Modulus-86 — Rev. 2.0

The Modulus-86 Rev. 2.0 is a composite 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-86 is a turnkey high-performance analog subsystem. It achieves state of the art performance using readily available off-the-shelf parts. The Modulus-86 uses an LME49710 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 THAT1200 differential receiver provides common-mode rejection rivaling that of a line transformer, thus, minimizing hum injection by eliminating ground loops in the signal path.

Note: Texas Instruments has decided to discontinue the DIP packed version of the LME49710 used in this project. The TO-99 (metal can) package version of the LME49710 remains in production and is pin compatible with the DIP version. The Mouser project BOM for this project now points to the LME49710HA (TO-99).  For instructions on how to fit the TO-99 on the DIP-8 footprint, see below.

Key features of the Modulus-86 Rev. 2.0:

  • Vanishingly low 0.000067 % THD (1 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00012 % THD (38 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00034 % THD+N (38 W, 8 Ω, 1 kHz).
  • 40+ W output power (8 Ω) when using the Power-86 and recommended transformer.
  • Ideally suited for multi-channel amplification.
  • Differential input with nearly 90 dB CMRR eliminates hum and ground loop issues.
  • Phenomenal power supply rejection ensuring consistent, high performance even with unregulated power supplies.
  • Ultra-low -120 dB inter-channel crosstalk greatly simplifies the power supply requirements.
  • 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 maximum 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.
  • Power and output terminal blocks accept wire sizes up to AWG 10 (5.2 mm2).
  • All leaded. Easy to solder. 90 × 70 mm board footprint.
  • Bill-of-materials includes a Mouser Electronics Project Link as well as Digikey part numbers.

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 (30+ page .pdf), including a full set of schematics, circuit description, assembly guide, and bill-of-materials will be provided to paying customers.


The assembled Modulus-86 Rev. 2.0 module is shown below. This module is the exact module used for the measurements on this page.

Modulus-86 Rev. 2.0 Assembled Board

The total parts cost for the board show above is approximately $45. A reasonable build budget for a complete stereo amplifier, including chassis, heat sinks, connectors, etc. is around $380.

Rev. 2.0 represents a major revision of the Modulus-86 circuit. Modulus-86 Rev. 2.0 offers the following key improvements over Modulus-86 Rev. 1.0:

  • Improved performance near clipping, resulting in an increase of approximately 10 % in maximum output power.
  • Improved ground layout resulting in a 10 dB reduction in mains-related signal.
  • Significantly improved DC servo. The 10 second settling time of the DC servo represents a 25× improvement over Modulus-86 Rev. 1.0.
  • Turn-on transient has been reduced by approximately 2.5× for thump-free power-up.
  • Same PCB size, connector locations, and mounting bolt pattern as Modulus-86 Rev. 1.0. This allows a Modulus-86 Rev. 2.0 board to be used as a drop-in upgrade in an existing Modulus-86 Rev. 1.0 amplifier.


Parts Availability

Texas Instruments has decided to discontinue the LME49710 by September, 2016. Currently, Newark/Farnell is the only distributor who has the LME49710NA (DIP) in stock. Once the stock has been depleted, there are a couple of options for replacements. The TO-99 can version of the LME49710, the LME49710HA, is still in production. It will require a little lead bending to get the TO-99 to work with the DIP-8 footprint, but it’s pretty straight forward. Pin 1 of the IC goes in Pin 1 on the footprint. Pin 2 to pin 2, etc. Pin 8 is marked by a metal tab on the TO-99. An example of a TO-99 adapted to a DIP-8 footprint is shown below. Do note that it’s pin 8 that’s marked with a tab on the TO-99, whereas all other IC packages have pin 1 marked.


Other candidates for drop-in replacements include the LMP8671 with an SMD-DIP adapter. Neurochrome remains fully committed to the Modulus-86 amplifier design and we will do our best to ensure a smooth transition as the LME49710NA sunsets.



The full set of specifications for the Modulus-86 Rev. 2.0 are tabulated below.

Output Power40 W8 Ω
THD0.000067 %1 W, 8 Ω, 1 kHz
THD0.00012 %38 W, 8 Ω, 1 kHz
THD+N0.00034 %38 W, 8 Ω, 1 kHz
IMD0.00069 %SMPTE, 60 Hz + 7 kHz, 4:1, 38 W, 8 Ω
Gain20.0 dB
Input Sensitivity1.75 V RMS38 W, 8 Ω
Bandwidth0.065 Hz – 85 kHz1 W, -3 dB
Full-Power Bandwidth84 kHz
Slew Rate14 V/µs 8 Ω || 1 nF load
Noise Floor-135 dBV
Residual Mains Hum-120 dBVPower-86 with recommended transformer
Total Integrated Noise and Residual Mains Hum40 µV RMS20 Hz – 20 kHz, unweighted
Dynamic Range (AES17)112 dB1 kHz
Common-Mode Rejection Ratio89 dB1 kHz
Common-Mode Rejection Ratio86 dB20 kHz
All parameters are measured using a the recommended power supply (±28 V).


Sound Quality

The first time I turned on the Modulus-86 and started the music, I went “WOW!” before even making it to my listening chair. It was immediately obvious that this amp was something special. What struck me was the level of clarity of the reproduction and the deep quiet during quiet passages in the music. Talk about a huge 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-86 really shines. The midrange is open and natural. The bass is precise and tight. What can I say? 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. I am certain the incredible sonic performance is due to the stellar supply rejection and flat distortion response of the Modulus-86.


Builder Feedback on the First Modulus-86 Revision

I have built your amps, and they sound beautiful! I am really very happy with how the amps came together, and the sound they are producing. I am also very impressed with how quiet they are at idle. It’s really quite incredible. – Mike from Canada.

I’ve built a few DIY amps. This is bar none the best build doc I have ever seen. Very well written and easy to understand. – Erik from California.

Got the boards populated, and ran a quick test with them with my Alpair 7As. I am beyond impressed with this design. Thank you again! – Brady from Minnesota.


Performance Characterization

The Modulus-86 is by far the best semiconductor-based amplifier I have ever designed, both in terms of measured performance and in terms of sound quality. The performance level in the Modulus-86 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.

The impressive performance is exemplified in the harmonic spectrum shown below. This spectrum was measured using an Audio Precision APx525 audio analyzer. The Modulus-86 was operating from a ±28 V Power-86 power supply. The full-power THD measures 0.00012 %. Click on any of the graphs below for a larger view.

Modulus-86 Rev. 2.0 Harmonic Spectrum @ 1 kHz, 38 W, 8 Ω

Even more impressive is the distortion performance at 1 W output power as shown below. For this measurement a precision 1 kHz oscillator was needed as the 0.0001 % THD of the APx525 source was dominating the THD measurement. The THD at 1 W output power measures to an impressive 0.000067 %, consisting mostly of even order harmonics. This distortion profile is reminiscent of that of a single-ended triode tube amplifier and is commonly considered to be the distortion profile that is the most pleasing to the ear.

Modulus-86 Rev. 2.0: Harmonic spectrum @ 1 kHz, 1 W, 8 Ω with precision oscillator

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

Modulus-86 Rev. 2.0 THD+N vs Output Power (1 kHz, 8 Ω)

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.

Modulus-86 Rev. 2.0: THD+N vs Frequency @ 38 W, 8 Ω

At the first watt, where the majority of listening takes place, the Modulus-86 shows an extremely flat, low THD+N profile. This is very likely the magic behind the very melodic, open, and natural presentation of this amplifier.

Modulus-86 Rev. 2.0: THD+N vs Frequency @ 1 W, 8 Ω

Finally, the noise floor is shown below. Note that the noise floor is completely free of hum components. This is responsible for the dead quiet during quiet passages in the music.

Modulus-86 Rev. 2.0: Noise Floor


Circuit Topology

The block diagram for the Modulus-86 is shown below.

Modulus-86 Rev. 2.0 Block Diagram
The LM3886 provides the output power and is controlled by an LME49710 precision audio opamp. This topology results in a composite amplifier where the small signal performance is controlled almost entirely by the LME49710 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 LME49710.

The input to the composite amplifier is provided by a THAT1200 differential receiver. 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-86 is typically about 200 µV. The DC servo in Modulus-86 Rev. 2.0 represents a significant improvement over the servo in Rev. 1.0. Specifically, Rev. 2.0 includes a DC servo with third-order filtering. This enables a fast settling time of only ten seconds while simultaneously maintaining stellar THD at 20 Hz. By comparison, the settling time in Modulus-86 Rev. 1.0 is nearly four minutes.

The Modulus-86 has a gain of +20 dB (10×). This value was chosen to ensure a good gain structure in the end application. Should a higher gain be needed, it can easily be modified for +26 dB (20×) at a minimal reduction in THD performance. Should a lower gain be desired for further optimization of the system gain structure, the Modulus-86 circuit supports the use of the THAT1203 and THAT1206 for a total amplifier gain of +17 dB (7x) and +14 dB (5x), respectively.




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.