TDA7052A buzzing when no input GND

Discussion in 'The Projects Forum' started by vol_, Apr 29, 2016.

  1. vol_

    Thread Starter Member

    Dec 2, 2015
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    Hello,

    i am breadboarding a TDA7052A power amp for instrumentation use. When the guitar jack isn't connected on the input socket i get a buzz sound, which is not too annoying, but i would like to know if this is normal or if its something wrong.

    the schematic i'm building is in this site:

    http://www.diystompboxes.com/smfforum/index.php?topic=99271.0

    I know that this occurs from the fact that there is no ground connected in the input stage because when i touch the gnd pin of the socket (unconnected with the jack) the buzz stops.
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
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    Input impedance too high?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I think you mis-understand the schematic. The input signal goes to the jfet transistor gate but the input ground should go to the circuit ground. This is assumed but not shown.
     
  4. vol_

    Thread Starter Member

    Dec 2, 2015
    93
    3
    On the breadboard, I connected the input gnd in a point where also the 3.9K, 3.3M and 10K resistors, all together, are connected to ground. TDA7052 pin 3 and pin 6 ground are next to this point. Is this what you mean by saying the input gnd should go to the circuit gnd? And actually, there is no other gnd than the circuit gnd.


    Danny as i saw in other guitar amps schematics 1M is the minimum input impedance and there are several schematics with 3M input impedance, especially for piezzo (acoustic guitar) pick ups.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A good solution to stop the buzzing when nothing is connected would be to have a 10k resistor automatically connect the jfet gate to ground when unplugging the cord with a switched connector (like a headphone jack that causes external speakers to cut out when the headphones are plugged in.

    right now, the 3.3M (and any nearby wires on your breadboard) are acting like an antenna and the circuit is amplifying any RF noise that comes in. The voice coil on the speaker is a low-pass filter and lets you hear any of the audio-range noise that passes through. A 10k (or even a dead short - bare copper wire) from the gat to the source will kill all of that noise. Note that even your finger (50k to 250k generally) kills most of the noise.
     
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  6. dannyf

    Well-Known Member

    Sep 13, 2015
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    probably designed by people who have no idea of what they are doing, :)

    Typically for audio, 47K is the upper end. I tend to go 10k or even 1k as a practical low end.

    In your case, put a resistor in that neighborhood on the input and see what happens.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Those resistor values do not work on a guitar. The passive pickups on an electric guitar are expecting at least 250k (Fender) or more (Gibson) input impedence. The piezo of an acoustic guitar (as the OP has) needs 1M to 5M (1 to 3.3M is typical).

    Knowing what they are doing is a question of knowing the purpose of the devices that are connected to it.
     
  8. vol_

    Thread Starter Member

    Dec 2, 2015
    93
    3
    Yes, excuse me, i didn't explained that the amp is to be used for guitar and other acoustical instruments with piezo pick ups.

    Danny thanks for the audio info, though. I will need this in the near future.
     
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