Audio amplifier DC offset protection

Discussion in 'General Electronics Chat' started by @android, Oct 23, 2012.

  1. @android

    Thread Starter Member

    Dec 15, 2011
    178
    9
    hi, I was wondering what above mentioned term exactly means? In OP-AMP we use input offset voltage to make output voltage zero. What is dc offset protection? Or OP-AMP offset term & power amplifier offset term is different? :confused:
     
  2. electron_prince

    Member

    Sep 19, 2012
    93
    3
    perhaps it means a method which prevent dc shift of the signal.

    offset, in general means compensation. So again i am forced to think of it as a battery which when connected with the input source gives the average value of output zero.

    I think it's done with that decoupling capacitor.

    Just a blind guess. Wait for some knowledgeable person reply.

    Edit: Decoupling capacitor is also called bypass capacitor.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Audio power amplifiers can be designed with split power supplies and therefore can eliminate the need for the large value output capacitor which tends to limit the low frequency response.

    For extra power, two channels of the power amp are bridged, also eliminating the output capacitor.

    If the DC offset at the output to the loudspeaker is excessive, there goes your speaker.

    A simple solution is to put a resettable thermal fuse or an incandescent light bulb in series with the loudspeaker.

    Once, when setting up for a gig, the rented system went into feedback mode and I saw the speaker light up. I was sure the speaker was blown. But it wasn't, thanks to the light bulb.
     
  4. @android

    Thread Starter Member

    Dec 15, 2011
    178
    9
    So basically dc offset protection is to protect speaker from dc input. How dc offset is generated at output of amplifier? What causes dc shift in the output of amplifier?
     
  5. electron_prince

    Member

    Sep 19, 2012
    93
    3
    maybe the thermal noise.
     
  6. timescope

    Member

    Dec 14, 2011
    298
    44
    Most modern amplifiers are dc coupled which means that the speaker is connected directly to the output transistors. If one of the output transistors fails and becomes short circuited between collector and emitter, the positive or negative supply voltage will appear at the output and burn the voice coil of the speaker. To prevent this, a relay circuit disconnects the speaker if the dc output of the amplifier is not zero volts. Some high power amplifiers use a triac to short circuit the output to ground if a dc offset occurs ( crowbar circuit), causing a fuse to blow.

    Timescope
     
  7. @android

    Thread Starter Member

    Dec 15, 2011
    178
    9
    Ok. So now I know that DC offset basically DC shift to music signal at amplifier o/p. Also know that 20mV-50mV is acceptable.

    Now, I have another question. I have a 2-CH car audio amplifier. Here is how I tested it. I hooked it up with 12" woofer (BTL) cranked volume to max (are you nuts!! :eek:) with below amp settings.
    Crossover: LPF
    Level: max
    Bass boost: max
    It ran until half an hour and after that it went into protection for two or three minutes & again ran for few minutes & again went into protection.....

    And one thing I know for sure this wasn't thermal protection. Because temperature wasn't over 55-56°C. Thermal protection is set around 70-74°C.

    If I run amp by only making crossover FULL, all other settings same as above then it won't shut off even after an hour!! :confused:

    How about this?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Temperature of what? Thermal protection usually protects output devices, so if you measure the temperature of the heatsink or something else, you are way off the actual temperature in the device.
     
  9. @android

    Thread Starter Member

    Dec 15, 2011
    178
    9
    Of course heat sink temperature. But I do know at what heat sink temperature my amp goes into thermal protection. Actual junction temperature would be much higher.
     
  10. @android

    Thread Starter Member

    Dec 15, 2011
    178
    9
    Is anybody here?
     
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