2.1 Hi-Fi Amplifier

A roundup of what I learned from my first big electronics project.


Circa. 2012

This was my first attempt at a “real” electronics project. It wasn’t as easy as I thought it’d be. Big surprise!

Goal: To create a hi-fi amplifier to drive a set of custom speakers.

General Specifications:

  1. Left and Right Channels: 2x 40W into 8Ω via LM3886 (X2)
  2. Subwoofer: 1x 100W into 4Ω (via LM4780)
  3. Headphone amplifier: Single op-amp “cMoy”
  4. Subwoofer Active Crossover: Stereo to Mono Mixer, Volume, Crossover Frequency


  1. Avoid using a wooden enclosure if you can (even if it looks cool).
    1. Most “panel-mount” electronic components are meant to be mounted to something like ~1/32″ steel/aluminum, not a piece of 1/8″ wood.
    2. If your device is mains powered (that is, directly from 120V or 230V), and is not double insulated, then you should use an earthed metal enclosure to protect the user in case something goes wrong.
      • For instance, if a high voltage wire within your product wiggles loose and comes in contact with the metal shaft of a potentiometer, you do not want the user to get zapped when he/she goes to adjust the pot.
      • In a device with a non-conductive enclosure (wood, plastic, etc.), a dangerous high voltage can remain on that pot indefinitely.
      • In a device with a metal enclosure, the pot’s shaft is tied to earth, via the earth-referenced enclosure. So, if a high voltage wire comes in contact with an exposed metal part (the potentiometer), a large current will flow from the high voltage source to earth. This fault current causes the device’s fuse to blow, safely disabling the unit.
    3. Wood does nothing to shield your circuitry from electromagnetic interference (EMI).
      • Electromagnetic interference is composed of electric and magnetic fields.
      • To terminate electric field lines, use a material with good electrical conductivity (most common metals).
      • To redirect/divert magnetic field lines, use a magnetically permeable material (ferrous materials).
      • Steel is usually a good pick!
  2. Try to think the whole design through before you try to put it together.
    1. Building a prototype is one thing, but making a good looking, final product is a whole other animal.
    2. It’s way harder to make changes to your design once you’ve already started physically building the hardware.
    3. Try not to leave any stones unturned during the design phase. Think about:
      • How you are going to assemble your product: Is it practical/easy to put together?
      • The user experience: Do the controls and layout make sense?
      • Internal layout/architecture: Are your internal components arranged in a way which make good use of space, while maintaining good thermal and EMI performance?
        • I bring this up because my internal layout decisions forced me to run line-level audio signals very close to the AC input, which necessitated the use of fancy (see: expensive), shielded twisted pair. This could have been avoided from the start.



cMoy Amplifier

Jim’s Audio LM3886 PCB

Jim’s Audio LM4780 PCB

EMI Shielding

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