Audio Circuit Breaker
(Doug sends the following information about himself: "I've never written before but with your help, I think I can share some ideas with fellow readers. I'm a guitar/keyboard player with mostly music training and no electronics training. A couple of years ago an amp didn't work right, so I took it to the best shop in town and ended up paying $30 just to have the power tubes changed! My Scottish blood couldn't deal with that so I started reading everything I could get my hands on about electronics (Craig's books and articles in Guitar Player and Modern Recording included). I'm currently enrolled in the Do-It-Yourself School of Electronics. My experimenting centers around troubleshooting audio problems on the road and generally making rock and roll shows easier to take.")
Here's something that saved my band lots of money by saving speakers. Picture this: Your band is cooking along, and everything is great... until someone trips over the AC power cord. Upon discovering this error, the well-meaning person jams the plug back into the wall. KABOOM! When the power comes back to the board, it sends a tremendous spike to the power amps. After a couple of accidents like this, you can either convert your damaged equipment into boat anchors, or prevent such problems in the future with the audio circuit breaker (ACB for short). While I borrowed the idea from home stereo speaker protection circuits, it works fine with PA and other musical applications.
The ACB plugs into the same power box used by your board; Should the AC power go off for any reason, relays interrupt the signals going from the board to the system's power amps. When power comes back on again, the audio signals remain interrupted until an additional reset button is pressed (hopefully after the power surge!).
The schematic shows a version using 4 relays to protect a triamped system (triamp systems have separate high, low, and midrange power amplifiers), and inserts in the system between the crossover and power amps. T1 steps down the AC power to 12 VDC. This is rectified by D1-D4, and filtered by C1. Do not use a larger filter capacitor, since larger caps will hold the relays on for a certain amount of time after the power goes off, and the relays should open up at the first sign of power loss.
IC1 regulates the unfiltered voltage down to a regulated 12V, with LED1 and current limiting resistor R1 indicating that power is reaching the unit. As soon as the reset button is pressed, RY1's coil receives power and its contacts close. This provides power to RY2 - RY4, and closes these contacts. Each set of contacts interrupts the crossover outputs going to the power amps, providing the audio circuit breaker function. Diodes D5 - D8 clamp the spikes released from the relay coils when power is cut.
If you have a mono system or do not use a crossover, eliminate RY3/RY4, D7/D8, and the corresponding jacks to create a single channel version.
This is a very non-critical circuit, so many substitutions are possible. If you don't have 12V DC relays, you can use 6V types by changing T1 to a 6V transformer and IC1 to a 6V regulator. 5V reed relays designed for computer logic applications will also work; again, use 6V transformer but change IC1 to a +5V regulator. In both cases, R1 and R2 should be 1K instead of 2.2K.
The ACB has served my band well, and hopefully it will do the same for you.
Feature by Doug Montcrieff
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