Modulus-186

ADVANCED PRODUCT INFORMATION (2018/10/20): The Modulus-186 modules are currently in production. The information provided below is based on the prototype build. Slight changes in performance (most likely for the better) should be expected.

The Modulus-186 is the most compact member of the Modulus family of amplifiers. Nimble, yet powerful would be the motto of the Modulus-186. Thanks to the Neurochrome Modulus error correction circuit topology, the Modulus-186 provides nearly 70 W into 4 Ω at vanishingly low distortion levels.

Outline

Key Features

  • Mono construction.
  • 40/65 W into 8/4 Ω, respectively @ THD < -120 dBc.
  • Tested for stability with reactive loads up to 1.0 µF || 8 Ω.
  • Multi-tone IMD residual: < -105 dBV.
  • Damping factor: >500 @ 1 kHz; >225 @ 20 kHz (8 Ω).
  • Integrated noise (20 Hz – 20 kHz): 17 µV (A-weighted); 22 µV (unweighted) @ 20 dB gain.
  • Integrated noise (20 Hz – 20 kHz): 23 µV (A-weighted); 28 µV (unweighted) @ 26 dB gain.
  • Balanced input (can be connected to unbalanced sources as well).
  • Default gain: 26 dB for ease of use with other HiFi components. 20 dB available upon request. Higher gain possible by a simple resistor swap.
  • Four-layer PCB, fully optimized for the highest performance.
  • Designed, manufactured, and assembled in Canada. All components sourced from reputable distributors (Mouser, Digikey, et al.)
  • Available for pre-order with a significant early adopter discount. Expected in stock by the end of November, 2018, at which point the into sale will end.

The image below shows the prototype build of the Modulus-186. The final version will feature a Neurochrome Blue circuit board.

 

Error Correction

The Neurochrome Modulus composite amplifier topology uses a precision amplifier to perform error correction on a less precise power amplifier. The Modulus-186 is a composite amplifier, which uses an LME49720 to perform error correction on an LM3886 power amplifier IC. This results in an amplifier which has the precision of the LME49720 and the power of the LM3886. 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-186 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 (LME49720 and associated components) has its own power supply rejection (the PSRR of the LME49720 due to its design and architecture). The end result is that the error correction circuit will correct for any distortion and supply-induced errors by the LM3886. This is done without introducing any errors of its own, while staying within the performance limitations of the LME49720. The end result is a 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-186 indifferent to the type of power supply used. When operated at levels below clipping, the Modulus-186 performs as well on a well regulated switching supply as it does on an unregulated power supply.

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

As mentioned in the Key Features, the Modulus-186 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-186. 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-186. These cables can easily be made by the savvy DIYer. They are also available commercially.

Recommended Power Supply & Heat Sink

The recommended power supply for the Modulus-186 is ±30 V with at least 5 A (RMS) available per channel. Thus, for a traditional unregulated power supply, I recommend a Power-86 or  Power-686 with a 2×22 VAC, 80-100 VA power transformer per channel. The Antek AN-1222 and Hammond 1182L22 are both good choices for mono builds. A stereo build requires a 2×22 VAC, 160-200 VA power transformer (for example Antek AS-2222 and Hammond 1182N22). Several additional choices are listed in the Modulus-186 Design Documentation.

One of the better choices for a switching supply for a stereo Modulus-186 build is the SMPS300RE from Connex Electronic.

The Modulus-186 is a Class-AB amplifier, thus dissipates some amount of heat. It will therefore need to be fitted with a sizeable heat sink. The recommended heat sinks are tabulated below. These recommendations are based on an ambient temperature of 25 ºC and a maximum heat sink temperature of 60 ºC.

Supply Voltage Load Impedance Crest Factor Thermal Resistance
±30 V 8 Ω 14 dB (music) 2.12 K/W
±30 V 4 Ω 14 dB (music) 1.24 K/W
±30 V 8 Ω 3 dB (sine wave) 1.53 K/W
±30 V 4 Ω 3 dB (sine wave) 0.85 K/W

Thankfully most of us listen to music rather than sine waves, so the 1.24 K/W figure is a reasonable target and many of the commercially available chassis options feature suitable heat sinks. For example, the 2U/300 size ModuShop Dissipante chassis offers heat sinks with a thermal resistance of 0.45 K/W, thus is very well suited for a stereo (or even quad) build of the Modulus-186, assuming a maximum of two Modulus-186 modules per heat sink.

The ModuShop enclosures are available in North America from the DIY Audio Store. They’re available in the EU from ModuShop directly.

Specifications

The specifications for the Modulus-186 are tabulated below. These were measurements of the prototype build shown above.

Parameter Value Notes
Output Power 40 W 8 Ω, < 0.01 % THD+N
THD TBD 1 W, 8 Ω, 1 kHz
THD TBD 40 W, 8 Ω, 1 kHz
THD+N TBD 40 W, 8 Ω, 1 kHz
Output Power 65 W 4 Ω, < 0.01 % THD+N
THD TBD 65 W, 4 Ω, 1 kHz
THD+N TBD 65 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 TBD 40 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 TBD 40 W, 8 Ω
IMD: DFD 917 Hz + 5.5 kHz @ 1:1 TBD 1 W, 8 Ω
Multi-Tone IMD Residual < -100 dBV AP 32-tone, 40 W, 8 Ω
Gain 26 dB Resistor programmable. +20 dB min.
Input Sensitivity 0.9 V RMS 40 W, 8 Ω
Input Impedance 48 kΩ Differential and single-ended
Bandwidth TBD 1 W, -3 dB
Slew Rate TBD 8 Ω || 1 nF load
Total Integrated Noise and Residual Mains Hum 23 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 28 µV RMS 20 Hz – 20 kHz, Unweighted
Output DC Offset Voltage < 2.0 mV Typical performance
Residual Mains Hum < -130 dBV
Dynamic Range (AES17) TBD 1 kHz
Common-Mode Rejection Ratio TBD 1 kHz
Common-Mode Rejection Ratio TBD 20 kHz
Damping Factor >500 1 kHz, 8 Ω
Damping Factor >225 20 kHz, 8 Ω
All parameters are measured at a supply voltage of ±30 V unless otherwise noted.

The gain of the Modulus-186 can be changed by changing one resistor. Lowering the gain lowers the noise floor of the Modulus-186. 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-186 are unaffected by the gain change.

Parameter Value Notes
Gain 20 dB R11 = DNP
Input Sensitivity 1.8 V RMS 40 W, 8 Ω
Total Integrated Noise and Residual Mains Hum 17 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 22 µV RMS 20 Hz – 20 kHz, Unweighted
Dynamic Range (AES17) TBD 1 kHz
All parameters are measured at a supply voltage of ±30 V unless otherwise noted.

Performance Graphs

The performance of the Modulus-186 exceeds that of my Audio Precision APx525 audio analyzer. Thus, the THD+N graphs show mostly the noise of the APx525 source and the noise floor of the Modulus-186. 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-186 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-186. 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. Click on the graphs for a larger view.

Graphs to come.