I have two stereo receivers. A Yamaha HTR-5280 and a Pioneer VSX-515. I have the Yamaha hooked up right now because it has a phono input. The Pioneer does not. Unfortunately, the Yamaha has a bad encoder on the volume knob. It jitters up and down no matter which way you turn it. I'm either going to have to find and replace the encoder or just build a preamp for the Pioneer.
Building a preamp seems to be the better project. I'm choosing a transformer to step down voltage to run the opamps. I need +15v and -15v rails. It'll have a full-wave bridge rectifier and LM7815/LM7915 voltage regulators which require approximately +2v above the output voltage. Having never chosen a transformer for a project I'm wondering if they, by design, provide extra voltage to overcome the forward voltage of the diodes and supply enough for the voltage regulators if I choose a 30Vac CT transformer or should I be getting a 36Vac (18-0-18) transformer?
I just want to make sure I'm choosing the correct part so that I'm not either underpowering the voltage regulators or overpowering them causing heat.
I'm thinking the 36Vac is the way to go. I'm also probably going to get a torroidal transformer by Hammond. They're pricey but I like the advantages, especially size.
I completely forgot that these values are RMS so I could almost go with a 24v CT transformer but that's cutting the +2v margin razor thin and may cause the regulators to cut on and off.
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