Modulus-86
Modulus-86 Modulus-86
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$79
Current inventory: In stock.

The Modulus-86 Rev. 3.0 is a 65 W composite amplifier which uses error correction to provide world class performance. It requires no adjustments and is built using readily available parts.

The Modulus-86 is optimized for ultra-low distortion – both harmonic distortion and intermodulation distortion. The result is a crystal clear and non-fatiguing sound that is revealing without being sterile. Due to the ultra-low noise floor of the Modulus-86, builders experience complete quiet between tracks even with high-sensitivity speakers. 

Modulus-86 Rev. 3.0 THD+N vs output power, load impedance

Builder Feedback

They are the best best amps I've heard and I've listened to top of the line Krell, Levinson, Meridian and Linn. Tom's amps with good, source components, speakers and room consideration make for outstanding music reproduction for me. - Henry from Connecticut.
I have built your amps, and they sound beautiful! I am really very happy with how the amps came together, and the sound they are producing. I am also very impressed with how quiet they are at idle. It's really quite incredible. - Mike from Canada.
I've built a few DIY amps. This is bar none the best build doc I have ever seen. Very well written and easy to understand. - Erik from California. 

The Modulus-86 has also been featured on Reddit's Kit Thursday. Read more here: Modulus-86 on Reddit

Key Features

  • Mono construction: One Modulus-86 board per channel.
  • Output Power at <0.1 % THD+N
    • 65 W into 4 Ω
    • 40 W into 8 Ω
  • THD at 1 W
    • -136 dBc (0.000016 %) into 8 Ω
    • -133 dBc (0.000022 %) into 4 Ω
  • Residual hum & noise: 13.7 µV RMS (20 Hz – 20 kHz, A-weighted).
    • World class performance ensured by an error correction circuit with local, low-noise power supply regulators.
    • Power supply agnostic by design. High performance is attained with unregulated power supplies and with switch mode supplies.
    • Differential (balanced) input which eliminates hum and ground loop issues. Can be used with single-ended (unbalanced) sources as well.
    • Four-layer PCB optimized for minimum power supply and ground impedance by the use of copper planes and pours. All ground management is handled on the PCB.
    • On-board EMI/RFI input filter and ESD protection.
    • All through-hole construction with socketed ICs. 
    • Bill-of-materials includes a Mouser Electronics Project Link for ease of parts ordering. Parts cost: $50.

    All Neurochrome circuit boards are fully electrically tested by the manufacturer. The boards are gold plated, RoHS compatible, and are made in Canada.

    Canadian flag

    Modulus-86 Build Process

    To get a feel for what is involved in a Modulus-86 build, please view the build video below.


    The table below lists the recommended power supply and heat sink sizes for a Modulus-86 intended for use with 4 Ω and 8 Ω speakers. 

    Item Specification Notes
    Minimum heat sink, stereo
    0.84 K/W Two channels per heat sink
    Minimum heat sink, mono 1.68 K/W One channel per heat sink
    Minimum power transformer rating 120 VA
    Recommended power transformer voltage 2 × 22 VAC Or: 44 VCT (44 VAC, centre tapped)
    Power supply PCB Power-86 ±30 V DC, nominal
    Suitable switching power supply SMPS300REh (±30 V) Connex Electronic
    Recommended chassis 2U × 300 × 440 mm

    Dissipante Series, ModuShop
    or 'BZ4309' from eBay

    Output inductor MOD86 Inductor
    One needed per Modulus-86 PCB
    Parts cost $50 Per Modulus-86 PCB
    Expected build budget, stereo amp.
    $550


    Note that the build budget includes all parts necessary for a basic Modulus-86 stereo amplifier. The chassis and power supply account for approximately 45 % of the build budget. 

    The build budget assumes that the chassis is based on either the ModuShop 2U Dissipante or the 'BZ4309' available from many vendors on eBay and that the builder is able to drill the necessary mounting holes in the chassis. Given that the unmodified chassis accounts for nearly half of the build budget, the budget can be reduced considerably by repurposing an existing amplifier chassis. Similarly, if the builder chooses to have a chassis professionally made the build budget should be increased accordingly.

    The Modulus-86 includes a Thiele network in series with its output. This ensures the stability of the amplifier even with a capacitive load. I provide instructions for how to wind the inductor in the design documentation. You will need a length of 18 AWG (1.0 mm Ø) transformer wire to make the inductor. Should you prefer an easier route, I do have professionally made inductors available here: Modulus-86 Output Inductor. You will need one inductor per Modulus-86 board.

    The full set of specifications for the Modulus-86 Rev. 3.0 are tabulated below.

    Parameter Value Notes
    Output Power 40 W 8 Ω, < 0.01 % THD+N
    THD

    -136 dBc

    (0.000016 %)

    1 W, 8 Ω, 1 kHz
    THD

    -134 dBc

    (0.000019 %)

    40 W, 8 Ω, 1 kHz
    THD+N

    -117 dBc

    (0.00014 %)

    40 W, 8 Ω, 1 kHz
    Output Power 65 W 4 Ω, < 0.1 % THD+N
    THD

    -133 dBc

    (0.000022 %)

    1 W, 4 Ω, 1 kHz
    THD

    -119 dBc

    (0.00011 %)

    60 W, 4 Ω, 1 kHz
    THD+N

    -114 dBc

    (0.00020 %)

    65 W, 4 Ω, 1 kHz
    IMD: SMPTE 60 Hz + 7 kHz @ 4:1 -106 dB 20 W, 8 Ω
    IMD: DFD 18 kHz + 19 kHz @ 1:1 -119 dB 20 W, 8 Ω
    IMD: DFD 917 Hz + 5.5 kHz @ 1:1 -109 dB 1 W, 8 Ω
    Multi-Tone IMD Residual < -139 dB Ref.: 40 W AP 32-tone, 40 W, 8 Ω
    Gain 20 dB
    Input Sensitivity 1.8 V RMS 40 W, 8 Ω
    Bandwidth 76 kHz 1 W, -3 dB
    Slew Rate 14.4 V/µs 8 Ω || 1 nF load
    Total Integrated Noise and Residual Mains Hum 13.7 µV RMS 20 Hz - 20 kHz, A-weighted
    Total Integrated Noise and Residual Mains Hum 17.2 µV RMS 20 Hz - 20 kHz, Unweighted
    Output DC Offset Voltage < ±2 mV Typical performance
    Dynamic Range (AES17) 119 dB 1 kHz
    SINAD 111 dB 5 W, 400 Hz
    Common-Mode Rejection Ratio > 70 dB 120 Hz (typ.)
    Damping Factor 642 1 kHz, 8 Ω
    Damping Factor 214
    20 kHz, 8 Ω
    PCB Dimensions 90.2 × 69.2 mm
    Finished Assembly Dimensions and Weight

    91.2 × 69.7 × 27.5 mm

    90 g

    (W × D × H)

     

    All parameters are measured using a Power-86 with the recommended transformer (±30 V DC).

     

    The Modulus-86 is one of the best power amplifiers I have ever designed, both in terms of measured performance and in terms of sound quality. The performance level of the Modulus-86 is obtained by the Neurochrome Modulus error-correcting architecture, meticulous attention to detail during the circuit design process, and, equally important, careful PCB layout.

    The block diagram for the Modulus-86 is shown below.

    Modulus-86 Rev. 3.0 block diagram

    The Modulus-86 uses one channel of an LM4562 precision audio opamp and an LM3886 power amplifier IC to form a balanced, composite power amplifier. In this configuration the LM4562 is used both to provide error correction for the LM3886 and to provide a balanced input for the composite amp. Another LM4562 completes the front-end of the amplifier.

    The Modulus-86 is power supply agnostic by design. This is a unique feature to the Neurochrome Modulus amplifiers. Due to the error correction provided by the Neurochrome composite amplifier topology, the Modulus-86 performs as well on a traditional unregulated linear power supply or a switch mode power supply (SMPS) as it does on a well-regulated laboratory power supply.

    The composite circuit topology also ensures a low DC offset for the Modulus-86. A typical build has less than ±2 mV of DC offset. Worst case the DC offset may reach ±7 mV. These values are guaranteed by design. Note that to achieve DC offsets this low, the PCB must be cleaned of flux and other residues after soldering.

    The PCB layout is crucial at the performance levels attained by the Modulus-86. The four-layer PCB employs differential signal routing and carefully designed copper pours and planes to ensure optimal board performance. All critical connections on the PCB were modelled and optimized using simulation as well as lab experiments. The result is an amplifier that performs impeccably both at high power levels and–perhaps more importantly–at low power levels.

    The Modulus-86 has a gain of +20 dB (10×), which is the lowest gain supported by this circuit. This value was chosen to ensure a good gain structure in the end application. Should a higher gain be needed, it can easily be modified for higher values by adding a resistor. The gain can be increased (within reason) without impacting any of the other performance parameters of the amp.

    For the measurements below, the Modulus-86 was operating from a ±30 V power supply. The graph below shows the THD+N vs output power for 8 Ω load. The amplifier delivers 40 W at the onset of clipping. Note that the sharp jump at 12-15 W is caused by range switching in the APx555B. The THD+N vs output power plots mostly show the THD+N floor of the measurement system.

    Modulus-86 Rev. 3.0: THD+N vs Output Power, 8 ohm

    Repeating the measurement with a 4 Ω load reveals:

    Modulus-86 Rev. 3.0: THD+N vs Output Power, 4 ohm

    The onset of clipping occurs at 65 W. The THD+N vs frequency plots for 40 W into 8 Ω and 60 W into 4 Ω, respectively, are shown below. Note that the measurement bandwidth was changed to 60 kHz to capture at least three harmonics of 20 kHz. This also increases the noise bandwidth, hence the THD+N, of the measurement.

    Modulus-86 Rev. 3.0: THD+N vs Frequency, 8 ohmModulus-86 Rev. 3.0: THD+N vs Frequency, 4 ohm

    The Modulus-86 operates in Class AB, so the plot below may appear a bit out of place as it shows the THD+N vs output power and frequency measurement commonly found in data sheets for Class D amplifiers. I am including it here to showcase that the Modulus-86 performs 10-100× better than most Class D amplifiers.

    Modulus-86 Rev. 3.0: THD+N vs Output Power, Frequency, 8 ohm

    While good performance at high output power is a good indicator of the quality of an amplifier, stellar performance at lower power is an even better indicator. After all, a typical stereo amplifier will often operate at power levels in the 0.1-10 W range during critical listening. Thus, I measured the output spectrum of the amp at 1 W into 8 Ω. The result is shown below.

    Modulus-86 Rev. 3.0: Harmonic spectrum @ 1 W, 8 ohm

    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 Modulus-86 accordingly. The measurement is shown below. Note that due to a limitation in the DFD IMD source of the APx555B, the frequencies used must be an integer multiple of each other. Thus, I measured at 917 Hz (5500/6) + 5.5 kHz. I performed this measurement at 1.0 W. The result is shown below. Note that the performance of the Modulus-86 is over 20 dB better than the performance of any of the amps shown on Linkwitz's site.

    Modulus-86 Rev. 3.0: 1+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 the amp. The 18k+19k IMD is indicative of the loop gain available in the amp near the end of the audible spectrum, which can be telling of an amplifier's sound quality. The Modulus-86 provides excellent performance on both of these measurements.

    Modulus-86 Rev. 3.0: SMPTE IMDModulus-86 Rev. 3.0: 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 levels just below clipping (40 W, 8 Ω, which is also the 0 dB reference in the plot). Note that even the tallest IMD components are 139 dB below clipping level! This is likely why the Modulus-86 sounds transparent. This measurement shows that it does not add anything (or at least extremely little) to the source signal, even at levels just below clipping where the amplifier is working the hardest. Also note that the amplifier output is completely free of mains-related hum or noise.

    Modulus-86 Rev. 3.0: Multi-Tone IMD

    The Modulus-86 shows only a tiny amount of residual mains hum. Note that this measurement was taken with the amplifier board sitting unshielded on a lab bench, thus, actual performance once enclosed in a metal chassis is likely to be better. The plot below shows the noise floor of the amplifier when powered by a pair of well-regulated laboratory power supplies (HP 6643A).

    Modulus-86 Rev. 3.0: Residual Mains Hum & Noise

    For completeness, I measured the amplitude response and gain flatness as shown below.

    Modulus-86 Rev. 3.0: Amplitude ResponseModulus-86 Rev. 3.0: Gain Flatness

    Finally, the output impedance and resulting damping factor for 8 Ω load are shown below.

    Modulus-86 Rev. 3.0: Output ImpedanceModulus-86 Rev. 3.0: Damping Factor, 8 Ω