Hey all:

Another way to handle this is to use a NPN BJT transisitor as the switch (i.e relay). You can use this either to source the trigger current downstream (you will need a wall wart or something similar) or to a relay.

The reason this works is that the input impedance looking into the base of the transistor is quite high (probably at least 100 k Ohm and probably even higher); the collector is tied off to the + of the wall wart and the emitter goes to the 12 V trigger inputs downstream (or the relay if you are switching that).

Since (looking into the base) the impedance is high, very little current is required of the 990's trigger outputs (using this approach it almost seems to the 990's trigger output that it is sourcing no current at all, which is a good thing). In short, the current flowing into the base of the NPN will be on the order of 120 microamps - at least three orders of magnitude less than what the 990's triggers are rated to source (50 mA). This is a good thing.

So, once the base is turned on, the operating current (current flowing into the downstream components' triggers) is sourced by the wall wart. The only potential issue that I can see is that the wall wart should be at least 700 mV above 12 V due to forward bias voltage drop in the transistor. I suspect that using a 13-15 VDC wall wart would be just fine.

The only caveat is that the wall wart must be able to source the operating current required by the triggers on the amplifiers. Keep in mind that the more devices you tie to the emitter, the more current the wall wart will have to source. Still, if you found one that would source 500 mA that would be more than you would ever need.

Additionally, if you want to adapt this circuit, you can have the emitter drive a relay - this is a nice way of switching AC mains while providing low current draw on the trigger output of the 990 as well as making relay selection (those capable of being switched by a source limited to 50 mA max) a bit broader.

A friend of mine used this approach (Darlinton transistor only) to get his projector to switch on - he did not have a 990, but had the same issue, namely, the trigger input impedance was low enough to pull the trigger voltage down to nearly zero (i.e. it looked for all purposes like a dead short). Anyway, the Darlington transistor (he found it at Radio Shack) approached worked like gangbusters for him. Inexpensive and effective... two of my favorite things.

Mark