on-off delay for amplifier

Discussion in 'The Projects Forum' started by kubeek, Aug 22, 2008.

  1. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    So here is the circuit for delaying a relay on the outputs of an amplifier.

    in this picture there is only the delay circuit. It is made to be powered straight from the 12V AC supply, but should be easy to redo for some other higher voltage.
    The relay which is switched by the transistor is then operating the power contacts. The second relay is there to ensure immediate discharging of the timing cap, so if you rapidly switch power on-off-on the time before on is always the same. It is a small DIP polarised relay.
     
  2. kubeek

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    Sep 20, 2005
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    And this is the whole project, it is a triple headphone amp for use in my recording studio.
    It has one A-class amplifier and two AB-class buffers.
    In this amp the delay is used to "short" output capacitors to prevent nasty sounds.

    The design off the A-class amp is made by Wavebourn from diyaudio.com
     
  3. blocco a spirale

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    Jun 18, 2008
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    That looks like a very complicated way of providing a power-on delay.
     
  4. kubeek

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    It certainly is a little overkill, but I want to be absolutely sure my 300$ headphones don´t get accidentaly destroyed, because the amp has quite a lot of power.
     
  5. blocco a spirale

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    I was thinking more along these lines:
    [​IMG][​IMG]
     
  6. kubeek

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    The links don´t work..

    I am not sure, but what does that circuit do when you switch it off for a short time and on again? I think it will start immediately depending on the charge of the cap, which is what I wanted to avoid.
     
  7. blocco a spirale

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    Q1 is responsible for discharging the power-on delay cap (C2) when power is removed, so even if power is only removed briefly the power-on delay should restart.

    When the circuit is powered, Q1 is off as the base and emitter are at the same potential. As soon as power is removed R1 quickly pulls the base low, switching on Q1 which discharges C2 causing the relay to drop-out at the same time.
     
    Last edited: Aug 23, 2008
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