I recently upgraded my home theater to bring it current with high-def content. My 12 year-old 50” standard-def rear-projection set was replaced by a 60” plasma display, and my non-HDMI capable A/V receiver was dumped in favor of an Onkyo 7.1-channel preamp/processor. (Naturally, the 9.2-channel version with Internet streaming audio came out a month later.) This gave me an opportunity to finally install the separate power amps I purchased as surplus from my employer several years ago.
After opening the pre/pro and perusing the manual I realized I’d need a method of controlling the amps. The Crest Audio Vs-450s have mechanical power switches and no trigger inputs. The Onkyo has three trigger voltage outputs that are programmable as to when they turn on. I needed some sort of interface between the pre/pro in my living room and the amps, which I located in a basement equipment rack. Basement installation was necessary because the amps have big cooling fans in their front panels. I went to the web and shopped around for trigger voltage-controlled power strips. Several companies make them, and they’re pricey (around $200 for a two-outlet unit). That was out of the question.
I then considered building my own trigger-controlled outlets. I found several schematics and blog posts from people who’d built their own, but I really didn’t want to tackle yet another project. Then inspiration hit me—I use X10 for home control, so why not use it to control the amps? I ordered three Powerflash modules and five X10 appliance switches, and sketched out how everything would connect. The X10 Powerflash modules have two screw terminals for an input, and can take either a control voltage or a simple switch closure (selectable by a switch on the module). The voltage input would come from an Onkyo trigger output. The module has three possible modes of operation; one mode will send an X10 ‘on’ command when voltage is first applied and an ‘off’ command when the voltage is dropped. Each amp would be plugged into an appliance module and be set to a unique address that matched one of the Powerflash interfaces. I could theoretically turn amps on and off when needed depending on the listening mode. For example, if I wished to listen to 2-channel music I could turn on only the front left – front right amp. When I switch to a movie surround mode other amps would come on. This would allow a degree of ‘green’ operation and save some money on the power bill. It was the perfect solution (or so I thought)!
I wired everything together and turned the system on for the first time, but the amps didn’t power up. I could turn them on with a wired mini-controller but the Powerflash modules didn’t seem to work. After a bit I had a forehead-slapping moment. My DirecTV DVR is connected to a UPS in the equipment rack, and UPSes are notorious X10 signal eaters. I dug out a filter specifically designed for signal-interfering devices and plugged the UPS into it. No change—the Powerflash modules still didn’t work. Then I realized that the APC home theater power bar I’d installed would also filter out X10 signals. To get around this I ran wires from the Onkyo trigger outputs through the wall and down into the basement. I mounted an unfiltered power strip on a floor joist below the living room and plugged the Powerflash modules into it. The power strip was then plugged into a basement AC outlet on a separate breaker from the entertainment system (but on the same phase in the breaker box). Success! The amps would then turn on with the pre/pro.
I spent some time listening to the new system and was very pleased with what I heard. Even without calibration it sounded fantastic. I was ecstatic! Then I turned the system off and went downstairs for something, and that’s when I discovered that the amps hadn’t turned off. After several hours of testing I again found that the amps would turn off with the mini-controller in the living room, so I used that as a stopgap measure while I thought the problem through. Several days later I realized what the problem was. The amps would turn on okay because I had the Onkyo programmed with a 1-second delay between triggers. For example, on power-up the first trigger voltage was activated. A second later the second trigger activated, and the third trigger finally activated two seconds after the first. This gave each module’s signal a chance to transmit. When the power is shut off, however, all the trigger voltages go off at once. All three Powerflash modules send their X10 commands at the same time and they collide; consequently the appliance modules never get their signals and the amps stay on. There’s no way to adjust for this condition in the Onkyo. It was time for more head-scratching. How could I delay the trigger voltages just long enough to ensure there would be no collisions?
The idea proved to be fairly simple.
The circuit I constructed consists of just two components, a blocking diode and an electrolytic capacitor. The capacitor, once charged by the trigger voltage, will retain that charge for a short time after voltage is removed before dropping it. This should be long enough for a non-delayed Powerflash to transmit its signal and avoid a data collision. The blocking diode prevents the capacitor’s voltage from bleeding back into the Onkyo’s trigger voltage output. Any other delayed Powerflash modules would have a different value capacitor to ensure the other modules transmitted first. I determined the capacitor value by trial and error. As it turns out, a 2200 uF, 25 volt capacitor will hold its voltage in this application approximately 22 seconds. A 220 uF cap holds its voltage for just over two seconds. So to calculate the time delay, multiply the delay time period (in seconds) by 100, then round up or down to a standard capacitor value (in microfarads). I built my delay circuit on a small piece of perfboard and used two mini screw terminal blocks to connect the input and output wires. I protected the entire assembly from shorts by placing a piece of heatshrink tubing over it.
If you do any work with X10 devices, especially Powerflash modules, this little circuit could come in handy.
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