Modulus-86 — Rev. 1.0

This circuit has been superseded by the Modulus-86 Rev. 2.0

The Modulus-86 is a composite amplifier. When implemented according to the build instructions provided, the performance of our composite amplifier is dominated by a precision audio op-amp. This allows for the implementation of a high-performance chipamp-based amplifier with vanishingly low THD and phenomenal sound quality.

The Modulus-86 uses an LME49710 precision audio op-amp to control an LM3886 power amplifier. An OPA277 precision op-amp is used as a DC servo, ensuring an output DC offset below 200 µV. 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.

Key features of the Modulus-86:

  • Ultra-low 0.00018 % THD (35 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00035 % THD+N (35 W, 8 Ω, 1 kHz).
  • 35 W output power (8 Ω) using a ±28 V power supply.
  • 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 using 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 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.

A fully assembled Modulus-86 board is shown below. This is the exact board used for the measurements shown on this page.


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 $370.


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

Parameter Value Notes
Output Power 35 W 8 Ω
THD 0.00018 % 1 W, 8 Ω, 1 kHz
THD 0.0002 % 35 W, 8 Ω, 1 kHz
THD+N 0.00035 % 35 W, 8 Ω, 1 kHz
IMD 0.00066 % SMPTE, 60 Hz + 7 kHz, 4:1, 35 W, 8 Ω
Bandwidth 0.002 Hz – 140 kHz 1 W, -3 dB
Full-Power Bandwidth 84 kHz
Slew Rate 14 V/µs  8 Ω || 1 nF load
Noise Floor -125 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 ±28 V power supply.

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”, for example, is out of this world. I am certain the incredible sonic performance is due to the stellar supply rejection and flat THD+N vs. frequency of the Modulus-86.

Builder Feedback

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 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 SYS-2712 audio analyzer. The Modulus-86 was operating from a ±28 V power supply consisting of a toroidal mains transformer, a KBPC2510 bridge rectifier, and two 22000 µF electrolytic capacitors. The full-power THD measures 0.0002 %.

MOD86_R1p0_FFT_1kHz35W8RJust as impressive is the distortion performance at 1 W output power as shown below. The THD at 1 W output power measures to an impressive 0.00018 %, consisting mostly of second and third 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.

MOD86_R1p0_FFT_1kHz1W8RThe 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.


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.


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.


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.


Circuit Topology

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

MOD86_BlockDiagramThe 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 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. Another advantage of the differential input is that greatly simplifies connecting two Modulus-86 boards into a bridge amplifier. Connecting two Modulus-86 amplifiers in a bridge configuration requires two identical boards; one board having its input connections swapped. On other amplifier circuits, several component changes are generally required to connect the amplifiers in bridge configuration. On the Modulus-86, you just swap two wires. Done! Connecting two boards in parallel, or four boards in bridge/parallel is just as easy.

To avoid capacitors in the direct signal path, a DC servo was implemented. The DC servo uses a OPA277 opamp, selected for its stellar DC performance. The DC offset on the output of the Modulus-86 is typically below ±10 µV.

The Modulus-86 has a gain of +20 dB (10x). 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 at a minimal reduction in 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.