Circuit Description

The Modulus-86 Rev. 2.2 is a composite amplifier providing true world class performance. This performance level is reached by using a specialized audio op-amp to perform error correction on a high-power chip amplifier. The Neurochrome composite amplifier topology results in a high-performance 65 W amplifier with vanishingly low distortion and phenomenal sound quality.

The Modulus-86 is a turnkey analog subsystem. It requires no adjustments and 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 perform error control on 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.

The Modulus-86 is power supply agnostic, which is a unique feature to the Modulus-86. Due to the error correction provided by the Neurochrome composite amplifier topology, the Modulus-86 performs as well on a traditional unregulated linear power supply or a switch mode power supply (SMPS) as it does on a well-regulated laboratory power supply.

Key features of the Modulus-86 Rev. 2.2:

  • 65 W (4 Ω), 40 W (8 Ω) output power when using the Power-86 and recommended transformer.
  • Mono construction: One Modulus-86 board per channel.
  • Vanishingly low 0.000061 % THD (1 W, 8 Ω, 1 kHz).
  • Ultra-low 0.000067 % THD (40 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00038 % THD+N (40 W, 8 Ω, 1 kHz).
  • Differential input with nearly 90 dB CMRR eliminates hum and ground loop issues.
  • Power supply agnostic. The phenomenal power supply rejection ensuring consistent, high performance even with unregulated power supplies and switch mode supplies.
  • Ultra-low -120 dB inter-channel crosstalk in stereo builds 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 thru-hole construction with socketed ICs. Easy to solder. 90 × 70 mm board footprint.
  • Bill-of-materials includes a Mouser Electronics Project Link for ease of parts ordering. In addition to the parts in the Mouser project, you will need about 90 cm (3′) of AWG 18 enamelled magnet wire for the output inductor.

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 circuit description, assembly guide, and bill-of-materials will be provided to paying customers.


The assembled Modulus-86 Rev. 2.2 module 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 just shy of $50. A reasonable build budget for a complete stereo amplifier, including chassis, heat sinks, connectors, etc. is around $400.

The build process was documented in great detail by one of my builders. Do note that this builder went completely overboard on component selection with “TCC Naked Z-foil” resistors and such. This is completely unnecessary from a technical performance perspective as the Modulus-86 will provide world class performance with the specified parts. The fancy components do make for a good looking amp and impressive marketing copy, however. His slide show is available here: Poseidon’s Voice – Neurochrome Modulus-86 Rev. 2.1 Build. Note that this build video refers to Rev. 2.1 of the Modulus-86.

The Modulus-86 has also been featured on Reddit’s Kit Thursday. To read the builder feedback on Reddit, please follow this link: Modulus-86 on Reddit. Some really nice pictures of a very clean build of a Modulus-86 + Power-86 amp are available here: Modulus-86 on Imgur.

Modulus-86 Rev. 2.1 used the TO-99 metal can version of the LME49710 due to Texas Instruments’ decision to temporarily halt production of the DIP version. Unfortunately the TO-99 version is about twice the cost of the DIP version and many builders found the TO-99 version harder to work with. As the two versions of the LME49710 contain the same active circuitry and provide identical performance, I switched back to the DIP version for Modulus-86 Rev. 2.2. I also took the opportunity to shave a few bucks off the BOM cost by implementing a few lessons learned from the Modulus-286 in the DC servo for the Modulus-86.

Compared to Modulus-86 Rev. 1.0, Rev. 2.2 offers the following key improvements:

  • 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 all previous Modulus-86 revisions. This allows a Modulus-86 Rev. 2.2 board to be used as a drop-in upgrade in an existing Modulus-86 amplifier.

The Modulus-86 Rev. 2.2 provides performance similar to that of the Modulus-86 Rev. 2.0 and 2.1 at a slightly lower parts cost.



The full set of specifications for the Modulus-86 Rev. 2.2 are tabulated below.

Parameter Value Notes
Output Power 40 W 8 Ω
THD 0.000061 % 1 W, 8 Ω, 1 kHz
THD 0.000067 % 40 W, 8 Ω, 1 kHz
THD+N 0.00038 % 40 W, 8 Ω, 1 kHz
Output Power 65 W 4 Ω
THD 0.000067 % 65 W, 4 Ω, 1 kHz
THD+N 0.00041 % 65 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 0.00069 % 40 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 0.00069 % 40 W, 8 Ω
Multi-Tone IMD Residual < -110 dBV AP 32-tone, 40 W, 8 Ω
Gain 20.0 dB
Input Sensitivity 1.77 V RMS 40 W, 8 Ω
Bandwidth 0.065 Hz – 85 kHz 1 W, -3 dB
Full-Power Bandwidth 84 kHz
Slew Rate 14 V/µs  8 Ω || 1 nF load
Total Integrated Noise and Residual Mains Hum 33 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 42 µV RMS 20 Hz – 20 kHz, Unweighted
Residual Mains Hum < -110 dBV
Dynamic Range (AES17) 112 dB 1 kHz
Common-Mode Rejection Ratio 89 dB 1 kHz
Common-Mode Rejection Ratio 86 dB 20 kHz
All parameters are measured using a Power-86 with the recommended transformer (±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.

It has been well over two years since I designed the Modulus-86 and it has been my “daily driver” amp ever since. I use a 4-channel Modulus-86 amp powered by a Power-86 to power a pair of Siegfried Linkwitz LXmini speakers. I still enjoy it every time I use it. I continue to be amazed by the level of detail and the imaging of the Modulus-86 and LXmini combination.


Builder Feedback

[The Modulus-86 and Parallel-86] are the best best amps I’ve heard and I’ve listened to top of the line Krell, Levinson, Meridian and Linn. Tom’s amps with good, source components, speakers and room consideration make for outstanding music reproduction for me. – Henry from Connecticut.

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.

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. Click on any of the graphs below for a larger view. The full-power THD measures 0.000067 %. 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 precision oscillator does create a bit of mains hum, resulting in some intermodulation products at multiples of 60 Hz away from the fundamental frequency. Modulus-86 R2.1_ Harmonic Spectrum (8 ohm, 1 kHz, 40 W)

Even more impressive is the distortion performance at 1 W output power as shown below. The THD at 1 W output power measures to an impressive 0.000061 %, consisting entirely of low 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 R2.1_ Harmonic Spectrum (8 ohm, 1 kHz, 1 W)

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.

Modulus-86 R2.1_ THD+N vs Output Power (8 ohm, 1 kHz, BW = 22 kHz)

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

Modulus-86 R2.1_ THD+N vs Output Power (4 ohm, 1 kHz, BW = 22 kHz)

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 40 W into 8 Ω.Modulus-86 R2.1_ THD+N vs Frequency (40 W, 8 ohm, 60 kHz BW)

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.

Modulus-86 R2.1_ THD+N vs Frequency (65 W, 4 ohm, 60 kHz BW)

The intermodulation distortion is vanishingly low as well. The graph below shows the DFD 18 kHz + 19 kHz (1:1) results. The IMD measures 0.00037 % at full output power (40 W) into 8 Ω.
Modulus-86 R2.1_ IMD - DFD (18 + 19 kHz @ 1_1; 40 W, 8 ohm)

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 0.0008 %.

Modulus-86 R2.1_ IMD - SMPTE (60 Hz + 7 kHz @ 4_1; 40 W, 8 ohm)

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 (40 W, 8 Ω). Note the IMD residuals (the “grass” most clearly visible towards 20 kHz) is below -110 dBV, a truly stellar result.

Modulus-86 R2.1_ Multi-Tone Test (40 W, 8 ohm)

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.

Modulus-86 R2.1_ Residual Mains Hum


Circuit Topology

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

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. Modulus-86 Rev. 2.2 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.

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.