Fully-balanced, dual-mono α10 pre-amplifierYou can build a fully-balanced, dual-mono version of the α10 pre-amplifier with two backplane boards (each one with a δ1 attenuator, a δ2 input/output selector and a pair of α20 line-stage boards).
One backplane board is used for the left channel, the other one is for the right channel. On each board, what's marked as "left" and "right" channels are used for the hot (+) and cold (-) phases of the balanced signal. Each channel has its own σ22 PSU, making it a "dual mono" configuration. You may use separate transformers for each σ22 PSU, or one larger one for both. There is only one σ25 PSU for the 5V digital power.
In the illustration above, the left channel α10 backplane is fully-populated similar to a regular (unbalanced) α10, and connected to the LCDuino-1, front panel switch and motorized potentiometer in the same way, shown in green in the diagram.
The right channel backplane, on the other hand, does not have the σ25 PSU board, and has no connectivity to the LCDuino-1, front panel switch and potentiometer. It also takes only the 15V-0-15V transformer input for the analog section. The 9V and SSR connections are not used.
A connection must be made from the left channel backplane's LJ4 connector to the right channel backplane's LJ4, so that the 5V power, ground and the I²C signals are sent to the latter board. This is shown in light blue in the diagram. There are 6 wires, I recommend using 22 AWG stranded copper for these, tack soldered to the left channel backplane's LJ4 solder pads, and then terminated with a Molex plug on the other end. The Molex plug would then plug into a matching Molex header at the right channel backplane's LJ4. If you stack-mount the left channel board on top of the right channel board (using long standoffs), then these wires can be quite short. Using the Molex connector allows the left channel backplane to be removed easily to access the right channel board.
Each of the α10 backplane's eight input/output ports can be configured as balanced or unbalanced.
You will need to determine the overall height of the two stacked α10 assemblies and choose an appropriate enclosure with enough room to accommodate the whole thing. XLR connectors are also larger so you should factor the rear panel layout into the enclosure choice.
Since there are twice the number of δ1 and δ2 boards in this configuration compared to the regular α10, the power transformer used with the σ25 PSU (for 5V digital power) should be rated at least 8VA to handle the additional current demands.
Alternative power relayInstead of a solid state relay (SSR) as specified in the Parts list, you may use a "buffered" electro-mechanical relay module. The benefit of using such a module is lower cost, but it does increase the overall current draw on the σ25 power supply when in the "on" state.
There are several vendors that make or sell these relays, including amazon.com and. ebay (search for "relay module 5V" and look for reasonable units). They are usually one or two electro-mechanical relays mounted on a small circuit board, with transistor drive. Some may have an opto-coupler input. You should choose one that's rated for 5V power supply operation, high-level trigger, with either one channel or two channels (SPST, SPDT, DPST or DPDT). With single-pole relays, you switch the "live" line of the AC mains just like how the SSR is usually wired. With double-pole relays, you can wire it such that the neutral line is also switched. With double-throw relays, wire your circuit in "normally open" configuration (i.e., contacts are open when untriggered).
These relay modules require an additional power supply and ground wire pair to the "always on" σ25.
The contact current rating should not be lower than 3A or greater than 10A. If the current rating is too low, the relay contacts may prematurely age. If the rating is too high, then chances are that it contains SSRs insread of electro-mechanical relays.
Here is an example 2-channel (DPDT) buffered relay, for 5V operation, and up to 10A of load current.
Note: Do not use an unbuffered, conventional electro-mechanical relay. The SSR trigger output from LCDuino-1 does not have sufficient output current capability to drive a relay coil.
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