Modulus-286

ADVANCED PRODUCT INFORMATION (2018/10/20): The Modulus-286 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-286 is the middle child of the Modulus family of amplifiers. It is perfect for those who want a high-performance amplifier that delivers more than 100 W into 4 Ω. Thanks to the Neurochrome Modulus error correction circuit topology, the Modulus-286 provides 125 W into 4 Ω at vanishingly low distortion levels.

Outline

Key Features

  • Mono construction.
  • 65/125 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: >560 @ 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): 27 µV (A-weighted); 32 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-286. 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-286 is a composite amplifier, which uses an LME49720 to perform error correction on two LM3886 power amplifier ICs operating in parallel. This results in an amplifier which has the precision of the LME49720 and the power of the 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-286 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 LM3886es. 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-286 indifferent to the type of power supply used. When operated at levels below clipping, the Modulus-286 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-286 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-286. 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-286. 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-286 is ±36 V with at least 6.5 A (RMS) available per channel. Thus, for a traditional unregulated power supply, I recommend a Power-86 or  Power-686 with a 2×25 VAC, 100 VA power transformer per channel. Thus, a stereo build requires a 2×25 VAC, 200 VA power transformer and the Antek AS-2225 would be a good choice here. An alternative would be the 2×24 VAC, 500 VA Hammond 1182S24. At 500 VA, it is a bit overkill, but unfortunately Hammond does not make a smaller 2×24 VAC or 2×25 VAC transformer. Several additional choices are listed in the Modulus-286 Design Documentation.

One of the better choices for a switching supply for a stereo Modulus-286 build is the ±36 V version of the SMPS300RE from Connex Electronic.

The Modulus-286 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
±36 V 8 Ω 14 dB (music) 1.53 K/W
±36 V 4 Ω 14 dB (music) 0.88 K/W
±36 V 8 Ω 3 dB (sine wave) 1.10 K/W
±36 V 4 Ω 3 dB (sine wave) 0.60 K/W

Thankfully most of us listen to music rather than sine waves, so the 0.88 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-286, assuming a maximum of two Modulus-286 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-286 are tabulated below. These were measurements of the prototype build shown above.

Parameter Value Notes
Output Power 65 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 125 W 4 Ω, < 0.01 % THD+N
THD TBD 125 W, 4 Ω, 1 kHz
THD+N TBD 125 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 TBD 65 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 TBD 65 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, 65 W, 8 Ω
Gain 26 dB Resistor programmable. +20 dB min.
Input Sensitivity 1.1 V RMS 65 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 27 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 32 µV RMS 20 Hz – 20 kHz, Unweighted
Output DC Offset Voltage < 2.0 mV Typical performance
Residual Mains Hum < -120 dBV
Dynamic Range (AES17) TBD 1 kHz
Common-Mode Rejection Ratio TBD 1 kHz
Common-Mode Rejection Ratio TBD 20 kHz
Damping Factor >560 1 kHz, 8 Ω
Damping Factor >225 20 kHz, 8 Ω
All parameters are measured at a supply voltage of ±36 V unless otherwise noted.

The gain of the Modulus-286 can be changed by changing one resistor. Lowering the gain lowers the noise floor of the Modulus-286. 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-286 are unaffected by the gain change.

Parameter Value Notes
Gain 20 dB R11 = DNP
Input Sensitivity 2.2 V RMS 65 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 ±36 V unless otherwise noted.

Performance Graphs

The performance of the Modulus-286 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-286. 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-286 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-286. 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.