The ε12 muting / protect circuit

Parts list

The table below lists the recommended parts for the ε12 v2 circuit. See the "Parts recommendations and options" section below for further details. Parts designated with an asterisk (*) should be omitted for single-supply amplifiers with no virtual ground (see the Instructions section for more details).

Items shown in italic are available from the AMB audio shop.

Circuit board
-1ε12 printed circuit board
VR1*13/8" multi-turn cermet trimpot 1KΩ
R1, R2*21/8W 1% metal film resistor 3.32KΩ
R3*, R4*, R5*, R6*, R7*, R8*, R1271/8W 1% metal film resistor 10KΩ
R9*11/8W 1% metal film resistor 100KΩ
R10*11/8W 1% metal film resistor 1KΩ
R11, R1321/8W 1% metal film resistor 475KΩ
C1, C3, C4*3low-ESR electrolytic capacitor 100µF 25V
C21multilayer ceramic capacitor 0.1µF 50V (X7R)
C5*1film capacitor 1µF 63V (MKT or MKS)
C6*, C72non-polar electrolytic capacitor 100µF 16V or 25V
Integrated Circuits
U11Texas Instruments TL082CP dual JFET-input operational amplifier
Q1, Q3*, Q4*3NPN small signal BJT BC546B
Q2*, Q62PNP small signal BJT BC556B
Q51N-channel small signal enhancement mode MOSFET 2N7000
D1124V 500mW zener diode 1N5252B or BZX55C24
12V 500mW zener diode 1N5242B or BZX55C12 for single supply, non virtual ground amplifiers
D2, D3*, D4*, D54small signal diode 1N4148
K1124VDC DPDT relay Omron, Fujitsu or Tyco
12VDC DPDT relay for single supply, non virtual ground amplifier
(see comments below)
*1DIP-8 socket
PWR12-circuit Molex KK 254 pin header and crimp housing
IN, OUT24-circuit Molex KK 254 pin header and crimp housing
-10Molex 254 crimp terminals
--hookup wire
--PCB standoffs, screws and other hardware

Parts recommendations & options

Most components are non-critical, and you may substitute with other parts with similar specs. Be sure the parts you choose are compatible with the footprints, lead spacings and pin-assignments of the PCB. There is no need to use "boutique-grade" components in this circuit.


For a standard ε12, the following 24VDC relays are recommended: For use with non virtual ground, single-supply amplifiers, a 12VDC relay should be used: These are "high sensitivity" types with low coil current. If you use a different relay, use one with the correct coil voltage specifications, and a coil current lower than 15mA to reduce Q1 and Q2 transistors' heat dissipation. The relay must fit the footprint and pinout configuration of the board.


The U1 opamp (TL082CP) may be substituted with others, but only FET-input dual opamps with low input bias current are recommended. Do not use opamps with very wide bandwidth and high slew rates because those characteristics are not necessary in this application, and may lead to unstable operation. Alternatives are TL072CP, LF353N or other low-cost dual FET opamps in a DIP-8 package.


  • Q1, Q3, Q4: BC546B (NPN small signal transistor, TO-92)
  • Q2, Q6: BC556B (PNP small signal transistor, TO-92)
  • Q5: 2N7000 (N-channel small signal enhancement mode MOSFET, TO-92)
If you use other transistor types than those listed above, be sure to verify the pin-out compatibility.


The ε12 PCB has 0.300" lead spacing for the D1 diode, and 0.200" lead spacing for D2, D3 and D4.
  • D1: 1N5252B, BZX55C24 or any 24V 500mW or greater zener diode with a DO-35 or DO-41 casing
    (1N5242B or BZX55C12 12V 500mW zener diode for amplifiers with single supply and no virtual ground)
  • All other diodes: 1N4148 or 1N914


Almost all 1/8W resistors fit nicely. 1% metal film or 5% carbon film types may be used. The ε12 PCB has 0.300" lead spacings for all resistors.


The VR1 trimpot is a 1KΩ 3/8" multi-turn cermet trimpot. Any of the following series are recommended:


  • C1, C3, C4: These should be low-ESR electrolytic type rated at least 25V. The lead spacing is 0.100" (2.5mm);, and the maximum body diameter is 0.240" (6.3mm).
  • C2: This should be a multilayer ceramic type (X7R). The lead spacing is 0.100" (2.5mm).
  • C5: This should be film-type. A metallized polyester (MKT or MKS) box capacitor is recommended. The holes on the PCB support a capacitor with 0.200" lead spacing and no larger than 7.5mm x 5.5mm footprint.
  • C6, C7: These should be non-polar (also known as "bi-polar") aluminum electrolytic type rated 16V or 25V. The lead spacing is 0.138" (3.5mm), and the maximum body diameter is 0.315" (8mm).


  • Molex KK 254 pin headers, friction lock (choose vertical or right angle, gold or tin plating):
    • Molex 22-11-2022 (2P, gold-plated, vertical)
    • Molex 22-23-2021 (2P, tin-plated, vertical)
    • Molex 22-11-2042 (4P, gold-plated, vertical)
    • Molex 22-23-2041 (4P, tin-plated, vertical)
    • Molex 22-12-2024 (2P, gold-plated, right angle)
    • Molex 22-05-3021 (2P, tin-plated, right angle)
    • Molex 22-12-2044 (4P, gold-plated, right angle)
    • Molex 22-05-3041 (4P, tin-plated, right angle)
  • Molex KK 254 crimp housings:
    • Molex 22-01-3027 (2P)
    • Molex 22-01-3047 (4P)
  • Molex KK 254 crimp terminals: (choose gold or tin plating):
    • Molex 08-55-0102 (gold plated)
    • Molex 08-50-0114 (tin plated)

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