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The ε22 amplifier backplane board


  • June 28, 2019: There is no longer any stock of ε22 boards. If there is sufficient demand, AMB may run another group buy at AMB DIY audio forum.
  • Please read the ε22 PCB v1.00 notes before using the board. These notes do not apply to v1.1.


ε22 ("epsilon 22") is a circuit board designed to serve as a "backplane" for the β22 stereo amplifier in 3-channel active ground configuration. The three β22 boards (left, right and ground channels) are mounted on the ε22 as "modules", using standoffs.

The ε21 backplane board is for the 2-channel passive ground and 4-channel balanced configurations.

The ε22 board contains circuit traces that takes the place of most physical wires, allowing the β22 amplifier to be built with minimal wiring clutter in the enclosure. Not only does this make the amplifier internals appear tidy, the large ground planes provide very low ground impedance and interference shielding properties, while the optimized trace routing reduce stereo crosstalk. ε22 also greatly improves the serviceability of the amplifier. Each β22 board can be removed as a module simply by removing the mounting screws and unplugging the board from the backplane.

The ε22 board provides the following connections:
  • Front panel connections

    • Connections to two stereo headphone output jacks (Molex KK 254 3-pin headers)
    • Connections to left and right volume control potentiometer or stepped attenuator (Molex KK 254 3-pin headers)
    • Connection to a power indicator LED, optional (Molex KK 254 2-pin header)

  • Rear panel connections

    • Connections to left and right audio inputs (Molex KK 254 2-pin headers)
    • Connections to left and right loop-out, optional (Molex KK 254 2-pin headers)
    • Connections to left and right speaker output, optional (Molex KK 396 4-pin header)
    • Connection to DC power V+, G and V- (Molex KK 396 3-pin header)

  • Internal connections

    • Audio/ground input connections to each β22 board (Molex KK 254 2-pin headers)
    • DC power V+, G, V- and amplifier output connections to each β22 board (Molex KK 396 4-pin headers)
    • Positions for Zobel network resistors and capacitors (RzL, RzR, CzL and CzL). These parts are needed if amplifier instability occurs.
    • Positions for Ground loop breaker resistor and capacitor (Rg and Cg).
    • The right-rear mounting screw hole connects to the chassis via a metal standoff. Input signal ground connects to the chassis through the Ground loop breaker at this point. If a direct chassis connection is desired, then do not populate Rg and Cz, and install a wire jumper across Rg.

Schematic diagram

The following is the schematic diagram of the ε22 board.

Parts list

The following is the parts list to populate the ε22 board.

QtydescriptionPart number
82P Molex KK 254 headerMolex 22-23-2021
82P Molex KK 254 crimp housingMolex 22-01-3027
43P Molex KK 254 headerMolex 22-23-2031
43P Molex KK 254 crimp housingMolex 22-01-3037
28Molex .100 crimp terminalMolex 08-50-0114
13P Molex KK 396 headerMolex 26-61-4030
13P Molex KK 396 crimp housingMolex 09-50-8033
44P Molex KK 396 headerMolex 26-61-4040
44P Molex KK 396 crimp housingMolex 09-50-8043
19Molex .156 crimp terminalMolex 08-56-0106
2RLED1,RLED2 10KΩ 1/8W or 1/4W resistor 
2RzL,RzR 22Ω 2W metal oxide resistorsee β22 parts list
2CzL,CzR 0.047µF 100V film or multilayer ceramic capacitorsee β22 parts list
1Rg 10Ω 5W wirewound resistorVishay-Dale CP series or Xicon CR series, see β22 parts list
1Cg 0.1µF 250VAC class X or Y film capacitorsee β22 parts list
-Metal standoffs, machine screws, washers, and other hardwareImperial 4-40 or Metric M3

  • All the Molex KK 396 headers and crimp terminals listed have gold contacts. Tin-contact versions are also available at lower prices.
  • For non-speaker use, one less 4P Molex KK 396 connector is needed.
  • Rg and Cg are not required if PSU is in separate case (jumper the Rg position to connect signal ground to the amp chassis).
  • If loop-out is not used, two less 2P Molex KK 254 connectors are needed.
  • If only one headphone jack is installed, one less 3P Molex KK 254 connector is needed.
  • If the extra panel-mount LED is not installed, one less 2P Molex KK 254 connector is needed, and RLED1/RLED2 may be omitted.
  • The value of RLED1 and RLED2 may need to be changed depending on the LED.
  • The height of the standoffs for mounting β22 boards on the ε22 board must be tall enough to clear RzL, RzR, CzL and CzR if these parts are populated.
  • The Rg resistor is erroneously marked as "Rz" on the board (next to the Cg capacitor). This should not be confused with the RzL and RzR resistors located elsewhere on the board

The circuit board

The ε22 board is made of high quality FR-4 glass epoxy, double-layer with plated-through holes as well as silkscreen and soldermask. The copper layers are 2 ounce heavy duty construction for low impedance. There is a ground plane on each side of the board for lowered ground impedance and improved shielding.

The dimensions of the board is 9" x 10" (228.6mm x 254mm). The thickness of the board is 0.062" (1.58mm). The following illustrates the ε22 mounting hole locations. Please refer to the β22 website for the β22 module mounting hole locations.

Clean both sides of the blank ε22 board with paper towel and isopropyl alcohol or electronics flux remover before soldering any parts on it.

Additional information

Please visit the β22 amplifier website and the AMB DIY audio forum for further information and support.

The ε22 board:

With β22 boards mounted:

Copyright © Ti Kan
All rights reserved. Commercial use of this design is prohibited without prior permission.