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    • #12063

        Hi all,

        Some days ago, a good friend of mine asked me about a problem with a Braun amplifier.
        After powering on, a loud “plop” was heard in the speakers.
        This is not normal as this amplifier has a so called “anti-plop” circuit.
        This little circuit connects the speakers to the output amplifiers via a time delayed relay.
        For some reason, this circuit failed in the Braun amplifier.
        This is the part of the original diagram with the “anti-plop” circuit :

        duagram extract

        After powering on, relay RS65 gets energised after about a second. The relay contacts close and connect the speaker terminals to the outputs of the amplifiers.
        There”s also a protection circuit around T651 & T652. In normal condition the amplified signal swings around 0V.
        R651 & R652 are connected to these outputs and whenever a pos. or neg. voltage is present (malfunctioning amplifier outputs), the relay quickly turns off and disconnect the speakers.

        To facilite the repair process, Idecided to build this circuit with the Lectron set.
        It’s build in no time …
        I have redimensioned the parts values so the circuit can be used with 9V.
        We also make use of a pos. 9V battery module and a neg. 9V battery module
        To simulate the pos. and neg. error voltages, I have used switches S1 (neg. voltages) and S2 (pos. voltages).
        Here’s the setup :

        speaker protection

        And this is how it works :

        After powering on with S3, the E of T3 gets aboout 1,58V (voltage divider around R8 and R11.
        This will cause a prolongued charging time of C2, since the B of T3 will need to be 0.6V higher than the E.
        Now, C2 starts to charge via R7. As soon the voltage at the B of T3 is about 2,4V, it starts to conduct and T4 is turned on.
        The relay is now energised and the led turns on (simulating that the speakers are connected).
        D1 is in the circuit to lead higher voltages than 9V to the pos. VCC of 9V.

        Simulation of’ a negative error signal on the amplifier’s output :
        As soon S1 is pressed (-9V), this neg. voltage flows through R2 to the E of T2.
        Since the B is more pos. than the E, it will conduct and immediately discharges C2.
        This turns off the relay very quickly.
        T1 remains blocked.

        Simulation of’ a positive error signal on the amplifier’s output :

        As soon S2 is pressed (+9V), this pos. voltage flows through R3 to the B of T1. This transistor conducts and shorts C2 to GND.
        This will turn the relay immediately off.
        T2 remains blocked.

        D2 (Schottky diode) protects C2 against neg. charging which would harm it.
        C1 helps to avoid false triggers (smoothing any AC voltage).

        And here a setup of the circuit.
        It performs amazingly well and is true to the original.
        With Lectron, it’s so easy to build and investigate electronic circuits.

        Many greetings

        speaker protect setup

      • #12089

          This is amazing Frank!  Mr. Greger would be so impressed with your creativity and solid design skills!  I would bet that he would hire you to be along side with Mr. Max Guerth!

          Thank you so much!

          • This reply was modified 2 weeks, 1 day ago by Michael.
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