Modulus-686

The key features of the Modulus-686 are listed below: 

• Fully balanced mono construction.
• 4-layer, gold plated, ROHS compatible circuit board. Professionally assembled in Calgary, Canada.
• Output power: 360 W (4 Ω) – 240 W (8 Ω).
• Ultra-low 0.00026 % THD+N (140 W, 8 Ω, 1 kHz).
• Ultra-low 0.00048 % THD+N (100 W, 4 Ω, 1 kHz).
• Ultra-low noise: 25.0 µV (RMS, A-weighted, 20 Hz - 20 kHz).
• Gain: +26 dB. Changeable by resistor option. Minimum gain: +20 dB.
• Differential/balanced input (can also be used with single-ended/unbalanced sources).
• On-board EMI/RFI input filter and ESD protection.
• Power supply agnostic circuit architecture. The Modulus-686 performs equally well on an unregulated power supply as it does on a well-regulated power supply.
• The finished module measures 8.35 × 2.50 × 1.35 inches (Approx. 212 × 65 × 40 mm).

Build Budget

The Modulus-686 circuit board is available as a fully assembled and tested module. The wiring harness is available separately: Modulus-686 Wiring Harness.

The Modulus-686 circuit boards are professionally assembled in Calgary. My assembly vendor uses a pick-and-place machine for component placement and solders the boards using a 13-zone solder reflow oven. This procedure eliminates the issues with component cracking that you often see from the overseas assembly houses, resulting in a high-quality, high-reliability product. Thus, by relying on the professional assembly, I am able to deliver a better product at a lower price, while also increasing the chances of success for the builders.

Error Correction

The Neurochrome Modulus composite amplifier topology uses a precision amplifier to perform error correction on a less precise power amplifier. The Modulus-686 is a fully balanced composite amplifier, which uses an LME49724 to perform error correction on six LM3886es configured as a bridge-parallel power amplifier. This results in an amplifier which has the precision of the LME49724 and the power of six LM3886es. This error correction is the central point of the Neurochrome Modulus composite architecture. The composite design will correct for many types of error, including distortion and power supply induced errors.

The error correction circuit in the Modulus-686 has its own regulated power supply. Consequently, the power supply for the error correction circuit is clean and free of ripple, even if there is some ripple voltage on the power supply to the board. In addition, the error correction circuit (LME49724 and associated components) has its own power supply rejection (the PSRR of the LME49724 due to its design and architecture). The end result is that the error correction circuit will correct for any distortion and supply-induced errors in the LM3886es. This is done without introducing any errors of its own, while staying within the performance limitations of the LME49724. The end result is a very powerful amplifier with vanishingly low distortion.

As mentioned, the error correction circuit also corrects for power supply induced errors in the power amplifier. This makes the Modulus-686 indifferent to the type of power supply used. When operated at levels below clipping, the Modulus-686 performs as well on a well regulated switching supply as it does on an unregulated power supply. The block diagram of the Modulus-686 is shown below.

Balanced Input (which can also be used with unbalanced sources)

As mentioned in the Key Features, the Modulus-686 has a differential (balanced, XLR) input. There are two reasons for this:

1. Differential signalling sounds better.
2. Differential signalling measures better as it rejects hum.

Using differential signalling moves the ground connection between the various pieces of equipment out of the signal path. This results in a reduction in mains hum of about 90 dB (31,600×), which is nearly as good as you would get from an input transformer (but without the distortion of the transformer). Thus, I recommend using a differential connection to the Modulus-686. Sadly, many consumer and prosumer sources do not feature differential outputs. In those cases, I suggest using a pseudo-differential cable between the single-ended (unbalanced, RCA) source and the differential (balanced, XLR) input on the Modulus-686. These cables can easily be made by the savvy DIYer. They are also available commercially.

The specifications for the Modulus-686 are tabulated below. These were measurements of the first production sample.

The gain of the Modulus-686 can be changed by changing one resistor. Lowering the gain lowers the noise floor of the Modulus-686. The lowest gain supported is 20 dB and the performance improvements at this gain setting are listed below. The remaining performance parameters of the Modulus-686 are unaffected by the gain change.

The performance of the Modulus-686 approaches that of my Audio Precision APx555 audio analyzer. Thus, the THD+N graphs show mostly the noise of the APx555 source and the noise floor of the Modulus-686. Similarly, the THD+N vs frequency is mostly dominated by the noise of the measurement gear, which is all state of the art. The biggest take-home message here is that the Modulus-686 contributes only a minuscule amount of distortion, intermodulation, and noise to the input signal. It is as close to a straight wire with gain as you can get, and the measurements below confirm this claim. Terms like "transparent" or "wire with gain" are overused, but those really are the best descriptors for the Modulus-686. It is difficult to describe what "The Source Material, Amplified" sounds like. Open and natural, I guess. But those words are overused too... Rather than devolving into marketing babble, I'll let the performance measurements speak for themselves.

The measurements shown below were performed using a pair of Mean Well SE-600-36 switching supplies forming a ±36 V @ 16.6 A power supply. Note that these supplies are equipped with fans, thus are not desirable for use in home HiFi systems. They do work incredibly well as laboratory supplies, though.

The graph below shows the THD+N vs output power for 8 Ω load. The amplifier delivers 225 W at the onset of clipping and 240 W when the THD+N crosses 0.1 %. 

Repeating the measurement with a 4 Ω load reveals:

Soft clipping starts at just over 300 W and 0.1 % THD+N is reached at about 360 W.

The THD+N vs frequency plots for 100 W into 8 Ω and 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.

The Modulus-686 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-686 performs 10-100× better than most Class D amplifiers.

While the performance of a power amp near clipping is relevant in the characterization of the amp, the amp will spend the vast majority of the time at much lower power levels. Thus, many look to the harmonic spectrum at 1 W output for clues about the sound quality of the amp. The harmonic spectrum at 1 W output into 8 Ω is shown below for the Modulus-686. Note that the distortion is very low and entirely even-ordered with just the second and fourth harmonics showing.

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-686 accordingly. The measurement is shown below. Note that due to a limitation in the DFD IMD source of the APx555, 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 W. The result is shown below. Note that the performance of the Modulus-686 is well over 20 dB better than the performance of any of the amps shown on Linkwitz's site.

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. 

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. Note that even the tallest IMD components are 130 dB below the reference level of 100 W into 8 Ω. This is likely why the Modulus-686 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. 

The Modulus-686 shows only a minuscule 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. This is the residual hum and noise of the Modulus-686 when powered by an SMPS:

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

As mentioned in the Key Features, the Modulus-686 features a differential input. The common-mode rejection of this input is shown below.

The output impedance and resulting damping factor for 8 Ω load are shown below. 

And finally the transient response is shown below. The first plot shows the amplifier output when provided a 10 kHz square wave input. As you can see, the output is completely clean and free of ringing.

The first sign of overshoot occurs when 100 nF is added in parallel with the 8 Ω load resistor. Note that 100 nF is a rather capacitive load.

Increasing the capacitive load further causes some ringing. This is due to the resonance between the output inductors and the load capacitor. While this looks dramatic, the key point is that the ringing dies out quickly. This is the sign of an amplifier with good stability.

I recommend using a 4U tall chassis with heat sinks in a full power Modulus-686. ModuShop in Italy provides some nice and affordable chassis. The CAD drawings provided below are intended for a full-power Modulus-686 build in a ModuShop 4U x 400 x 430 mm Dissipante chassis. The drawings are provided as-is. The perforated internal plate is an option on the ModuShop Dissipante chassis. These designs do not use the internal plate.

Modulus-686 Rev. 1.1 with Guardian-86 Rev. 2.0 and Antek power transformer: 

• Neurochrome MOD686 R1p1 Stereo wAntek - ModuShop 04 400 - REV B.pdf
  
• Neurochrome MOD686 R1p1 Stereo wAntek - ModuShop 04 400 - REV B.drw
  

• Neurochrome MOD686 R1p1 Stereo wToroidy - ModuShop 04 400 - REV B.pdf
• Neurochrome MOD686 R1p1 Stereo wToroidy - ModuShop 04 400 - REV B.dwg

Modulus-686 Rev. 1.1 with Guardian-686 and Antek power transformer: 

• Neurochrome MOD686 R1p1 Stereo wAntek - ModuShop 04 400.pdf
• Neurochrome MOD686 R1p1 Stereo wAntek - ModuShop 04 400.dwg

Modulus-686 Rev. 1.1 with Guardian-686 and Toroidy power transformer: 

• Neurochrome MOD686 R1p1 Stereo wToroidy - ModuShop 04 400.pdf
• Neurochrome MOD686 R1p1 Stereo wToroidy - ModuShop 04 400.dwg

A "Safe-n-Sane" version of the Modulus-686 using two Mean Well RPS-400-27 power supplies. This allows for 130 W into 8 Ω and 200 W into 4 Ω. It also allows the amp to fit in a nice and compact 3U x 330 x 300 mm ModuShop Mini Dissipante chassis. The CAD drawing for this is provided below. 

• Modulus-686 Rev. 1.1 Safe-n-Sane: Neurochrome MOD686 MINI dissipante 03 33 300 - R1p3.pdf
• Modulus-686 Rev. 1.1 Safe-n-Sane: Neurochrome MOD686 MINI dissipante 03 33 300 - R1p3.dwg

Drill templates:

The Modulus-686 needs to be mounted on a heat sink. To make life easier for the builders, I provide the drill templates below. Do note that the drill pattern varies between revisions, so please ensure you choose the correct template.

Modulus-686 Rev. 1.1: Modulus-686_R1p1_DrillTemplate.pdf

Modulus-686 Rev. 1.0, fully assembled: ModulusModule_DrillTemplate.pdf

Modulus-686 Rev. 1.0, SMD-only (no backplate): ModulusBracket_DrillTemplate.pdf