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External trigger standby
External trigger standby
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Old 6th January 2010, 07:55 PM   #1
sbear is offline sbear  United States
diyAudio Member
Join Date: Jul 2004
Location: Lawrence, KS
Default External trigger standby

I'll soon be finishing some preamps that will provide a 12v trigger signal to power on my Aleph AX100 clones. My power amps are now switched on/off manually with a DPST switch on the positive and negative outputs after the filter caps. (The filter caps are always charged when the amps are plugged in.)

I have searched for some simple circuits, but most circuits I have found are over complex with soft start, digital control, blinking LEDs, etc. and usually require a seperate transformer.

So, I figure I'd just draw one up and see if it is adequate. In the schematic below I haven't provided values, but I think the idea is apparent.


SW2 is the manual on/off switch that would be placed on the front panel of the amplifier. When the Switch is "on", the trigger is disconnected.

R6 would be calculated to provide the proper current through the relay when 12v is applied to the mosfet gate.

R5 might or might not be needed in order to ensure the mosfet operates in the triode region when the trigger voltage is applied. I am thinking if I wanted the circuit to work with lower trigger voltages like 5v, it would be necessary.

R7 and R8 are a voltage divider that would provide 12v to the mosfet gate when the switch is "on". When "off", R7 pulls the gate voltage to zero if there is no trigger voltage, turning off the mosfet. When there is trigger voltage, R7 would be calculated to draw minimal current from the trigger source.

After the explanation, I have a few questions. Mainly, does anyone know what specific relay would work best in this circuit. Searching through mouser and digikey, there are relays that would seem to fit the spec that range from a couple $ to $70. I'm somewhat confused on the subject. My Amps are biased to about 7A, so I figure I would need to overrate the relay to 10A at least.

Comments appreciated.
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