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question about 16ohm to 8ohm speaker

Discussion in 'Electronic Basics' started by Joe, Jan 14, 2004.

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  1. Joe

    Joe Guest

    I have a small (digital, I guess) recorder/player that I bought from the rat
    shack last week. It runs on a 9 volt battery and you can record 20seconds
    worth of sound and then play it back. It has a 16 ohm speaker on the output
    (a really fragile piezo type (?), with a clear plastic film over it). My
    problem is that it does not play back loud enough. I am measuring about 2V
    peak to peak signal on my oscope with the speaker disconnected (unloaded). I
    would like to change the 16 ohm speaker to a better 8ohm speaker and maybe
    be able to boost the signal along the way, maybe have a pot to control the
    volume. When I connect the 8ohm speaker in place of the 16ohm speaker, the
    volume drops substantially (no surprise there, I guess).

    I first tried connecting the output to a LM386 audio amp, and then
    outputting that to an 8ohm speaker (LM386 was wired for minimum parts count,
    gain of 20). There was a lot of distortion, enough so that I could not
    really hear the sound that had been recorded originally. All kinds of weird
    noises were coming out of it, but it was really loud!

    Then I thought of replacing the LM386 with a common collector type amp to
    act as a buffer for the signal. I used a 2N2222 transistor as a common
    collector, but volume was still low. It did improve a little with a 2N4401,
    but is still not loud enough.

    Any ideas on how I could boost the signal enough to tweak the volume with a
    pot, and drive an 8ohm speaker ? I don't know the wattage of the 8ohm
    speaker, it was salvaged from something, but it is about 2 inches in
    diameter.


    TIA,
    Joe
     
  2. tempus fugit

    tempus fugit Guest

    When I connect the 8ohm speaker in place of the 16ohm speaker, the

    I would think it would be louder - lower Z = more current coming out of the
    device.

    A small signal transistor like these is not really capable of delivering
    enough power to drive a speaker to any appreciable level.

    Why not go back to the 386 and drop the gain to, say, 5 or 10 and see what
    the sound is like then? You can easily connect a volume control to the 386
    as well. The 386 was developed specifically for applications like yours, I
    think.
     
  3. Goran Tomaš

    Goran Tomaš Guest

    Actually, the volume should have got louder (less resistance, more
    current) but I guess the 8 Ohm speaker is much less sensitive...
    Go back there and either reduce the gain or (better) put a pot between
    output and LM386 input to act as a volume control.


    Regards,
    Goran Tomas
     
  4. Joe

    Joe Guest

    Thanks tempus, I can try that. I am not sure how to connect the LM386 for a
    gain of 5 tho. I followed the data sheet and it shows how to connect for a
    gain of 20, but I dont see a feedback and source resistor like when using a
    regular opamp. Which components would I change for a gain of 5?

    Joe
     
  5. Joe

    Joe Guest

    Hi Goran,

    thank you for the advice, when I do that, I get a squealing noise coming
    from the speaker at certain ranges of the 10K pot. I still have it at a gain
    of 20 tho, because that's the circuit that is shown in the data sheet. If I
    knew how to reduce the gain, I think it would work. I am not sure which
    components to change to bring the gain to about 5.

    Joe
     
  6. Ian Bell

    Ian Bell Guest

    I think you were on the right lines with the LM386. However you really do
    not need the gain of 20. I suspect the output impedance of the RS device
    is relatively high which is why you get lees when you use an 8ohm speaker.
    Try again with unity gain.

    Ian
     
  7. Joe

    Joe Guest

    Hi Ian,

    Which components do I change to achieve a lower gain?
    Is there a formula for the LM386? The data sheet I have says "the gain is
    internally set to 20 to keep external part count low, but the addition of an
    external resistor and capacitor between pins 1 and 8 will increase the gain
    to any value between 20 and 200."

    So if I read that right, 20 is the lowest gain I can achieve with the LM386.
    Is that right?

    Thanks,
    Joe
     
  8. The LM386 is not guaranteed to be stable for closed loop gains less
    than 9 (page 3 of the data sheet talks about how to go that low), but
    you can easily divide the input signal by any amount including 20
    before applying it to the amplifier. Just connect your signal to one
    end of a pot with the other end grounded, and feed the amplifier from
    the wiper. See the first figure under Typical Applications, page 5.
    http://www.nari.ee.ethz.ch/teaching/PPS/PPS02/doc/LM386.pdf
     
  9. Goran Tomaš

    Goran Tomaš Guest

    Joe,

    the total gain in the system is product of pot attenuation and
    amplifier gain. So, if the gain of the amplifier is 20 and you adjust
    a potentiometer so that it creates a voltage drop of 1/20, you will
    have 20 * 1/20 = 1, unity gain.

    A pot before the amp will give you a range from complete attenuation
    (silence) when wiper is turned all the way to the one side (and that
    side is grounded) to max amplification of 20 when the wiper is turned
    all the way to the other side (and that side is connected to the
    output of a device).


    Regards,
    Goran Tomas
     
  10. Joe

    Joe Guest

    Hi John,

    Thanks for the link. I think that is the same data sheet I was reading from.
    I just didn't read down far enough. The pot at the input works as long as I
    don't turn it up too far. If I do, the speaker starts squealing and I see
    very noisy waveforms on the scope. I am using a single turn pot tho. Later
    on today, I was going to use a 10 turn pc mount pot and see if I can tweak
    it a little better.

    Thank you,
    Joe
     
  11. Joe

    Joe Guest

    Thanks Goran,
    I am working on it, I think I need a multiturn pot, because there's a spot
    on the single turn pot where the output is very low and then suddenly jumps
    loud with lots of noise. I was going to try a 10 turn 10k pot later on
    today.

    Joe
     
  12. John Fields

    John Fields Guest

    ---
    Sounds like you've got a bad pot.

    In any case, since your R/S thing is designed to drive a low impedance
    (16 ohms, you say) you may just be buying yourself trouble with that 10k
    pot. Try 1000 ohms or even 100 ohms to see what happens. I'll bet if
    you do your problem will go away.
     
  13. Or even a 20 ohm speaker fader control.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  14. Joe

    Joe Guest

    Thanks John, I'll give that a try.

    Joe
     
  15. Just add a fixed resistor in series with the input end of the pot,
    till the desired maximum volume corresponds to the pot turned all the
    way up.
     
  16. Goran Tomaš

    Goran Tomaš Guest

    A multiturn pot or trimmer won't do it. Linear pot won't either. You
    need logarithmic potentiometer!

    With linear pot you'll get exactly what you're getting - nothing,
    nothing and then suddenly very loud. Ear responds to volume in
    logarithmic manner. Therefore you need logarithmic potentiometer. A
    10k log pot should do it...


    Regards,
    Goran Tomas
     
  17. Joe

    Joe Guest

    Hi Goran,

    I guess an audio taper is close to what you are calling logarithmic. I have
    one of those. With all these suggestions, my results are definitely
    improving. Thanks for the advice.

    Joe
     
  18. Joe

    Joe Guest

    Hi John,

    Thanks I will try that. The problem is working itself out as I get these
    suggestions. I am going to try an audio taper pot also. I think maybe I also
    need a better quality speaker.

    Joe
     
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