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I picked up a used Rotel RMB1075 5-channel power amp for a good price and got a HK AVR235 to use as a processor for the amp. The combination sounds great. I chose the 235 because I wanted modern processing features and high sound quality and couldn't find anything good (new or used) with pre-amp outs in the $2xx price range. At $350 new, the AVR235 supported all the modern formats, offered ProLogic IIx, had 7.1 capability in case I want it to power 2 more channels in the future, and had the triple crossover feature so I could set individual xover freqs for the center, mains, and rears.


Anyhow to the main purpose of this post: The AVR235 doesn't have a 12V trigger output and I got tired of manually switching the power amp on/off and feeling guilty of wasting power when I forgot to turn it off.


I'd read about the possibility of using a 12V wall-wart power transformer plugged into the switched outlet of the receiver, but couldn't find any details of the polarity of the 12V trigger input on the Rotel power amp in the amp's user manual or on Rotel's site. I finally found some info in another Rotel user manual that said the tip is positive and the ring is ground.


The 12V trigger connector on the Rotel power amps is a mono 2-conductor 1/8" phone plug like this one:

Radio Shack link

[I used a different type with a right angle plug like this one here.



I dug around in my big box of discarded wall-wart power transformers (I knew they would come in handy some day...) and found the one with 12V DC output and the lowest amperage rating (200mA) since I figure the trigger wouldn't draw too much current. I tested it with my meter and under no load it actually puts out 16.8V DC but the Rotel manual also said that the 12V trigger was rated from 3V to 30V so I figured 16.8V would be ok.


I then cut off the power connector from the wall-wart cord and tested the polarity to find the ground and hot wires. (the ground was the wire with the stripe, but YMMV) I did a temporary connection with the wires wrapped around the terminals and it worked, so I went ahead and soldered the wires in good.


I did a final test with the wall-wart plugged into the switched outlet on the back of the HK and the new 3.5mm connector in the Rotel amp (12V trigger switch set to on) and everything works as expected: Turn on the receiver and it switches the power amp on; Turn off the receiver and the power amp goes off. Too bad the blue LED on the front of the amp stays on all the time...I hope it doesn't burn out.


Anyhow, I hope this is helpful for anybody in a similar situation. The key piece of information in this post is the polarity of Rotel amps' 12V trigger input. The tip is positive and the ring is ground.
 

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Discussion Starter #2
does anybody know if this setup would also work with a lower voltage supplied to the trigger? I'm wondering why the Rotel manual says that anything from 3V to 30V will work and if that is the case, why is it called a 12V trigger?


If a lower voltage will work then perhaps I can find a smaller wall-wart to do the same job.
 

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Quote:
Originally Posted by collin
why is it called a 12V trigger?


Rolls off the tounge better than "Low voltage direct current trigger"?


Try it with a lower voltage transformer. Even if it doesnt work, you wont hurt it.
 
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