AMB Laboratories
[f] AMB on Facebook
The ε21 amplifier backplane board


  • Mar 30, 2015: There is no longer any stock of ε21 boards. If there is sufficient demand, AMB may run another group buy at AMB DIY audio forum.


ε21 ("epsilon 21") is a circuit board designed to serve as a "backplane" for the β22 stereo amplifier. Two β22 boards (channels A and B) are mounted on each ε21 backplane as "modules", using standoffs.

The ε21 backplane can be used for two different amplifier configurations (see the β22 website for details):
  1. Use one ε21 backplane board to implement a conventional 2-channel "passive ground" amplifier.
  2. Use two ε21 backplane boards to implement a 4-channel "fully balanced" amplifier.
The ε22 backplane board is for the 3-channel active ground configuration.

The ε21 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. ε21 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.

Each ε21 board provides the following connections:
  • Front panel connections

    • Connections to two stereo TRS headphone jacks or two XLR headphone jacks. (Molex KK 254 3-pin headers)
    • Connections to the 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 audio input RCA or XLR input jacks (Molex KK 254 2-pin headers)
    • Connections to loop-out RCA or XLR jacks, optional (Molex KK 254 2-pin headers)
    • Connections to speaker output binding posts, optional (Molex KK 396 4-pin header)
    • Connection to DC power V+, G, V- and OG (Molex KK 396 4-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. These parts are needed if amplifier instability occurs.
    • Positions for optional Ground loop breaker resistor and capacitor.
    • 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.
The channel assignments are as follows:

Configuration 2-channel
passive ground
fully balanced
ε21 Board 1 Channel A Left Left+
Channel B Right Left-
ε21 Board 2 Channel A n/a Right+
Channel B Right-

Schematic diagram

The following is the schematic diagram of the ε21 board.

Parts list

The following is the parts list to populate the ε21 board.

QtydescriptionPart number
72P Molex KK 254 headerMolex 22-23-2021
72P 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
26Molex .100 crimp terminalMolex 08-50-0114
44P Molex KK 396 headerMolex 26-61-4040
44P Molex KK 396 crimp housingMolex 09-50-8043
16Molex 396 crimp terminalMolex 08-56-0106
2RLED1,RLED2 10KΩ 1/8W or 1/4W resistor 
2 or 1Rz 22Ω 2W metal oxide resistorsee β22 parts list (also see notes below)
2 or 1Cz 0.047µF 100V film or multilayer ceramic capacitorsee β22 parts list (also see notes below)
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

  • The ε21 connections marked "OG" are unused in a 4-channel fully balanced configuraion.
  • 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.
  • If amplifier instability occurs, install the Zobel (Cz and Rz). For the 2-channel passive ground configuation, install CzA/RzA and CzB/RzB, but leave CzC/RzC empty. For the 4-channel fully balanced configuration, install CzC/RzC, but leave CzA/RzA and CzB/RzB empty.
  • The height of the standoffs for mounting β22 boards on the ε21 board must be tall enough to clear the Rz resistors and Cz capacitors (if these parts are populated).

The circuit board

The ε21 board is made of high quality FR-4 glass epoxy, double-layer with plated-through holes as well as silkscreen and soldermask. Large copper planes on both sides provide low ground impedance and improved shielding.

The dimensions of the board is 9" x 7" (228.6mm x 177.8mm). The thickness of the board is 0.062" (1.58mm). The following illustrates the ε21 mounting hole locations. Please refer to the β22 website for the β22 module mounting hole locations.

Clean both sides of the blank ε21 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.

A 3D rendering of a partially-populated ε21 backplane board:

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