quality

I designed the Universal Buffer to be sonically transparent and firmly believe I have achieved that goal. I generally prefer precise amplifiers as they render the source material naturally, without any colouration, and offer a spacious and realistic sound stage.

Measuring the -140 dB THD of the Universal Buffer is a significant challenge, in particular as the analyzer section of the Audio Precision APx525 analyzer is only good to -125 dB and its source to -112 dB. Thus, I use a precision oscillator with a THD below -140 dB and a notch filter to attenuate the fundamental frequency. The resulting measurement of the distortion residual is shown below. As shown, the third harmonic is just below -140 dB at 2 V RMS output voltage.

Neurochrome Universal Buffer: Distortion residual

The graph below shows the THD+N vs output level. The Universal Buffer delivers just over +27 dBu (differential) at the onset of clipping. With single-ended input, the maximum clean output is 6 dB lower. Note that the sharp jumps (aside from when the buffer clips) are caused by range switching in the APx525. The THD+N vs output power plots mostly show the THD+N floor of the measurement system.

Neurochrome Universal Buffer: THD+N vs I/O configuration and output level.

The Universal Buffer can be operated at a power supply voltage of ±2.5 to ±17 V. The absolute maximum supply voltage is ±18 V. The power supply should be regulated for the best performance. 

Naturally, the maximum clean output voltage provided by the Universal Buffer depends on the applied supply voltage. The relationship is shown in the graph below.

Neurochrome Universal Buffer: THD+N vs power supply voltage and output level

The THD+N versus frequency measurement below basically shows the noise floor of my APx525 audio analyzer.

Neurochrome Universal Buffer: THD+N vs frequency (2 V RMS)

Siegfried Linkwitz argues that the 1 kHz + 5.5 kHz intermodulation distortion (IMD) measurement is one of the measurements which is more indicative of the perceived sound quality. He bases this argument on the fact that IMD products in this measurement fall in the frequency range where the ear is the most sensitive (see the Fletcher-Munson curves for more detail). I think this argument carries a good amount of weight, so I measured the Universal Buffer accordingly. The measurement is shown below. Note that due to a limitation in the IMD source of the APx525, the frequencies used must be an integer multiple of each other. Thus, I measured at 917 Hz (5500/6) + 5.5 kHz. The result is shown below. 

Neurochrome Universal Buffer: 1+5.5 kHz intermodulation distortion

The more conventional IMD measurements are shown below. The two plots show the SMPTE (60 Hz + 7 kHz @ 4:1) IMD and DFD (18 kHz + 19 kHz @ 1:1) IMD, respectively. Poor SMPTE IMD is often indicative of thermal issues or power supply issues in a circuit. The 18k+19k IMD is indicative of the loop gain available in the buffer near the end of the audible spectrum, which can be telling of the sound quality. The Universal Buffer provides exemplary performance on both of these measurements.

Neurochrome Universal Buffer: SMPTE intermodulation distortionNeurochrome Universal Buffer: 18+19 kHz intermodulation distortion

Audio Precision has developed a multi-tone test signal, which contains 32 tones from 15 Hz to 20 kHz, logarithmically spaced in frequency. This test signal sounds a bit like an out-of-tune pipe organ. It is basically the closest I can get to music with a deterministic test signal. Thus, I argue that this multi-tone signal should be used in an IMD test for the best correlation between measurements and perceived sound quality. I run this test at 2.0 V RMS (which is also the 0 dB reference in the plot). Note that even the tallest IMD components are more than 147 dB below line level. This is likely why the Universal Buffer sounds transparent. This measurement shows that it does not add anything (or at least extremely little) to the source signal. Also note that the output is completely free of mains-related hum or noise.

Neurochrome Universal Buffer: Multi-tone intermodulation distortion

The plot below shows the residual mains hum and noise of the Universal Buffer when powered by a regulated power supply. The result shows the buffer outputs only minimal mains-related signal components (which are most likely from the test setup).

Neurochrome Universal Buffer: Residual mains hum and noise

As the Universal Buffer is a stereo circuit, I measured the channel separation. As shown below, the channel separation is truly exemplary, measuring nearly 140 dB midband.

Neurochrome Universal Buffer: Channel separation

When used with a differential source, the Universal Buffer provides rejection of any signals imposed equally on both wires in the differential signal pair. Common sources of such common-mode noise includes mains hum. As shown below, the Universal Buffer provides more than 85 dB of CMRR at the mains frequency.

Universal Buffer: Common-Mode Rejection Ratio

For completeness, the amplitude response and gain variation are shown below.

Neurochrome Universal Buffer: Amplitude (frequency) responseNeurochrome Universal Buffer: Gain flatness

Transient Response Measurements

One of the many applications of the Universal Buffer is as the output stage in a preamplifier. Thus, the Universal Buffer should be able to drive the capacitance of the interconnect cable connecting the Universal Buffer to the power amp or load. This cable capacitance tends to measure roughly 33-50 pF/m. Thus, as shown below, the Universal Buffer is capable of driving over 100 m of interconnect cable cleanly.

The measurements below show the transient response of the Universal Buffer for increasing load capacitance. On its single-ended output, the Universal Buffer drives 10 nF cleanly and 22 nF with controlled overshoot. Even at higher load capacitances, the Universal Buffer remains stable.

Universal Buffer: Transient Response (CL = 1.0 nF)Universal Buffer: Transient Response (CL = 4.7 nF)Universal Buffer: Transient Response (CL = 10 nF)Universal Buffer: Transient Response (CL = 22 nF)

The measurements for the differential output are shown below. As shown, the Universal Buffer can drive 4.7 nF cleanly on the differential output and even drive 10 nF with controlled overshoot.

Universal Buffer: Transient Response, Differential (CL = 1.0 nF)Universal Buffer: Transient Response, Differential (CL = 4.7 nF)Universal Buffer: Transient Response, Differential (CL = 10 nF)