12V trigger problems

[nyatt]

HI. I have what I hope will be a simple problem. I have an preamp with a 12V trigger output (outputs 12v @ max 100mA when the unit is "on", and 0V when the unit is in "standby"). I have an amplifier with trigger inputs, but it uses a scheme where, when the 1/8" mini plug trigger input is "open" or disconnected, the amp is on, and twhen the plug is shorted, the amp is in standby. I figured I could take a small relay and hook up the 1/8 mini to the amp on the Normally closed pins, so the amp is in standby when the relay is not in use. When 12V is applied to the coil, the contacts should open and the amp should turn on. Unfortunately, I can't get this scheme to work. The relay has a DCR of 610 ohms, so the current draw from the preamp would be only 20 mA or so, certainly less than the 100mA max rating. Yet I can hear and feel that the relay is not working. Any idea why this would not work? Is there a better way to do this? (I suppose anything that DOES work would be better...).
Thanks!

 

[nigelwright7557]

HI. I have what I hope will be a simple problem. I have an preamp with a 12V trigger output (outputs 12v @ max 100mA when the unit is "on", and 0V when the unit is in "standby"). I have an amplifier with trigger inputs, but it uses a scheme where, when the 1/8" mini plug trigger input is "open" or disconnected, the amp is on, and twhen the plug is shorted, the amp is in standby. I figured I could take a small relay and hook up the 1/8 mini to the amp on the Normally closed pins, so the amp is in standby when the relay is not in use. When 12V is applied to the coil, the contacts should open and the amp should turn on. Unfortunately, I can't get this scheme to work. The relay has a DCR of 610 ohms, so the current draw from the preamp would be only 20 mA or so, certainly less than the 100mA max rating. Yet I can hear and feel that the relay is not working. Any idea why this would not work? Is there a better way to do this? (I suppose anything that DOES work would be better...).
Thanks!

Why not just drive a transistor in open collector mode ?
So long as you put a resistor in the front end of the transistor to reduce base volts to 0V7 it should be OK.
Not sure if you need an inversion stage or not.

 

[nyatt]

If you can show me what you mean, and what to use, I am happy to try. I can solder up a tube amp from a schematic, but (sadly), I do need instructions.
Thanks!

 

[nigelwright7557]

If you can show me what you mean, and what to use, I am happy to try. I can solder up a tube amp from a schematic, but (sadly), I do need instructions.
Thanks!

 

[nyatt]

Beautiful. What is the transistor model? MPSA42?
Are their equivalents I can use?

 

[nigelwright7557]

Beautiful. What is the transistor model? MPSA42?
Are their equivalents I can use?

They dont come much cheaper or more common than a MPSA42 !
I use them in handfulls on my amps.

Any NPN transistor will probably do that can stand 30v across it and has a gain of 40 or more.

 

[nyatt]

That's great. Thanks for your help. I'll give it a try!

 

[whoandcar]

You'll need 2 transistors, because with that scheme the transistor "closes" contacts with 12 V present, and opens with 0 V, just the contrary of what is needed.

 

[nyatt]

to be clear, I would like the output open when 12V is on the input. Can you draw me a schematic?
Thanks!

 

[unclejed613]

try this. the transistor type isn't critical. a BC548 or a 2SC1318 would work here just as well.

 

[nyatt]

thanks. what is the value or R3?

 

[d3imlay]

Why not sub a PNP (2n3906) and use one transistor. To do that reverse the collector/emitter connections.