I'm working on a 200W audio amplifier. I've made several mistakes along the way, but the most recent leaves me puzzled. I wired the output wrong and it caused two resistors to burn up, but I can't figure out where that much current came from.
It's designed as a bridge amplifier. The image below shows just the output sections of each side of the bridge, from the simulation I've been using. I wired the outputs wrong, so that both NPNs went to one output and both PNPs went to the other. That's reflected in the image below as the "outp" and "outn" outputs. The power transistors and 0.1Ω resistors are off the PCB. About 15 seconds after I flipped the power switch (no input signal and no speaker load), R42 let the smoke loose, and I even saw a small red flash come from it. Upon further inspection, R14 is discolored as well. Everything else is fine.
These are 1/4W resistors, so it had to have been over 50mA to burn them up. In the simulation these resistors only get 3mA through them when the output is wired wrong. I didn't get a chance to measure any voltages before the smoke. The power rails for the power amp section are unregulated and come from a 48V center tapped transformer. I've attached the LTSPICE file with the power supply section and output section. There's more to the whole circuit, but there's no DC connection between the preamp and power amp sections.
What could have caused that much current to go through only those two resistors, and not the other two 100Ω resistors? I want to figure out if there's any other problems before I fix the wiring and try again.
It's designed as a bridge amplifier. The image below shows just the output sections of each side of the bridge, from the simulation I've been using. I wired the outputs wrong, so that both NPNs went to one output and both PNPs went to the other. That's reflected in the image below as the "outp" and "outn" outputs. The power transistors and 0.1Ω resistors are off the PCB. About 15 seconds after I flipped the power switch (no input signal and no speaker load), R42 let the smoke loose, and I even saw a small red flash come from it. Upon further inspection, R14 is discolored as well. Everything else is fine.
These are 1/4W resistors, so it had to have been over 50mA to burn them up. In the simulation these resistors only get 3mA through them when the output is wired wrong. I didn't get a chance to measure any voltages before the smoke. The power rails for the power amp section are unregulated and come from a 48V center tapped transformer. I've attached the LTSPICE file with the power supply section and output section. There's more to the whole circuit, but there's no DC connection between the preamp and power amp sections.
What could have caused that much current to go through only those two resistors, and not the other two 100Ω resistors? I want to figure out if there's any other problems before I fix the wiring and try again.
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