The ε12 muting / protect circuit
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  • January 2, 2011: ε12 has been updated to version 2. See the Revisions section for details. ε12 v2 boards are now available at AMB audio shop.
  • The default ε12 website (this site) now pertains to v2. If you are building the v1.2 board, the old ε12 v1.2 website is still available for your reference.


ε12 ("epsilon 12") is a turn-on delay muting and DC offset protection circuit designed specifically for use with headphone amplifiers and pre-amplifiers. It prevents the turn-on "thump" and turn-off noise present with some amps, as well as provide protection to downstream devices (i.e., headphones or power amplifier and its connected speakers) in case the output DC offset goes awry during a malfunction.

ε12 can be used with a variety of headphone amplifiers, such as the following: The above list is just an example, many more amplifiers are compatible. The Cavalli SOHA II, Millett Hybrid MAX and MiniMAX amplifiers include modified versions of the ε12 circuit.

ε12 provides DC offset protection on all channels of the traditional 2-channel passive-ground, 3-channel active-ground and 4-channel balanced amps. This circuit is intended to be powered from the amp's DC rails (between ±12V to ±30V) and has its own voltage regulation and adjustable virtual ground reference.

For the single-supply, non virtual ground amps, ε12 provides muting delay functionality but not DC offset protection. See the Instructions and Wiring sections for more details.

The wide power supply voltage range (see the Instructions section) allows the ε12 to be compatible with most solid-state and some low-voltage vacuum tube amplifiers.

The headphone outputs from the amp is routed through a relay, which keeps the headphones disconnected for a few seconds delay after power up. The circuit will also disconnect the headphones if any of the channels being monitored produces approximately ±70mV or more of DC offset, and will reconnect the headphones if the offset drops to a safe threshold. The trigger point is chosen to be sensitive enough to protect most headphones from damage under normal circumstances, but not too sensitive to get false triggering. Compare this to many DC offset protection circuits for speaker amps, which would not trigger until over 600mV. The relay as specified is a high quality Omron, Fujitsu or Tyco unit with gold-clad silver contacts rated at 2A.

The circuit is designed to be simple, low in parts count and cost, yet provide all the functionality desired.

To obtain the blank printed circuit board, visit the AMB audio shop. Other parts are available from online vendors. See the Parts list section.

Please also visit the AMB DIY audio forum regarding the ε12. AMB and the community provides assistance to builders there.

An assembled ε12 v2 circuit board (click to enlarge)

What skills and tools are needed

You should know how to read a schematic diagram and correlate it to the circuit board layout, identify electronic parts and their pin-outs, have decent soldering skill, and be proficient with a multimeter. You need at least a basic set of tools for electronics work, such as a good soldering iron and accessories, screw drivers, needle nose plier, diagonal wire cutter, etc. For mounting the board in the chassis of a headphone amplifier,

Why not offer a full kit

AMB is not prepared to stock all the needed parts and offer them as a full kit. Currently, only the blank circuit board is offered. All the other parts are easily obtainable from online electronics parts suppliers, such as Mouser or Digi-Key.

Why not offer pre-assembled boards

This is intended to be a project for DIYers, and AMB is not set up to manufacture pre-assembled boards.

Where to get help

Be sure you read all the information at this site. If you cannot find an answer to a question, or encounter a problem you cannot solve, the best place to get support is the AMB DIY audio forum. Search the forums for an answer before asking a question. Your question may have already been asked and answered.

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