Skip to content

Purifi 1ET400A / Hypex NC500 Input Buffer

Availability:
5+ in stock, ready to be shipped
Original price $129 - Original price $129
Original price
$129
$129 - $129
Current price $129

This input buffer is intended for use with the Purifi 1ET400A and Hypex NC500 Class D amplifier modules.

Unlike other buffers, including the Purifi EVAL1 and the Hypex NC500 Evaluation Board, this Input Buffer will provide a differential output for the Class D amplifier, even if the input to the Buffer is single-ended. This ensures that even a single-ended source can drive the 1ET400A and NC500 to clipping, something the Purifi EVAL1, Hypex NC500 Evaluation Board, and many other input buffers cannot do. Many also report improvements in sound quality when the Purifi 1ET400A and Hypex NC500 are provided a differential input.

Read Positive Feedback's review of the Input Buffer and comparison with Purifi's EVAL1 here: Neurochrome Purifi 1ET400A / Hypex NC500 Input Buffer Review.

  • Description
  • Specifications
  • Circuit Topology
  • Performance Graphs
  • The buffer is a single-channel (mono) buffer, with the following features:

    • Differential (balanced, XLR) input, which can easily be configured for single-ended (unbalanced, RCA) operation as well.
    • True differential output with common-mode feedback, which ensures a differential output, even when used with a single-ended source. 
    • Jumper-selectable gain: 0 dB and 13.2 dB for a total amplifier gain of 12.8 dB and 26.0 dB, respectively, when used with the Purifi 1ET400A.
    • Gain can be further customized by adding a resistor (1206 size SMD and leaded/PTH supported).
    • Plug-and-play connection with Hypex SMPS1200 and future Purifi power supply.
    • Opamp power supply: Use the AUX supply provided by the SMPS1200, or add your own regulator. A small daughterboard with support for the Hypex HPR/HNR, LM7812/LM7912, and LM78L12/LM79L12 is available as a product option.
    • Status indicator LED output (NC500: CLIP and READY; 1ET400A: READY).
    • Speaker output is provided both on 6.3 × 0.81 mm quick-connect spades and a terminal block accepting up to AWG 10 (5.2 mm2) in size.

    In addition, the following options are provided for advanced users:

    • The buffer can be disabled by removing two resistors. A three-pin connector footprint provides direct access to the Purifi 1ET400A / Hypex NC500 input.
    • A two-pin connector footprint provides access to the remote sensing pins of the Purifi 1ET400A / Hypex NC500.

    Please refer to the Purifi 1ET400A and Hypex NC500 data sheets for further information on how to use these two advanced features.

    The Purifi 1ET400A / Hypex NC500 Input Buffer circuit board is fully assembled, RoHS compliant, and is designed and manufactured in Canada.

    Canadian flag
  • The full specifications for the Purifi 1ET400A / Hypex NC500 Input Buffer are tabulated below.

    Parameter Value Notes
    Quiescent Power Supply Current
    20 mA
    27 mA
    Nominal
    Maximum
    THD TBD
    (TBD dB)
    Vin = 2.0 V RMS
    THD+N < 0.00025 %
    (< -112 dB)
    Vin = 2.0 V RMS
    THD+N 0.00025 %
    (< -112 dB)
    Vout = 9.6 V RMS
    IMD: SMPTE 60 Hz + 7 kHz @ 4:1 -107 dB
    Vin = 2.0 V RMS
    IMD: DFD 18 kHz + 19 kHz @ 1:1 -116 dB
    Vin = 2.0 V RMS
    IMD: DFD 917 Hz + 5.5 kHz @ 1:1 -111 dB
    Vin = 200 mV RMS
    Multi-Tone IMD Residual < -146 dB Ref.: 9.12 V RMS AP 32-tone, Vin = 2.0 V RMS
    Gain 0 dB (1.0×)
    13.2 dB (4.8×)
    Can be further customized by resistor option.
    Gain Flatness ±0.02 dB 20 Hz - 20 kHz
    Bandwidth DC – 385 kHz 100 mV RMS, -3 dB
    Slew Rate ±20 V/µs Fully symmetric 
    Common-Mode Rejection Ratio (CMRR)

    > 100 dB

    > 65 dB

    60 Hz, typical

    20 kHz, typical

    Total Integrated Noise and Residual Mains Hum

    4.0 µV RMS (13 dB)

    1.6 µV RMS (0 dB)

    20 Hz - 20 kHz, A-weighted
    Total Integrated Noise and Residual Mains Hum

    5.0 µV RMS (13 dB)

    2.0 µV RMS (0 dB)

    20 Hz - 20 kHz, Unweighted
    Dynamic Range (AES17) 127 dB 1 kHz, Vin = 3.1 V RMS
    Input Impedance 48 kΩ
    PCB footprint 64 × 64 mm
    Full module dimensions

    64 × 66 × 28 mm

    40 g

    (W × D × H)

     

    All parameters are measured at an opamp power supply voltage of ±12 V, 4.4 kΩ differential (2.2 kΩ from signal to ground) load impedance, differential input, and 13.2 dB gain unless otherwise noted.

    Sound Quality

    I designed the Purifi 1ET400A / Hypex NC500 Input 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. This aligns well with the design philosophy of Purifi and Hypex as well.

  • The Purifi 1ET400A / Hypex NC500 Input Buffer features a balanced differential input. This input can accept both differential (balanced) signals and, with a simple wire link, single-ended (unbalanced) signals. The Buffer provides a balanced differential output for the Class D amplifier module. The Buffer supports two gain settings, selectable by a jumper: 0 dB and +13.2 dB. This allows for an input-to-output amplifier gain of 26.0 dB when used with the Purifi 1ET400A. The gain of the Buffer can also be customized by adding a resistor (1206 SMD size or leaded/PTH).

    The speaker output of the Purifi 1ET400A / Hypex NC500 is brought out on ¼" (6.3 × 0.81 mm) quick-connect spades and on a terminal block supporting the use of speaker wires up to AWG 10 (5.2 mm2) in size.

    In addition to the audio signals, the Buffer also provides the CLIP and READY status outputs with appropriate LED drivers. Note that only the Hypex NC500 provides the CLIP signal, whereas both the Purifi 1ET400A and the Hypex NC500 provide the READY signal.

    A simplified block diagram of the Purifi 1ET400A / Hypex NC500 Input Buffer is shown below.

    Purifi 1ET400A / Hypex NC500 Input Buffer Block Diagram

  • Measuring the THD of the Purifi 1ET400A / Hypex NC500 Input 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. I normally use a precision oscillator and notch filter for this, but I do not yet have differential versions of these circuits. I will add the FFT plot here once it becomes available. I expect the THD of the Input Buffer to be around -130 dBc at 13.2 dB gain and below -140 dBc at unity gain.

    The graph below shows the THD+N vs output level for the two gain settings of the Buffer. The Input Buffer delivers just over +25 dBu (differential) at the onset of clipping, which results in 3.2 dB headroom when used with the Purifi 1ET400A. The difference in THD+N between the two gain settings is caused by the amplification of the APx525 signal source's noise floor. 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.

    Purifi 1ET400A / Hypex NC500 Input Buffer: THD+N vs Output Level

    As noted, the difference in THD+N between the two gain settings is caused by the buffer amplifying the noise of the signal source in the APx525. You can easily convince yourself of this by examining the plot below, which shows the THD+N vs input level.

    Purifi 1ET400A / Hypex NC500 Input Buffer: THD+N vs Input Level

    Observant readers will note that the input impedance of the Hypex NC500 is lower than that of the Purifi 1ET400A (3.6 kΩ vs 4.4 kΩ, differential, respectively). Thus, I measured the THD+N vs Output Level for the two different impedances. The results are basically line-on-line as shown below.

    Purifi 1ET400A / Hypex NC500 Input Buffer: THD+N vs Output Level, Load Impedance

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

    Purifi 1ET400A / Hypex NC500 Input Buffer: THD+N vs Frequency, Gain

    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 Input 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. 

    Purifi 1ET400A / Hypex NC500 Input Buffer: 1 kHz + 5.5 kHz IMD

    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 Input Buffer provides exemplary performance on both of these measurements.

    Purifi 1ET400A / Hypex NC500 Input Buffer: SMPTE IMD - 60 Hz + 7 kHzPurifi 1ET400A / Hypex NC500 Input Buffer: 18+19 kHz IMD

    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 Input 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.

    Purifi 1ET400A / Hypex NC500 Input Buffer: Multi-Tone IMD

    The plot below shows the residual mains hum and noise of the Input 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).

    Purifi 1ET400A / Hypex NC500 Input Buffer: Residual Mains Hum

    The common-mode rejection ratio (CMRR) describes a differential input's ability to reject common-mode signals, such as mains hum. The CMRR of the Purifi/Hypex Input Buffer is shown below.

    Purifi 1ET400A / Hypex NC500 Input Buffer: Common-Mode Rejection Ratio - CMRR

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

    Purifi 1ET400A / Hypex NC500 Input Buffer: Amplitude ResponsePurifi 1ET400A / Hypex NC500 Input Buffer: Gain Flatness / Deviation

    Transient Response Measurements

    The transient response of the Purifi 1ET400A / Hypex NC500 Input Buffer is shown below. As shown, the response is clean and completely free of ringing and overshoot.

    Purifi 1ET400A / Hypex NC500 Input Buffer: Transient Step Response - 10 kHz square wave