The ε12 muting / protect circuit

Initial Setup & adjustments

After building the circuit and carefully checking all wiring/soldering connections to make sure they are done correctly, use the following steps to check and set up the ε12 for proper operation. This assumes a correctly-functioning amplifier that does not have a problem with high DC offset. The ε12 is designed to protect your headphones in the event of a malfunction, not to mask an existing DC offset problem.

Read through all steps first and familiarize yourself with what you're about to do before proceeding. Do not turn on the power until called for.

You will need a multimeter for this procedure. If at any point your measured voltages do not match what is described, immediately remove power and check the board and wiring for errors. Be careful not to short circuit any component with your test probe leads.

If you haven't already, install the opamp into its socket, taking care not to reverse the orientation.
  1. Connect the V+ and V- DC power connections to the amplifier's supply rails (±12V to ±30V for split rail or virtual ground amplifiers, 12V to 30V for single rail amplifiers with no virtual ground). However, do not connect the input and output wiring yet.
  2. For single rail amplifiers with no virtual ground, turn on the power and you should hear a "click" from the relay contact closing after approximately 6 seconds. If not, then the circuit is not functioning properly and you should immediately power-off and inspect your work. Otherwise, connect the input and output wiring and the setup procedure is complete. For split rail or virtual ground amplifiers, continue with the steps below.
  3. Pre-set the VR1 trimpot to approximately the middle of its adjustment range (i.e., if it is a 20-turn trimpot, turn the adjustment screw all the way counterclockwise until it reaches the stop, then turn it clockwise 10 turns).
  4. Turn on the power. After approximately 10 seconds you should hear a "click" of the relay contacts closing. If not, your circuit is not functioning correctly, and you should immediately power off and inspect your work before proceeding further.
  5. With your multimeter set to DC volts scale, measure the voltage between pins 8 and 4 of U1. It should be slightly less than 24V.
  6. We will now make an initial adjustment of VR1, the zero-reference adjustment trimpot. Measure the voltage across U1 pins 8 and the test point labeled "VG". It should be approximately half of the voltage reading taken in step 4. For example, if your reading was 22.80V, then this measurement should yield 11.40V. If not, adjust VR1 until you get within 50mV of the desired voltage. Check the voltage across pins 4 and VG, and this reading should be the same (but reversed in polarity).
  7. Turn off the power, connect the outputs of the amplifier to the inputs of the ε12 and connect the outputs of the ε12 to your headphone jack(s), then turn on the power again.
  8. We will now adjust VR1 again to fine-tune the basic zero-reference.
    • For a 2-channel amplifier (e.g., 2-board β22, unbalanced dynalo/dynahi, SDS Labs, Eaton, etc.), just adjust the pot until the voltage between the amp's ground and ε12's VG is zero.
    • For a 3-channel amplifier (e.g., 3-board β22, M³, PPA), adjust VR1 until the voltage between the amp's ground channel output and ε12's VG is zero.
    • For a 4-channel amplifier (with fully balanced outputs), adjust VR1 until the voltage between the amp's ground and ε12's VG is zero.
  9. The setup procedure is now complete.

Notes

  • If the relay oscillates on and off, and adjusting the VR1 trimpot will not make it stop, then you may reduce the sensitivity of the circuit to tame this behavior by increasing the value of resistors R5-R8 to 22KΩ to 47KΩ.
  • If you encounter false cut-outs when playing program material with loud low frequency content, you may reduce the sensitivity of the circuit as above. Additionally, You can increase the value of C5 to 3.3µF to lower the input filter frequency.

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