Modulus-286

The Modulus-286 is a composite stereo amplifier providing true world class performance. This performance level is reached by using a high-performance audio op-amp to perform error correction on a high-power chip amplifier. The Neurochrome composite amplifier topology results in a high-performance chipamp-based amplifier with vanishingly low distortion and phenomenal sound quality.

The Modulus-286 contains two channels of Modulus-86 on one board. When configured as a stereo amplifier, the Modulus-286 provides 40 W (8 Ω), 65 W (4 Ω). For higher output power, resistor options allow for the Modulus-286 board to be built as a mono amplifier. In mono configuration, the Modulus-286 provides 60 W (8 Ω), 100 W (4 Ω).

The Modulus-286 is a turnkey high-performance analog subsystem. It achieves state of the art performance using readily available off-the-shelf parts. The Modulus-286 uses an LME49720 precision audio op-amp to control an LM3886 power amplifier. An OPA2277 precision op-amp is used as a high-end DC servo with third order filtering for an ultra-low output DC offset and fast settling time. The DC offset is less than 200 µV and is reached within ten seconds of power-up. A differential receiver based on the OPA1612 provides common-mode rejection rivaling that of a line transformer, thus, minimizing hum injection by eliminating ground loops in the signal path.

Key features of the Modulus-286:

  • Stereo: 2 × 65 W (4 Ω), 2 ×40 W (8 Ω) output power when using the Power-86 and recommended transformer.
  • Mono (parallel option): 100 W (4 Ω), 60 W (8 Ω) when using the Power-86 and recommended transformer.
  • Ultra-low 0.000068 % THD (40 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00036 % THD+N (40 W, 8 Ω, 1 kHz).
  • Ideally suited for multi-channel amplification.
  • Excellent 96 dB isolation between channels.
  • Differential input with 85+ dB typ. CMRR eliminates hum and ground loop issues.
  • Phenomenal power supply rejection ensuring consistent, high performance even with unregulated power supplies.
  • Elaborate use of planes and copper pours to maximize circuit performance by minimizing supply and ground impedances.
  • Low-inductance signal ground connects to power ground at one point only for maximum performance.
  • On-board Zobel and Thiele networks for best stability even with capacitive loads.
  • On-board EMI/RFI input filter and ESD protection.
  • On-board low noise voltage regulators for the driver op-amp and DC servo.
  • Molex Mega-Fit 23 A rated connectors used for power supply and speaker out connections. BOM includes pre-crimped wire leads and mating connectors.
  • 115 × 80 mm board footprint.
  • Bill-of-materials includes a Mouser Electronics Project Link.

Circuit boards are available for sale. Payment is handled via Paypal. You can pay with any of the major credit cards and do not need an account with Paypal to complete the purchase. The complete Design Documentation (40+ page .pdf), including a full set of schematics, circuit description, assembly guide, and bill-of-materials will be provided to paying customers.

BuyItNow_Button

The assembled Modulus-286 prototype is shown below. This module is the exact module used for the measurements on this page.

The total parts cost for the board shown above is approximately $100. A reasonable build budget for a complete stereo amplifier, including chassis, heat sinks, connectors, etc. is about $400.

The BOM cost for the mono option is $90.

In addition to the components listed on the BOM, you will need approximately 1 m (3.3′) of enamelled 18 AWG magnet wire for the inductors. The inductors are also available ready-made in DIY friendly quantities from www.iwindcoils.com.

To watch a Modulus-286 being built in less than ten seconds, view the video below.

 

Specifications

The preliminary specifications for the Modulus-286 are tabulated below. These were measurements of the prototype and are subject to (slight) change.

Parameter Value Notes
Output Power 40 W 8 Ω
THD 0.000068 % 40 W, 8 Ω, 1 kHz
THD+N 0.00036 % 40 W, 8 Ω, 1 kHz
Output Power 65 W 4 Ω
THD TBD 65 W, 4 Ω, 1 kHz
THD+N 0.00042 % 65 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 0.00060 % 40 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 0.00010 % 40 W, 8 Ω
Multi-Tone IMD Residual < -105 dBV AP 32-tone, 40 W, 8 Ω
Gain 26.0 dB
Input Sensitivity 885 mV RMS 40 W, 8 Ω
Channel Separation 96 dB 1 kHz, 40 W, 8 Ω
Bandwidth 0.065 Hz – 85 kHz 1 W, -3 dB
Full-Power Bandwidth 95 kHz
Slew Rate 15 V/µs  8 Ω || 1 nF load
Total Integrated Noise and Residual Mains Hum 23.5 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 29.5 µV RMS 20 Hz – 20 kHz, Unweighted
Residual Mains Hum < -115 dBV PSU: Connex SMPS300RE
Dynamic Range (AES17) 115 dB 1 kHz
Common-Mode Rejection Ratio 85 dB 1 kHz
Common-Mode Rejection Ratio 65 dB 20 kHz
All parameters are measured at a supply voltage of ±30 V unless otherwise noted. This is the expected performance when using a Power-86 board with the recommended power transformer.

By a few simple component changes, the Modulus-286 can be configured to use the two channels in parallel. This allows for operation on up to ±36 V power supply rails, thus, higher output power. The specifications for mono operation are tabulated below. Parameters not mentioned are unchanged from stereo operation.

Parameter Value Notes
Output Power 60 W 8 Ω, THD+N < 0.0005 %
THD 0.000078 % 55 W, 8 Ω, 1 kHz
THD+N 0.00026 % 55 W, 8 Ω, 1 kHz
Output Power 100 W 4 Ω, THD+N < 0.001 %
THD 0.00015 % 95 W, 4 Ω, 1 kHz
THD+N 0.00051 % 95 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 0.00050 % 55 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 0.00012 % 55 W, 8 Ω
Multi-Tone IMD Residual < -110 dBV AP 32-tone, 40 W, 8 Ω
Gain 26.0 dB
Input Sensitivity 1.0 V RMS 55 W, 8 Ω
All parameters are measured at a supply voltage of ±35 V unless otherwise noted. This is the expected performance when using a Power-86 board with the recommended power transformer.

 

Sound Quality

Modulus-286 is fully transparent. It neither adds to nor subtracts from the source material. The result is an amazing level of clarity, detail, and enormous dynamic range. I have played a few instruments in my life – including the trumpet and a brief stint with a drum set. It is especially important to me that metallic instruments (brass wind and cymbal for example) sound metallic and natural. This is an area that challenges many amplifiers and where the Modulus-286 really shines. The midrange is open and natural. The bass is precise and tight. I really like it… The detail reproduced from Dire Straits, “Brothers in Arms” and “On Every Street”, as well as many of Mark Knopfler’s more recent albums for example, is out of this world.

 

Performance Characterization

The performance level in the Modulus-286 is obtained by meticulous attention to detail during the circuit design process, and, just as important, careful PCB layout. The PCB layout is crucial at these performance levels. The PCB employs differential signal routing and carefully designed copper pours and planes to ensure optimal board performance. All critical connections on the PCB were modeled and optimized using simulation as well as lab experiments. The result is an amplifier that performs impeccably both at high power levels and, perhaps more importantly, at low power levels.

For the measurements shown below, the Modulus-286 was operated from a ±32 V Connex SMPS300RE power supply.

To measure the THD of the Modulus-286, a precision 1 kHz oscillator was needed as the 0.0001 % THD of the APx525 source was dominating the THD measurement. The full-power THD measures 0.000068 %.

The graph below shows the THD+N versus output power for an 8 Ω load. Note that this is THD+N, hence, the ratio of the fundamental signal to the harmonics plus the noise present in the audio band.

The THD+N versus output power with a 4 Ω load applied is shown below.

Examining the THD+N versus frequency reveals a very flat distortion profile, i.e. the amount of distortion is nearly constant regardless of input frequency. At higher output power, such as the maximum output power shown below, there is a slight rise in THD towards the high-frequency end. The output power was 45 W into 8 Ω.

When a 4 Ω load is applied, the THD+N rises slightly, most notably at higher frequencies. Note that the THD+N is still well below the perceptible limit for humans.

The intermodulation distortion is vanishingly low as well. The graph below shows the DFD 18 kHz + 19 kHz (1:1) results. The IMD measures 0.00010 % at full output power (45 W) into 8 Ω.

Another common IMD test is the SMPTE test. This test uses 60 Hz mixed with 7 kHz at a ratio of 4:1. The result is show below. The IMD at 40 W, 8 Ω measures 0.00060 %.

The most challenging IMD test is a multi-tone test. I used an Audio Precision test signal with 32 tones distributed logarithmically from 16 Hz to 20 kHz. The result is shown below (45 W, 8 Ω). Note the IMD residuals (the “grass” most clearly visible towards 20 kHz) is below -105 dBV, a truly stellar result.

Finally, the residual mains hum is shown below. Note that the noise floor is nearly completely free of hum components. This is responsible for the dead quiet during quiet passages in the music. Also note that this was measured using the Connex SMPS300RE. The Modulus-286 performs as well on a switch-mode power supply as it does on a linear supply.

Mono Option

The Modulus-286 may be configured to connect the two channels in parallel. This is accomplished by changing a couple of components. Operating the two channels in parallel allows for operation at higher supply voltages (up to ±36 V) and results in higher output power.

For the measurements below a Connex SMPS300RE adjusted to ±35 V was used. The THD+N vs output power for the Modulus-286 (mono option) is shown below for 8 Ω load. The jumps in the curve near 5 W and 20 W are caused by the range switching on the APx525 audio analyzer.

The 4 Ω load case is shown below.

 

Circuit Topology

The block diagram for one channel of the Modulus-286 is shown below.

The LM3886 provides the output power and is controlled by one half on an LME49720 precision audio opamp. This topology results in a composite amplifier where the small signal performance is controlled almost entirely by the LME49720 precision audio amplifier. The composite amplifier will, thus, exhibit vanishingly low distortion, stellar power supply rejection, and have a sonic signature very close to that of an LME49720.

The input to the composite amplifier is provided by a differential receiver consisting of an OPA1612 and the other half of the LME49720. The differential input is ideally suited for differential connections, such as the XLR connections used in professional audio, but can also be configured to accept single-ended connections, such as the commonly available RCA connectors. Differential signaling provides many advantages, the most prominent advantage being the rejection of hum. This results in complete quiet during quiet passages of the music and results in a vast, wide, and open sound stage.

To avoid capacitors in the direct signal path, a DC servo was implemented. The DC servo uses an OPA2277 opamp, selected for its stellar DC performance. The DC offset on the output of the Modulus-286 is typically about 200 µV. The DC servo has third-order filtering which enables a fast settling time of only ten seconds while simultaneously maintaining stellar THD at 20 Hz.

The Modulus-286 has a gain of +26 dB (20×). This value was chosen to ensure good compatibility with a wide range of sources. Those who wish to further optimize the gain structure in their system can reduce the gain to +20 dB (10×) by removing a resistor. Gains higher than  +26 dB can be accommodated by changing one resistor. Unlike most amplifiers, the Modulus-286 allows the gain to be changed without any impact to amplifier stability.

BuyItNow_Button

Resources

Charles Kitchin, Scott Wurcer, and Jeff Smith. Composite Audio Power Amplifiers. Electronics Now, Nov 1992: 38-44. (.pdf link).

Modulus-86 on DIY Audio: Modulus-86: Composite amplifier achieving <0.0004 % THD.

Sergio Franco. Design With Operational Amplifiers and Analog Integrated Circuits. McGraw-Hill, 2001. ISBN: 0072320842. (Amazon link).

Walt Jung. Op Amp Applications Handbook. Newness, 2004. ISBN: 0750678445. (Amazon link). Also available as a free download from Analog Devices’ website.

Texas Instruments. Active Low-Pass Filter Design. TI, 2002. TI SLOA049B App. Note.