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A little too basic, maybe?

Discussion in 'Electronic Basics' started by rA-nD-oM-iZ-eR, Sep 19, 2003.

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  1. I know this might be very basic to some of you but I really would
    appreciate an answer.

    I have made an extension to my telephone handset to plug in my
    headphones (speaker and mic) and use it as a hands-free phone {don't
    ask, it's not important to know why}. The problem is that I get low
    volume in the headphone speaker. The other party hears me fine so the
    mic is ok. What I need is a way to enhance the volume in the speaker
    line. I've tried so many speakers (some with volume controlers, some
    without) and all gave the same result: low volume.

    I don't claim to be an expert but I know a little about electronics;
    little but enough for making connections, and for fixing stuff like
    radios and other small things (haven't tried the TV yet). I'm
    learning, or trying to, more about electroncis but I just finished
    high school and before now school wasn't giving me enough time to
    practice electronics or read more about it.

    I know this is possible, and I have knowledge of basic electronic
    components and I know how they work (these physics lessons in high
    school weren't so bad after all) but I still donno how to make a
    circuit to increase the voltage/volume. Please help me with this...

    I'd appreciate all your replies.

  2. hAVe a LOok aT iMPEdaNces iN ur BookS
  3. You need an amplifier in the speaker line.
    It can be a very simple amplifier, and it can be battery driven to
    simplify the construction.

    Try to find a readymade small amplifier, or build one from a
    schematic, there are lots of such schematics on the web.

    A single op-amp might be enough.

    An amplifier raises the signal strength and makes the signal suitable
    for the loudspeakers in your headset, that is what the other person
    meant with his talk about impedances.
    The signal needs more "muscles" to handle a loudspeaker.
    It needs the ability to deliver more current.

    You have a mono signal, so if you have a stereo headset you can
    connect the two speakers in series, or in parallell if series is
    impossible. Or maybe you are happy with sound in one ear only.
  4. Ben Weaver

    Ben Weaver Guest

    If they're stereo headphones, try wiring the two speakers in series
    instead of in parallel. That'll increase the impedence presented to the
    phone and make it seem a bit more like what was there originally.
    Thoughts anyone?

  5. Joey

    Joey Guest

    I had a problem with a set of studio phones once where the amplifier
    would distort before the phones got loud enough. I ran the signal
    through a multitap audio transformer and it raised the voltage going
    into the phones which effectively raised the wattage that the phones
    drew from the amp. Radio shack sells tiny audio transformers that you
    might try experimenting with, however I don't know if they are the
    right impedence for the job. The transformer I used was big but then I
    had the space for it.
  6. John Fields

    John Fields Guest

    If I remember correctly, your telephone handset receiver looks about
    like 600 ohms, while your headphones probably look more like 8 ohms. If
    that's true, then a passive solution would be to use a 600 ohm to 8 ohm
    audio transformer. Failing that, a single audio power opamp like an
    LM386 would prob'ly do it.
  7. I got it parallel from the start, so that's supposed to lower the
    resistance; am I right? I tried connecting the speaker line to my pc speaker
    that uses AC power, and most probably has some complicated amplifier, and I
    got good volume. If u'd be generous enough to point me to a diagram of a
    circuit for this amplifier I'd appreciate your help.

    Thank you for responding...
  8. Yes, when you parallell two equal resistances (impedances) the result
    is half the resistance.
    When you put them in series the result is double the resistance.
    If your signal source is having trouble with low resistance loads you
    may get higher volume if you connect the headphones in series.

    Other amplifiers are made for low resistance speakers, and then
    parallell connection is good because it gives much more effect.
    I don't have a link, but search the web for LM386 and you will
    probably find a schematic of how it is to be connected.
    It is a good little amplifier for speakers or headphones.

    But that means you have to buy a LM386 chip and a few other components
    and put it all together. Or two if you want stereo.

    A simpler solution could be to use an amplifier you already have, or
    can get easily.
    Most stereo systems have an input and outputs suitable for this
    purpose, so you can get and old stereo and put between your signal
    source and your headphones.
    Use the loudspeaker outputs from the amplifier if it doesn't have a
    headphone output.

    You only need a mono amp actually, if you can't find a stereo, so you
    might be able to use a radio with input and output, or whatever kind
    of amplifier you find.

    If you want to use any op-amp you can get hold of you should put the
    headphones in series as op-amps cannot handle low impedances well.
    LM386 is an exception, it is made for loudspeakers or headphones.
  9. The Captain

    The Captain Guest

    A phone line impedance, at audio frequencies varies between about 1000
    or more ohms at lowest frequencies (300Hz) to about 400 ohms at 4 KHz
    although this will vary all over the place depending on how long your
    phone line is, type of cable you phone company uses and on and on.

    Telephone instruments use a nominal 600 ohm resistance across the line
    as a cheap and cheerful method of more or less matching the line
    impedance for maximum signal reception. This is really a lousy match
    and causes all kinds of problems with echoes at the exchange, but is
    what, for historical reasons, we're stuck with.

    Your headset speakers in parallel, assuming they are 8 ohm speakers,
    will present 4 ohms to the line. This means that almost all the voice
    frequency power sent from your local exchange over your telephone line
    will develop in the line or, if the line is short, in the 600 ohm
    "matching" impedance at the exchange itself, leaving very little for
    your speakers. I'm actually surprised you can hear anything!

    Now, assuming that your headset is connected to the line, which is not
    too clear from your description, you need to match your earphones to
    the impedance of the line. A 600 ohm to 4 ohm transformer will do
    this, but they tend to be bulky, expensive and hard to find. Also,
    this will affect the operation of your telephone and might piss of the
    phone company considerably. If you have your speakers connected after
    the line connection, that is inside the phone instrument after the
    transformer, then use a matching buffer amplifier as described below.

    A buffer amplifier will take care of the problem. It need not have a
    high gain, in fact a gain of 1 should be sufficient, but it should
    have a near 600 ohm input impedance,if you are connected to the line,
    and a low output impedance. This will ensure maximum power transfer
    between line and speakers and least distortion in the speaker output.
    If the input connection comes from the interior of the phone, then
    it's probably safe to leave the 600 ohm matching resistor out of the
    circuit. If you are interested, I can send you a circuit diagram of
    just such an amplifier, although I would need to know exactly how your
    connection was made. If you are still interested, and I can find the
    time, I can send you construction details.

    As an interesting aside, the reason for the choice of 600 ohms for the
    telephone instrument, which matches the impedance of no cable
    presently in use anywhere in the world, is that 600 ohms was the
    impedance at voice frequencies of old style telephone wires on poles
    separated by wooden cross members. This was, of course, back in the
    days when only the rich had telephones and the need for multiple
    connections was not foreseen. Sort of an earlier version of: "I don't
    see why anyone would want more than 20 megabytes on a hard disk."


  10. Guess what, It worked. I changed the connections to series and it gave
    my nearly the same volume that I got from the normal handset. Although
    the connections were a little funny, being only 3 wires not 4, but I
    did manage to do it.

    I don't know how to thank you enough, guys, This was really helpful.
    I'm sry I didn't reply sooner but I intended to do so when I wrote
    half way through the reply and the electricity went off. I got
    interested more about amplifiers, though, and I have read and will be
    reading more about them in the coming days.

    Once again, I thank you from all my heart for helping me; all of
    you... I really do appreciate it.

  11. JeffM

    JeffM Guest

    If it has a 3-terminal plug, they will be out of phase in series.
    You have to do major surgery on the wiring.
  12. JeffM

    JeffM Guest

  13. Guy Macon

    Guy Macon Guest

    He should try putting them in parallel to get ~4X the power.
    It is likely that it still won't be enough (4X power isn't 4X
    louder because of the logarithic nature of the human ear) but
    it's cheap and easy, and thus worth a try.
  14. Guy Macon

    Guy Macon Guest

    I don't know... I personally like having my mono headphones
    be out of phase. It makes it seem like the voices are in my
    head. (and yes, I know that I just provided the perfect straight
    line for a joke!)
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