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Audio feedback problem

Discussion in 'General Electronics Chat' started by Space Varmint, Oct 16, 2008.

  1. Space Varmint

    Space Varmint New Member

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    I has to go back and re-bias some audio amps so I could maintain a ball park level for AGC in a radio receiver. Well, now I got more audio than I know what to do with. I tried bypassing the speaker pre-driver stage and didn't have quite enough so I'm leaving it in. If schematic is needed I can have done shortly as to what it looks like at the moment.
    I have tried heavy bypass caps across the power buss but it just changed the frequency of the feedback. Any suggestions?

    Thanks
     
  2. Mikebits

    Mikebits Well-Known Member

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    Not sure what your saying. Are you getting audio into your power or modulating your RF? Can you decrease audio gain? Feedback could be layout, is this a PCB or BB?
    If the latter, you might isolate grounds and tie at common point on chassis. Seperate supplies maybe?
     
  3. Space Varmint

    Space Varmint New Member

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    Oh, you think it's a ground loop? You could be right. It's on breadboard. I cannot decrease the audio up to where the AGC taps in. Then I have a pot and two amps. One is chip audioguru suggested. Great chip! A TDA 2822.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You might have feedback through the power supply.
    The audio amp is supposed to have a volume control at its input. Then you can decrease the audio to zero.

    Aren't you making a SSB shortwave radio? Please post a schematic.
     
  6. Mikebits

    Mikebits Well-Known Member

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    Well, AG knows his stuff so I would think his recomendation is a good one. I would run you audio on a seperate supply, and use a star ground configuration. Or perhaps try one then the other and maybe both. You are now working in the realm of voodootronics :)

    Experimentation is the best approach...

    Also sheilding the power audio from sensative areas might help. Anywhere that is high Z and gain will be easy targets for interference.
     
    Last edited: Oct 16, 2008
  7. Space Varmint

    Space Varmint New Member

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    At the input of the chip? I have it at the input of the chip's driver. A transistor stage.
     
  8. Mikebits

    Mikebits Well-Known Member

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    The TDA is the driver, right? Well there's your problem... Max gain on the TDA? I guess this is AG's area, so I will shut up now :)
     
  9. Space Varmint

    Space Varmint New Member

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    Yeah the speaker driver. It's a hot chip!
     
  10. Space Varmint

    Space Varmint New Member

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    OK, I moved the pot and it did help. I will go after power bus to improve it.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The TDA2822 power amplifier has a voltage gain of 100 which is plenty for a radio. It has a supply current drain of up to 1.2A. The 1.2A pulses cause the power supply voltage to sag and that is where the collector resistor of the transistor preamp (that is not needed) is connected.
    I have seen high gain amplifiers vibrating ceramic coupling capacitors (that should not be used for coupling audio) which caused feedback.

    The schematic will show exactly where the feedback is coming from.
     
  12. Space Varmint

    Space Varmint New Member

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  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An AM radio should have enough gain in its front end and IF to play weak signals loudly. Then AGC reduces the gain of the IF and then the gain of the front end for very strong signals so that weak and strong signals are the same volume.

    The TDA2822 and all other IC power amplifiers have an absolutely flat frequency response. Low frequencies and high frequencies have exactly the same amount of gain.

    Your problem is probably caused because the radio does not have enough RF gain so you are using an excessive amount of audio amplifier gain instead, then feedback from the power supply is causing hum, then feedback.

    Without seeing your schematic then we are only guessing.
     
  14. Space Varmint

    Space Varmint New Member

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    Well your right about the rf gain. It was done deliberately. I do not want much rf gain because of blocking & intermod. I prefer to use front end filtering for my rf gain which has not been done yet. I have two IF's where I did as much amplification as possible. I got the second IF up to about 1 volt P-P for weakest signal input. That was done to reduce the amount of audio amplification needed. Here is what my audio looks like:

    I have the whole radio in like five cat food cans and so there are power wires coming out of each and are excessive in length. I know these are AC 60 cycle antennas. I should be able to cut them off today.

    Anyway....grade me on my audio. you are the king there audioguru. ;)

    Correction: the chip driver is not biased like that. I accidentally pulled up the wrong file. Here are the corrections:

    1. R98 = 10K

    2. R95 =10K

    3. R96 =920 ohms

    4. R100 = 100K

    There are a couple of changes to the first audio amp too, like that 1500pf cap was removed. Very minor changes. I just don't have time to cut, paste and re-size the whole dern thing.
     

    Attached Files:

    Last edited: Oct 19, 2008
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What's R99? - assuming it's supposed to be the volume control it's connected completely wrongly, also decouple your split supply for the opamp.
     
  16. Space Varmint

    Space Varmint New Member

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    Oh hey Nigel. Didn't think anyone would be up. Forgot you are in the UK.

    Anyway the OP Amp is decoupled. Thanks for reminding me. I need to put that in the schematic.

    What do you mean by split supply? It is single supply.

    Actually my volume control is different. I need to get the right file and post it.

    Hang on...
     
  17. Space Varmint

    Space Varmint New Member

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    This is the correct file. This is precisely what I have now.
     

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  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You're generating a split supply with two resistors, it needs to be decoupled with a capacitor.

    It's still completely wrong, how can you have never seen how to wire a volume control?.
     
  19. Roff

    Roff Well-Known Member

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    I can't believe how much gain you are throwing away. Here is a list of the resistors that are at the beginning of each attenuator, along with the approximate attenuation of each divider:

    R135 -9dB
    R147 -24dB
    R57 -33dB
    R58 -12dB
    R100 -22dB

    You are throwing away approximately 100dB of gain! Throwing away gain means more noise and more distortion at the output.
     
    Last edited: Oct 19, 2008
  20. Roff

    Roff Well-Known Member

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    All three of your transistor amplifiers almost certainly have gain from Vcc to the output. Any noise on Vcc will be amplified at the output of each stage. You need your audio amps to have good power supply rejection.
     
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    All in all it's a pretty bizarre circuit, most of which isn't required.
     

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