Modulus-286 Kit

Outline:

Introduction

The Modulus-286 Kit allows you to build a high-performance amplifier in a very small package for a very competitive price. As the name indicates, the Modulus-286 Kit uses two Modulus-286 amplifier modules along with a custom made Connex Electronic SMPS300RE power supply to form a high-end amplifier that provides 2 × 65 W (8 Ω) and 2 × 125 W (4 Ω) at distortion levels well below audible.  The two channels may be bridged to form a mono amplifier capable of delivering 140 W into 8 Ω. Bridged operation into 4 Ω is not recommended.

My prototype build is shown below. Note that the final version will feature a discreet Power ON indicator (2.0 mm in diameter) on the front panel. On top of the amp is the CD cover of Mark Knopfler’s latest album for scale.

The rear panel of the amplifier is shown below. As you can see, it is quite compact. It features XLR inputs and speakON outputs. The outputs are also provided on 5-way binding posts placed on ¾” centres to allow the use of banana plug adapters. Note that the final version will have the power switch integrated into the IEC mains power inlet on the rear panel.

 

 

Tools & Skills Needed for Assembly

Assembling the Modulus-286 Kit requires fundamental mechanics and electronics assembly skills. The main challenge of the assembly is the compactness of this amp, which will challenge those with big hands. The compactness may also test your patience. If this is the case for you, I suggest taking frequent breaks and working slowly.

I recommend having these tools on hand for the assembly:

  • 5.5 and 7 mm nut drivers or sockets
  • 4 mm hex key
  • Philips #1 and #2 screwdrivers
  • Torx-Plux IP8 (or Torx T8) and Torx T10 screwdrivers
  • 2 mm and 3 mm flat blade screwdrivers
  • Long needle nose pliers
  • Regular needle nose pliers
  • End cutters
  • Wire strippers
  • Utility knife
  • Soldering iron & solder
  • Heat gun (for shrinking heat shrink tubing)
  • Two-part epoxy (5-minute type recommended)

A machinist’s ruler or slide caliper can be handy as well.

 

The Kit Includes

  • Two Modulus-286 amplifier modules
  • One Connex SMPS300RE power supply custom made for Neurochrome
  • Chassis with all holes cut and threaded and digitally printed front and rear panels
  • Mezzanine plate for power supply
  • Connectors, binding posts, mains IEC inlet
  • All necessary screws, nuts, washers, and chassis feet
  • Power ON indicator LED and series resistor
  • All wiring internal to the chassis
  • Detailed assembly instructions

 

Specifications

The specifications for the Modulus-286 Kit are tabulated below. These were measurements of the prototype build shown above.

Parameter Value Notes
Output Power 65 W 8 Ω, 1 kHz, < 0.01 % THD+N
THD 0.00032 %

-110 dBc

1 W, 8 Ω, 1 kHz
THD 0.00045 %

-107 dBc

65 W, 8 Ω, 1 kHz
THD+N 0.00044 %

-107 dBc

65 W, 8 Ω, 1 kHz
Output Power 125 W 4 Ω, 1 kHz, < 0.01 % THD+N
THD 0.0011 %

-99 dBc

125 W, 4 Ω, 1 kHz
THD+N 0.00098 %

-100 dBc

125 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 -100 dB 65 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 -115 dB 65 W, 8 Ω
IMD: DFD 917 Hz + 5.5 kHz @ 1:1 -96 dB 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 1.15 V RMS 65 W, 8 Ω
Input Impedance 48 kΩ Differential and single-ended
Bandwidth 0.8 Hz – 75 kHz 1 W, -3 dB
Slew Rate 13 V/µs 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 < -112 dBV
Dynamic Range (AES17) 120 dB 1 kHz, A-weighted
Common-Mode Rejection Ratio 88 dB 1 kHz
Common-Mode Rejection Ratio 63 dB 20 kHz
Damping Factor >250 1 kHz, 8 Ω
Damping Factor >140 20 kHz, 8 Ω
Mains Voltage 120 V and 230 V options available
Dimensions Weight 240 × 240 × 100 mm – 3200 g

 

Performance Graphs

Click on any of the graphs for a larger view.

The results of the THD+N vs output power and THD+N vs frequency measurements are shown below for 8 Ω load. This distortion level is well below audible at all frequencies within the audible spectrum. Soft clipping sets in just above 65 W.

Note that for the THD+N vs frequency measurements, the measurement bandwidth was changed to 60 kHz to capture at least three harmonics of 20 kHz. This increases the noise bandwidth of the measurement, hence, shows slightly higher THD+N than the 1 kHz measurement, which was performed with 20 kHz measurement bandwidth.

The graphs below shows the same measurements repeated with 4 Ω load. As expected, the distortion does increase slightly with the heavier load. With one channel driven, soft clipping is reached at 125 W. 

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-286 Kit 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 and at the full output power. The results are shown below. Note that the performance of the Modulus-286 Kit is more than 10 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-286 Kit provides excellent 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 -100 dBV level. This is likely why the Modulus-286 Kit 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 mains-related signal components are down at the -110 dBV level – even when the amplifier is running at clipping levels on the signal peaks.

For an amplifier as compact as the Modulus-286 Kit, it shows amazingly low mains hum. All mains-related components are below -110 dBV (3 µV). Even on efficient speakers I hear no hum. The measurement result is shown below.

For completeness, the gain flatness is shown below. The -3 dB bandwidth measures 0.7 Hz – 75 kHz. The gain flatness is +0/-0.3 dB from 20 Hz to 20 kHz.

The Modulus-286 Kit features differential inputs by default. The main advantage of this type of input is that it rejects any hum, noise, and interference coupling onto the interconnect cable connected to the input. The amount of rejection is expressed as the common-mode rejection ratio, which is shown below.

And finally, the output impedance and damping factor are shown below. Note that these measurements were performed at the 5-way binding posts, which represents the worst case output impedance. Even then the output impedance at 1 kHz is just a tad over 30 mΩ, corresponding to a damping factor of 250.