The γ3 high resolution DAC

Power supplies for γ3

γ3 requires three independent regulated DC power supplies as listed below. The current ratings assume that you use one of the recommended low-power LCD modules and the Alps RK16812 motorized potentiometer with LCDuino-1, plus some added headroom.

#Supply nameSpecificationsRecommended PSUUse
1AVDD / AVSS±10VDC 300mA minimumσ22For DAC analog sections
2GVDD+5VDC 400mA minimumσ11For DAC digital sections, on the "clean" side of the isolation barrier
3LVDD+5VDC 400mA minimum ("always on")σ11For the other side of the isolation barrier*

* Including the LCDuino-1, its ancillaries, the ζ1 Audio Widget module, and some other logic. Supply #3 is wired to be always-on (as long as the AC power cord is plugged into a live outlet), so that LCDuino-1 retains its power even when the unit is in the "off" state. The remote control or front panel momentary pushbutton can be used to turn it on again.

The separate power supplies and galvanic isolation eliminates ground loops and noise interference between separate circuit sections, ensuring maximum performance. For best results, linear power supplies with low noise, low output impedance, high PSRR, good line and load regulation characteristics should be used. AMB recommends the σ22 for supply #1, and two σ11s for supplies #2 and #3. AMB also recommends that the power supplies, including their power transformers, be located in a separate enclosure than the γ3 backplane board and its plug-in modules. An 8-circuit umbilical cord with suitable connectors is then used to connect the power supplies to the γ3 backplane board.

See the Parts list section for more details about the σ11 and σ22 power supplies, pertaining to their use for the γ3 DAC.

Here are photos of AMB's reference γ3's power supply chassis, built in a Par-Metal 20-12122N (same model as the enclosure used for the γ3 DAC section), with custom front/rear panels fabricated by Front Panel Express (click to enlarge):

AMB recommends 18 AWG stranded copper wires for all AC mains and transformer primary side wiring. For all other wiring in the power supply chassis, 22 AWG stranded copper wires may be used (this is the maximum wire size for the Conxall Mini-Con-X connectors used for the umbilical cable on AMB's reference γ3).

Schematic diagram - Three toroidal transformers

The following is the schematic diagram of the power supplies for use with γ3. Use this diagram to assist you with the wiring. This diagram shows the use of three power transformers (T1, T2 and T3), each of them having two primaries and two secondaries, so that at build time, you may set up the power supplies for 120V or 240V AC mains as shown.

Here is PDF format version of the above schematics (48KB).

Schematic diagram - One multi-secondary R-core and one toroidal transformer

In AMB's reference γ3, a single R-core transformer with four secondaries is used in place of T1 and T2. A toroidal transformer is used for T3. These options are listed in the Parts list section. The R-core transformer can be easily found on ebay with the following specifications:
  • Model name: RN-20
  • 30VA rating
  • Primary taps: 0V, 115V, 230V
  • Secondary 1: 15V (0.5A)
  • Secondary 2: 15V (0.5A)
  • Secondary 3: 9V (0.8A)
  • Secondary 4: 9V (0.8A)
  • SCN wire: connect to chassis

Note: There are "RN-20" R-core transformers with different primary and secondary specifications. Please check against the list above when choosing the correct one to purchase. If you use this R-core transformer, the wiring should look like the following diagrams. Both 120V and 240V AC mains versions are shown:

Here is PDF format version of the above schematics (49KB).

Note: Supply #1 needs to have its secondaries connected in series whereas supply #2 needs to have its secondaries in parallel. The RN-20 R-core transformer uses the same color for both leads of each secondary. It is not obvious how to connect two secondaries together in the proper phase. To determine the phase, you can do the following:
  1. Connect the two 15V transformer secondaries in series with no load (i.e., twist one yellow wire to one brown wire, then use the other two (untwisted) yellow and brown wires as test points.
  2. Connect AC to the transformer primary and measure the AC voltage across the test points. CAUTION: You're handling live AC mains! See the disclaimer.
  3. If you measured above 30V, then they are connected in-phase. If you have a very low voltage, then the two secondaries are connected out-of-phase. Reverse the connection of one of the windings (i.e., only the yellow wires) and re-test.
  4. Use a permanent ink pen to mark the insulation of the yellow wire not twisted to a brown wire. Then mark the insulation of the brown wire that is twisted to a yellow wire. You can now treat the marked wires as the "15V" leads of each secondary. The other (unmarked) brown and yellow wires will be the 0V leads.
  5. Untwist the yellow and brown wires. You are now done with the two 15V secondaries.
  6. You can repeat steps 1 through 5 for the two 9V secondaries to determine their phasing. The only difference is the voltage and the wire colors (blue and white).
An example inter-chassis umbilical connectors and cable pin assignment is shown below. It assumes the use of an 8-circuit circular connector, such as the Conxall Mini-Con-X used in AMB's reference γ3.

Alternate two-chassis configuration

An alternative to the scheme described above is to locate the σ11 and σ22 regulator boards in the same enclosure as the γ3 DAC backplane board. The power transformers are to be mounted in a separate chassis. This requires a larger DAC enclosure and a smaller power supply enclosure. Note that the umbilical cable will now carry AC instead of DC (the transformers' secondary wiring). You will need a minimum of 9 circuits in your umbilical connector for this configuration. Three for supply #1, two for supply #2, two for supply #3, and two for SSRTRIG and G. You would need to use a connector other than the Conxall Mini-Con-X series, as 8 circuits is the maximum available.

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