Modulus-186

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: < -125 dB ref. 40 W (8 Ω).
  • 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): 23.0 µV (A-weighted); 28.5 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.)

The Modulus-186 amplifier module is shown below.

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.

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 VoltageLoad ImpedanceCrest FactorThermal Resistance
±30 V8 Ω14 dB (music)2.48 K/W
±30 V4 Ω14 dB (music)1.35 K/W
±30 V8 Ω3 dB (sine wave)1.71 K/W
±30 V4 Ω3 dB (sine wave)0.91 K/W

Thankfully most of us listen to music rather than sine waves, so the 1.35 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.

ParameterValueNotes
Output Power40 W8 Ω, < 0.01 % THD+N
THD-115 dB

0.00019 %

1 W, 8 Ω, 1 kHz
THDTBD40 W, 8 Ω, 1 kHz
THD+N-107 dB

0.00045 %

40 W, 8 Ω, 1 kHz
Output Power65 W4 Ω, < 0.01 % THD+N
THDTBD65 W, 4 Ω, 1 kHz
THD+N-104 dB

0.00065 %

65 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1-99.5 dB20 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1-116 dB20 W, 8 Ω
IMD: DFD 917 Hz + 5.5 kHz @ 1:1-96.5 dB1 W, 8 Ω
Multi-Tone IMD Residual< -129 dB

Ref. 40 W

AP 32-tone, 40 W, 8 Ω
Gain26 dBResistor programmable. +20 dB min.
Input Sensitivity0.9 V RMS40 W, 8 Ω
Input Impedance48 kΩDifferential and single-ended
Bandwidth< 1 Hz – 75 kHz1 W, -3 dB
Slew Rate15.9 V/µs8 Ω || 1 nF load
Total Integrated Noise and Residual Mains Hum23.0 µV RMS20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum28.5 µV RMS20 Hz – 20 kHz, Unweighted
Output DC Offset Voltage< 2.0 mVTypical performance
Residual Mains Hum< -128 dBV
Dynamic Range118 dB1 kHz, A-weighted
Dynamic Range115 dB1 kHz, unweighted
Common-Mode Rejection Ratio85 dB1 kHz
Common-Mode Rejection Ratio66 dB20 kHz
Damping Factor5451 kHz, 8 Ω
Damping Factor22020 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.

ParameterValueNotes
Gain20 dBR11 = DNP
Input Sensitivity1.8 V RMS40 W, 8 Ω
Total Integrated Noise and Residual Mains Hum17.0 µV RMS20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum21.2 µV RMS20 Hz – 20 kHz, Unweighted
Dynamic Range121 dB1 kHz, A-weighted
Dynamic Range118 dB1 kHz, unweighted
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.

The graph below shows the THD+N vs output power for 8 Ω load. The amplifier delivers just over 40 W at the onset of clipping and 45 W when the THD+N crosses 0.01 %. Note that the sharp jumps (aside from when the amplifier clips) are caused by range switching in the APx525. The THD+N vs output power plots basically show the THD+N floor of the measurement system.

Repeating the measurement with a 4 Ω load reveals:

Soft clipping starts at 65 W and 0.1 % THD+N is reached at a hair over 70 W.

In most settings, the amplifier will deliver relatively low output power most of the time, hence, many argue that the THD at 1.0 W is more indicative of the quality of the amplifier. Thus, I measured the THD at 1.0 W into 8 Ω. The THD of the Modulus-186 is below that of the APx525 signal source, hence, I used a precision oscillator. Unfortunately, this oscillator adds a bit of mains hum, but the harmonic distortion components of the 1 kHz test tone are now clearly visible.

The THD+N vs frequency plot for 40 W into 8 Ω is 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-186 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-186 performs significantly better than most Class D amplifiers.

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-186 accordingly. The measurement is shown below. Note that due to a limitation in the DFD 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. I performed this measurement at 1.0 W, similar to the measurements on Linkwitz’s site. The three results are shown below. Note that the performance of the Modulus-186 is 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. The Modulus-186 provides world class performance on both of these measurements.

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 down at the -110 dBV level. This is likely why the Modulus-186 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-186 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 even better.

As mentioned in the Key Features, the Modulus-186 features a differential input. The common-mode rejection of this input is shown below.

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

Needless to say, the Modulus-186 is a truly world class amplifier.

Feel free to join the discussion of the Modulus-186 on DIY Audio: Modulus-186 (DIY Audio).