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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, Modulus-186, 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 W of output power.

When powered by the SMPS-86, the Modulus-86 will provide 28 W into 8 Ω. Due to the 2.5 A current limit of the SMPS-86, the amplifier will also deliver 28 W into 4 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.

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.

The key features of the SMPS-86 are tabulated below.

  • 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.

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.

The specifications for the SMPS-86 are tabulated below.

Parameter Value Notes
Input Mains Voltage 85 - 264 VAC
Input Mains Frequency 47 - 440 Hz
Max. Inrush Current 2.0 A 85 - 240 VAC
Output Voltage ±24 V ±2.5 %
Output Current, Continuous 2.5 A RMS ±24 V option
Output Power, Continuous 120 W Outputs evenly loaded
Line Regulation > 0.1 % Typical, 0.5 % max.
Load Regulation 0.5 % Typical
Output Impedance 50 mΩ DC
Output Noise 2.85 mV RMS 20 Hz - 20 kHz
Output Noise 2.90 mV RMS 20 Hz - 90 kHz

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

SMPS-86 block diagram

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).

Output Voltage Options

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 Voltage Continuous Output Current Notes
±5 V 10 A
±12 V 5 A
±15 V 4 A
±24 V 2.5 A Default 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. The power on indicator LED is connected by a EURO style connector. All necessary mating connectors are listed on the project BOM.