Members DeepEnd Posted June 17, 2015 Members Share Posted June 17, 2015 Here's my dilemma: I built a box to go in the "Pre Out/Power In" loop on my Fender FM65R amp. I wanted to be able to turn down the amp's volume while preserving a strong signal to go to a DI box. The box has three 1/4" jacks--Input, Attenuated Output, and Bypass--and a 5K audio pot. The Input jack is wired to input of the pot and to the Bypass jack. The Attenuated Output jack is wired to the output of the pot. But there's no signal to the Attenuated Output jack at all and the pot affects the volume to the Bypass jack but in reverse; turning the pot clockwise reduces the volume. I've checked the connections and everything seems okay. A continuity tester indicates that the connection between the Input and Bypass jacks is good. I thought it would be like a simple "Y" connection going to the Bypass jack and the pot, so both simultaneously or either by itself should have a signal, and only the Attenuated Output jack should be affected by the pot, which shouldn't be exhibiting backwards behavior. What's going on? Do I need to isolate the two outputs? If so, how? Should I simply rewire it so there are two attenuated outputs? Thanks in advance. Here's the design as is. The pot is seen from "below" (i.e., from the side opposite the shaft): Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 19, 2015 Moderators Share Posted June 19, 2015 5K strikes me as being too low. The preamp out impedance is 1K, so I'd want the attenuator pot to be at least 10K from that aspect. The input impedance for the power amp in is 10K. Try a larger potentiometer. And of course, double check your wiring. Link to comment Share on other sites More sharing options...
Members DeepEnd Posted June 20, 2015 Author Members Share Posted June 20, 2015 Thanks for replying and for suggesting the higher value pot. onelife replied over in the Amp Forum and I currently suspect a shorted pot: http://www.harmonycentral.com/forum/forum/guitar/acapella-28/31514442-help-with-attenutaor-box. I'm actually modifying a design WRGKMC linked to for me: https://www.gearslutz.com/board/attachments/newbie-audio-engineering-production-question-zone/400359d1401116406-newbie-important-questions-about-yamaha-spx90-connectivity-levels-line-attenuator.jpg. My only contribution was to add the "Bypass" jack. The original design was for use with a Yamaha SPX-90 reverb unit and I figured the pot value would be close enough. Since I'm replacing the pot anyway, I'll switch to 10K. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 20, 2015 Moderators Share Posted June 20, 2015 Good luck. Link to comment Share on other sites More sharing options...
Members DeepEnd Posted June 23, 2015 Author Members Share Posted June 23, 2015 Success! I checked all my connections and found a shorted jack. I corrected the problem and everything works as expected. I also replaced the pot with a 10K one on principle. Thanks again. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 24, 2015 Moderators Share Posted June 24, 2015 Glad it all worked out for you. Link to comment Share on other sites More sharing options...
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