The ε12 muting / protect circuit

Schematic diagram

The following is the ε12 v2 schematic.

Circuit Description

D1, Q1 and Q2 form a simple voltage regulator to provide the ±12V rails for use by the circuit. One half of U1, a TL082 dual opamp, operates as a voltage follower with unity gain, and is used as a virtual ground rail splitter. The VR1 trimpot adjusts the virtual ground's zero reference voltage relative to the rails. The use of a rail splitter here allows the ε12 to support monitoring of amplifiers with active ground outputs. A neat by-product of the rail splitter, is that the circuit will draw the same amount of current from each rail. Thus it will not cause an imbalance in the drain-down time of the positive and negative supply rail capacitors after turn-off.

Q5 and Q6 serve to drive the relay. They are arranged such that the +12V rail sources the relay coil current and the -12V rail sinks it. This is an improvement over ε12 v1 (where the relay coil current goes to virtual ground). This change removes the need for the OPA551PA high-current opamp used in v1 for the rail splitter, and eliminates the virtual ground potential shift when the relay coil is engaged and disengaged.

The time delay is accomplished by the charging of C7 through R11 after turn-on. The default values of these gives approximately 6 seconds delay. To increase the time, increase the value of R11 or C7. R13 forms a voltage divider with R11 to prevent Q5's Vgs from exceeding one half of the +12V rail voltage. R13 and D5 also helps to discharge C7 after power off. R13's value should be the same as R11.

The other half of U1 is configured as an inverting summing amplifier with a voltage gain of 10. It is also limited in frequency response with a low-pass corner frequency of 1.6Hz. This amplifies the DC offset from all the channels being monitored, which is then fed to Q3 and Q4. These two transistors provide positive and negative DC offset detection, respectively, and shunts C7 if the offset exceeds their turn on thresholds, causing the relay to disengage. The diodes D3 and D4 clamps the opamp's inputs for overvoltage protection.

The schematic shows four inputs and four outputs (L+, R+, L-, R-) which corresponds to a four-channel (balanced output) amp configuration. For a three-channel amp, just use one of the "-" inputs and outputs for the shared ground channel. For a traditional two-channel amp, you should skip the L- and R- connections.

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