Circuit Description

The Modulus-86 Rev. 2.3 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. This 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 one channel of an LME49720 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. 2.3:

  • 70 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.
  • On-board error correction circuit with local low-noise power supply regulators.
  • Ultra-low 0.000054 % (-125 dB) THD (1 W, 8 Ω, 1 kHz).
  • Ultra-low 0.000088 % (-121 dB) THD (40 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00042 % (-108 dB) THD+N (40 W, 8 Ω, 1 kHz).
  • Differential input with nearly 90 dB CMRR eliminates hum and ground loop issues.
  • Differential/balanced/XLR input can be used with single-ended/unbalanced/RCA sources as well.
  • 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.
  • 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 ($50), you will need one output inductor ($8) per Modulus-86 board (or about 60 cm (2′) of AWG 18 enamelled magnet wire if you would rather wind your own).

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. 2.3 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 $58, including the output inductor. 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.



The full set of specifications for the Modulus-86 Rev. 2.3 are tabulated below.

Parameter Value Notes
Output Power 40 W 8 Ω, < 0.01 % THD+N
THD -125 dB (0.000054 %) 1 W, 8 Ω, 1 kHz
THD -121 dB (0.000088 %) 40 W, 8 Ω, 1 kHz
THD+N -108 dB (0.00042 %) 40 W, 8 Ω, 1 kHz
Output Power 70 W 4 Ω, < 0.1 % THD+N
THD -121 dB (0.000085 %) 65 W, 4 Ω, 1 kHz
THD+N -104 dB (0.00067 %) 70 W, 4 Ω, 1 kHz
IMD: SMPTE 60 Hz + 7 kHz @ 4:1 -105 dB 20 W, 8 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:1 -112 dB 20 W, 8 Ω
IMD: DFD 917 Hz + 5.5 kHz @ 1:1 -104 dB 1 W, 8 Ω
Multi-Tone IMD Residual < -105 dBV AP 32-tone, 40 W, 8 Ω
Gain 20 dB
Input Sensitivity 1.8 V RMS 40 W, 8 Ω
Bandwidth 66 kHz 1 W, -3 dB
Slew Rate 14 V/µs 8 Ω || 1 nF load
Total Integrated Noise and Residual Mains Hum 32.0 µV RMS 20 Hz – 20 kHz, A-weighted
Total Integrated Noise and Residual Mains Hum 40.0 µV RMS 20 Hz – 20 kHz, Unweighted
Output DC Offset Voltage < 200 µV Typical performance
Residual Mains Hum < -117 dBV
Dynamic Range (AES17) > 113 dB 1 kHz
Common-Mode Rejection Ratio 84 dB 1 kHz
Common-Mode Rejection Ratio > 65 dB 20 kHz
Damping Factor 1900 1 kHz, 8 Ω
Damping Factor 660 20 kHz, 8 Ω
All parameters are measured using a Power-86 with the recommended transformer (±30 V).


Sound Quality

I designed the Modulus-86 in 2014 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.

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

[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 Graphs

The Modulus-86 is one of 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 the Neurochrome Modulus error-correcting architecture, meticulous attention to detail during the circuit design process, and, equally 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 below, the Modulus-86 was operating from a ±30 V power supply. Click on any of the graphs below for a larger view.

The graph below shows the THD+N vs output power for 8 Ω load. The amplifier delivers 40 W at the onset of clipping. 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 mostly show the THD+N floor of the measurement system.

Repeating the measurement with a 4 Ω load reveals:

The onset of clipping occurs at 70 W.

The THD+N vs frequency plots for 40 W into 8 Ω and 65 W into 4 Ω, respectively, are 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-86 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-86 performs 10-100× 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-86 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 100 mW, 1.0 W, and as function of signal level. The three results are shown below. Note that the performance of the Modulus-86 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-86 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 -105 dBV level. This is likely why the Modulus-86 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 amplifier output is completely free of mains-related hum or noise.

The Modulus-86 shows only a tiny 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 better. The top plot shows the noise floor of the Modulus-86 Rev. 2.3 when powered by a Connex Electronic SMPS300RE. The bottom plot shows the noise floor of the amplifier when powered by a pair of well-regulated laboratory power supplies (HP 6643A).

For completeness, I measured the amplitude response and gain flatness as shown below.

As mentioned in the Key Features, the Modulus-86 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-86 is truly an exceptional amplifier.


Circuit Topology

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

The LM3886 provides the output power and is controlled by 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 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. 2.3 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.