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Simple question in audio

  1. Jul 7, 2013 #1
    So I've used a chip like this before with a center tabbed transformer useing the tab as the ground for the speaker and everything.
    I want to run this on a 9v, what do I use as ground?
    is ground the -v on the battery or will just connecting all ground points together do it or what? I know that if I have 2 batteries in series I can use the middle - and + connection as ground but I am trying to refrain from doing this.
     
  2. jcsd
  3. Jul 8, 2013 #2
    Use 2 identical resistors to make a voltage divider that produces half the supply voltage. Use a capacitor parallel to one of the resistors to make the output impedance of the voltage divider very small at audio frequencies. (time constant of the capacitor, together with the resitors of the voltage divider about 0.1 s)

    Use this to bias the input of your amplifier. Your output will be sitting at half the supply voltage as well, so you need a large capacitor in series with your speaker, so no dc goes through it.

    look for "single supply operation" in this datasheet of the lm1875

    http://www.ti.com/product/lm1875
     
  4. Jul 8, 2013 #3

    sophiecentaur

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    It's important to make sure that the resistor chain is of low enough value (along with the decoupling capacitors) to supply enough current for the rails to be unaffected by the current supplied to the final load. This is sort of assumed when you use a transformer but the requirement is still there.
    Problem is that the bias resistors are a constant drain on the battery. It may be better to use three 1.5V cells each for positive and negative supplies.
     
  5. Jul 8, 2013 #4
    http://www.circuitstoday.com/car-stereo-circuit-using-tda2040

    I found this after I posted, (this is the chip I want to use) It seems to be what you guys are saying but will it work with 9v, and still get good output power? or would it be beter to just use two batteriese in seriece and the center as ground?

    I really need at least 10 watts output
     
    Last edited: Jul 8, 2013
  6. Jul 8, 2013 #5
  7. Jul 8, 2013 #6

    sophiecentaur

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    Unless you have a source of free 9V batteries, I can think of no reason not to use the power supply that you actually need, i.e. + 0 -. Any other solution will either be potentially unbalanced ( high source resistance) or drain the battery faster. For 10W of audio, you will be needing significant current from the supply yet still maintain the +/-4.5V swing.
     
  8. Jul 8, 2013 #7
    The circuit you posted is for a single supply. Notice the voltage reference formed by R1, R2 and C4 to bias the input.

    Accorcing to the TDA2040 datasheet, you can get out 4 watts with an 8 ohm loudspeaker and 7 watts with a 4 ohm loudspeaker, with a 9V supply. You'll need something heavier than a pp3 9v battery tough. With any battery, you might need to deal with a supply voltage of 6 volts or so when it discharges, and then you'll get even less power.
     
  9. Jul 8, 2013 #8
    I would probably use your chip but I am going HI-FI in this project, for example my pre-amp has .002% THD, and the tda2040 has max of .08%, which is as good as some hundreds of dollar yamahaa amps/receivers.
    I will need to put the circuit together and see if it has the loudness I am looking for, I am putting this together with 2 tda2040s for stereo, how much power would this draw? at max?
     
  10. Jul 9, 2013 #9
  11. Jul 9, 2013 #10
    Ok thanks, If i find my entire circuit consumes too much currrent and the battery drains faster than I like, will putting a battery in parallel double the life of my circuit without effecting things like LEDs or the amplifiers for that matter? I learned when I was a beginner that it shouldn't but something weird happened once which now makes me question it.
     
    Last edited: Jul 9, 2013
  12. Jul 9, 2013 #11
    yes see figure 7 for more detail on that
     
  13. Jul 9, 2013 #12

    sophiecentaur

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    Batteries can let you down drastically if you try them in parallel. They have to be from the same batch and in the same state of discharge or loads of mAhr will be wasted and the good one will drain into the old one.
     
  14. Jul 9, 2013 #13
    Ok good thank you! ok another thing, This project I am going to use rechargable batteries and have an dc wall adaptor to charge this mobile amp, is chargeing as simle as putting posotive from the female socket to posotive on the battery (maybe with a diode) and negative to negative, bada-bing-bada-boom?
     
  15. Jul 9, 2013 #14

    sophiecentaur

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    BOOM, if you use batteries and don't charge them as they like to be charged. This is a new can of worms you're bringing in. Read up about battery chargers - there's a lot in that subject.
     
  16. Jul 9, 2013 #15
    LOL ok I will look into it, so a little off the subject but I was wondering if there is a type of circuit that acts like a crossover, or like the Baxandall tone controller, that I could use to cut out everything above a calculated
    frequency. Cause I am curiouse in makeing a small low power sub woofer amp and I need a way to chop down everything above about 150hz to then be amplified.
     
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