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This is applicable to the PMP powered mixers regarding an undocumented issue of polarity of speaker connections.
Failure of the "A" side of these units can happen if Speakon to 1/4 inch speaker cables are used and IF the barrel of the 1/4 inch plug becomes grounded. This can happen from careless handling or from intentionally trying to parallel another device from the output which results in a ground being placed on the barrel of the 1/4 inch jack on the "A" side of these mixers. The barrel of a 1/4 inch on the "A" side is the "hot" from the amp.
Upon analyzing the circuitry, these mixers internally invert the audio signal to the "A" side and then the speaker connections reverse the polarity again. Behringer does this to easily allow "bridged" operation of the amplifier.
This creates a hazard that can result in blowing the "A" side amp of these mixers.
If one ONLY uses Speakon connectors everywhere it is NOT a problem, but anytime 1/4 inch plugs are used in the speaker cable system, the hazard exists.
In stereo terms your system is "clipping" which means you are pulling more amps than the system can handle. If it happens only when you use the mixer, I'm thinking the mixer cant handle the power. Check the specs, It should show maximum amp input.
The fault may not be in the power supply area! The other area of blame is the main amp section. Suspect any of the devices on the heat sink (also any device in the power supply with an heat sink). Transistors can be tested easily with this problem. An Ohm Meter on the resistance setting, on the terminals (even in circuit) of the transistor, will act like you have touched the test probes together. Just like a piece of wire, because that's what it has become and why they blow the fuse!
Fuse blowing is due to excessive current drain and the the first clue is the blown fuse itself. If the fuse blows with a flash a bang and splatters copper across the inside of the glass you have a hard blow caused by a dead short on the power supply, check for power switch flashing over internaly. Short circuit main filter capacitors. Shorted turns in power transformer primary winding. If fuse just separates check for short on output transistors C to E. Try isolating fault by disconnecting LT pos/neg supplies to output stage. Also check for partial shorts on speakers and speaker wires using an analog ohm meter on low ohms setting.
Hi, If you have small electronics knowledge this might help you. First fine in each mixer the amplifiers line In if your mixer has two amplifiers then you have four line In. Disconnect this line from your mixer and build a circuit Line out multiflier. You can use op amp for this application this is a simple circuit. You made two input and four outputs. If you are interested i can send you a schematic diagram.