Why do I hear my voice coming from my howebrew amplifier.

Discussion in 'Amateur Radio Amplifiers' started by M0KGV, Mar 22, 2015.

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  1. M0KGV

    M0KGV Ham Member QRZ Page

    I have just finished a 1KW solid state amplifier project. During testing I could hear my voice coming from the amp as I spoke into the microphone on the transceiver, anybody come across this before? LPF installed and working great -62db on the third harmonic.

    I would hate to think I have built a lemon!!. Any help greatfully received.

    Clive M0KGV
     
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    YOur power supply is obviously singing. YOu can try potting your power transformer or using some other acoustic isolation.
     
  3. M0KGV

    M0KGV Ham Member QRZ Page

    Forgot to mention the power supply is a Eltek Valere Flatpack 2 (2KW) 53 volts
     
  4. KM1H

    KM1H Ham Member QRZ Page

    Remote the PS from the amp and be sure of the voice source.

    Also is it band specific or otherwise point to a RFI problem?

    Carl
     
  5. M0KGV

    M0KGV Ham Member QRZ Page

    Cheers Carl the problem seems to affect all bands when the amplifier is driven above 300 watts.
     
  6. KW4EK

    KW4EK Ham Member QRZ Page

    There are a couple of possible explanations that come to mind. Firstly, anything with magnetic fields (inductors, transformers) can experience magnetostriction in the presence of a varying current -- if any part within that field is not absolutely rigid then they can vibrate at the same frequency, or a harmonic of such, as is flowing through them. Secondly, many types of capacitors, but especially ceramics, can exhibit microphonics (essentially a piezo electret effect) causing them to vibrate at the same frequency, or on a harmonic of a frequency, as the current flowing through them -- they can both give off audio (speaker) as well as introduce audio (microphone) into a circuit.

    Either, both, or something else entirely, (I think I've covered all the bases with that caveat) are likely responsible for the audio you are hearing.

    Good luck with your SS PA, hope to get around to building my own sometime in the next few years (famous last words, I know).
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm unfamiliar with that power supply but my guess is it's the supply that's singing with your voice. A lot of 48V power supplies here are telco supplies which are often called 'rectifiers' because they were mostly designed to charge 48V battery systems and while very powerful and regulated, they weren't designed for greatly fluctuating loads such as a linear amplifier and I've seen them have all sorts of mysterious maladies when so used.
     
  8. N8CMQ

    N8CMQ Ham Member QRZ Page

    Depending on the frequency, the amplitude, and duration
    of the vibration, it could cause a component failure.

    You may want to use silicone tubing as a stethoscope to
    locate what is singing, to ascertain if it is a critical part.

    I have had potted power transformers fail due to poor
    potting allowing coil turns to vibrate and short out to
    the case of the transformer. Ceramic caps have been
    known to crack due to vibration as well.
     
  9. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    It could be your tank circuit talking.
    In the tuning house of an AM broadcast
    antenna tower, you can sometimes hear
    program audio in the inductors.

    In the air-wound copper wire of my
    homebrew 100 watt AM transmitter,
    I could hear my own voice.

    I'm told it's from rectification at
    the junction of dissimilar metals.
     
  10. AA9G

    AA9G Premium Subscriber QRZ Page

    I'm glad I saw this thread because if ( when I get around to having one) I had heard a voice coming from my amp I would have gone straight to the ER and checked myself into the hospital funny wing.
     
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