Parts listThe table below lists the recommended parts for the ε12 rev 1.2 circuit. See also 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.
Parts recommendations & optionsMost 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.RelayThe Omron G5V-2-H1-DC12 or G6A-234P-ST15-US-DC12, Fujitsu-Takamisawa RY-12W-K or RA-12W-K relays are recommended. These are "high sensitivity" types with low coil current. If you use a different relay, use one with a DC 12V coil, and a coil current lower than 20mA to prevent false triggering and to reduce Q1 and Q2 transistors' heat dissipation. The relay must fit the footprint and pinout configuration of the board.OpampsDo not substitute the U1 opamp with another type. The OPA551 is chosen specifically for its high output current and low output impedance, and low input bias current characteristics, which are essential for this application.The U2 opamp (TL081CP) may be substituted with others, but only FET-input 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 to the TL081 are TL071, LF411, LF351, LF356, LF357, or other low-cost FET opamps in a DIP-8 package. TransistorsFor Q1, Q3 and Q4, virtually any small-signal NPN TO-92 transistor will work. Alternatives to the 2N3904 are 2N4401 or 2N5088. For Q2, you may use 2N4403 or 2N5087 instead of 2N3906.For Q5, this should be a darlington NPN TO-92 transistor with Vce greater than 25V, Hfe greater than 5000 and Ic greater than 100mA. Alternatives to the MPSA14 are: KSP13, KSP14, KSP25, KSP26, KSP27, MPSA13, MPSA27, MPSA28, MPSA29, 2N6426, 2N6427, 2N7052, 2N7053, BC372 or BC373. If you use other transistor types than those listed above, be sure to verify the pin-out compatibility. DiodesThe ε12 PCB has 0.300" lead spacing for the D1 and D2 diodes, and 0.200" lead spacing for D3 and D4.
ResistorsAlmost all 1/8W resistors fit nicely, and some 1/4W resistors should also fit. The Vishay-Dale RN55 series will barely fit, but the RN60 won't. The resistors specified are 1% metal film type, but the budget-minded can use 5% carbon film type. The ε12 PCB has 0.300" lead spacings for all resistors.TrimpotThe VR1 trimpot is a 1KΩ 3/8" multi-turn cermet trimpot. Any of the following series are recommended:
Capacitors
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