The SMPS-86 is a switch-mode power supply intended as a simple turnkey power supply solution for LM3886-based amplifiers, such as the Modulus-86 and LM3886 Done Right. It uses two sealed switch-mode power supply modules to provide ±24 V. Offering 120 W of continuous output power, it is designed power a stereo Modulus-86 or LM3886DR amplifier for audio reproduction. It is equally suitable for audio reproduction using Class D mono amplifiers with up to approx. 100-150 W of output power.

Key Features

  • Better performance and 30 % lower component cost than a typical unregulated linear supply.
  • No mains wiring required! On-board IEC mains inlet, mains fuse, and power switch.
  • International mains voltage input (85 – 264 VAC @ 47 – 440 Hz).
  • Well-regulated ±24 V output.
  • Up to 2.5 A of continuous output current at ±24 V.
  • Can also be configured for ±5 V, ±12 V, or ±15 V.
  • Galvanically isolated output.
  • LC filtered output to minimize noise.
  • On-board 320 Joule surge suppressor.
  • Soft-start limiting the inrush current to 2.0 A.
  • Power on indicator LED output.
  • Small 107 × 153 mm (4.2 x 6.0 inches) board size. 35 mm assembled height.


A fully assembled SMPS-86 power supply module is shown below. The SMPS-86 is intended to be panel mounted on a chassis panel no thicker than 4 mm.


Build Budget

The component cost of the SMPS-86 is approx. $65; 30 % less than the cost of a typical unregulated linear supply.


The specifications for the SMPS-86 are tabulated below.

Input Mains Voltage85 – 264 VAC
Input Mains Frequency47 – 440 Hz
Max. Inrush Current2.0 A85 – 240 VAC
Output Voltage±24 V±2.5 %
Output Current, Continuous2.5 A RMS±24 V option
Output Power, Continuous120 WOutputs evenly loaded
Line Regulation> 0.1 %Typical, 0.5 % max.
Load Regulation0.5 %Typical
Output Impedance50 mΩDC
Output Noise2.85 mV RMS20 Hz – 20 kHz
Output Noise2.90 mV RMS20 Hz – 90 kHz

This project contains two surface mounted components: The inductors used in the output filter. As shown in the image at the bottom of this page, the inductors are rather large (12.7 × 12.7 mm) and the terminals come out to the edge of the inductor package. All other components are leaded.

When powered by the SMPS-86, the Modulus-86 will provide 28 W into 8 Ω. This is plenty of power for most uses and is perfect for a compact desktop stereo amp. However, those who like to listen to very loud music or have low-efficiency 4 Ω speakers, should consider building the SMPS-86 + Modulus-86 as mono blocks.

Simplified Schematic

The block diagram of the SMPS-86 is shown below.


Note that the switching regulator modules have built-in mains filters and are transformer isolated. The power switch is a double-pole switch (shown as single pole for simplicity).

Options for Other Supply Voltages

Output voltages of ±5 V, ±12 V, ±15 V, and ±24 V are directly supported by changing the two switching power supply modules. The following options are available for the output voltage and current.

Supply VoltageContinuous Output CurrentNotes
±5 V10 A
±12 V5 A
±15 V4 A
±24 V2.5 ADefault configuration

The pinout of the output connectors is shown below. The terminal blocks are rising cage clamping types and accept cables up to AWG 10 (5.2 mm2). In addition to the terminal blocks, the output voltages are provided on a Molex MegaFit connector which is compatible with the power connector of the LM3886DR. The power on indicator LED is connected by a EURO style connector. All necessary mating connectors are listed on the project BOM.