“Upgrading” a Pimeta v1 Headphone Amplifier

Like I’ve mentioned in the M³ post, there are a few headphone amplifiers that I was always fond of. They weren’t always expensive or exotic amplifiers, but they simply sounded right to my ears. One of these is the Pimeta from Tangent. I first came across the Pimeta quite a few years ago when one of my friends had a portable unit powered from a battery. A few tears later, one of the amplifiers I’ve built was a Pimeta, that ran from a 24V regulated PS. Over the years that amplified has been modified a few times to suit the needs of the time, including a reduction of gain and PS voltage to fit more sensitive headphones and a smaller case. Recently, after a few years of not using it, I’ve had a renewed need for, and decided it was as good of a reason as any to give it a little “upgrade”. This post is meant to share those modifications, as well as give the Pimeta some more attention, as I think its a great little amp that isn’t getting enough love on the forums.

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Enhanced DC Protection Circuit

A DC protection circuit is typically included at the output of most audio amplifier, and is meant to disconnect the loudspeakers if a significant DC component is present at the amplifiers output. This is important as excess DC current through the loudspeaker (or headphones) will generate significant heat and can damage it. Unlike commercial products, most DIY builds I’ve seen over the years, don’t include a DC protection at the output. This of course leaves the loudspeaker/headphones connected to it vulnerable in case of a problem in the amplifier. Therefore, when I was planning one of my previous amplifier builds, I’ve decided I should first design, a DC protection circuit to add the output of the amplifier. I’ve decided to slightly enhance the circuit to include a few extra features other than just DC protection, and make it as versatile as possible:

Main Features/Specifications:

  • Supply voltage of +/-12V to +/-75V (or single 24V-150V supply)
  • Wide input swing of +/-55V
  • Support single-ended/balanced/active-ground amplifiers
  • 2 channels input per board
  • Adjustable sensitivity
  • Independent detection per channel for a robust design
  • Support 2 outputs (A/B/A+B) with relay switching
  • Visual notification(LED) of active output, and fault
  • Delayed start-up
  • Accelerated shut-down for reduced “popping” noise
  • 40mA supply current with single relay energized
  • Up to 8A load current with default relay

The circuit can obviously be modified if needed for a simpler build (not output selection for instance), and can be extended as far as voltage range is concerned. For instance, I have since used the same board with a few less parts with a single 12V supply for the output of a small headphone amplifier.

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