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Convert stereo to mono

Discussion in 'Electronic Basics' started by amdx, Sep 18, 2011.

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  1. amdx

    amdx Guest

    I have a new internet radio that has a a stereo headphone jack.
    I want to drive a single speaker with both channels.

    Will I cause harm to the amplifiers if I just tie the outputs together
    and use the that to drive the speaker?


    Do I need to add some resistors to isolate the amps from each other
    before the speaker?

    Thanks, Mikek

    Hmm. I'm thinking headphone jacks often have series resistors to
    limit headphone volume, is this right? The problem could ready be solved.
  2. amdx

    amdx Guest

    I used 23 ohm resistors on each channel, but the volume is to low.
    I shorted them to check the level, still to low.
    I wanted to drive a pillow speaker from the headphone jack. The radio
    has a line out and a headphone jack, but unless I open the radio and
    modify the wiring to the internal speaker, I don't think I'll have
    the power I need for a pillow speaker. The internal speaker has good
    sound and volume.
    I have the Tangent Quattro MKII internet radio. I've only had it a few
    days but it is pretty neat to have 10,000 radio stations available with
    no static.
  3. Kaz Kylheku

    Kaz Kylheku Guest

    What is the impedance of the speaker? Headphones range from around 30
    ohms to around 600 ohms, but loudspeakers can be way less. Is there a
    published minimum impedance that can be driven by this device's
    headphone jack?
    It could.

    If the amplifiers are playing nearly the same signal, there won't be an
    issue because the output transistors will be at the same voltage. This
    situation of tracking almost exactly the same voltage means that the
    left output looks like a high impedance to the right one, and vice

    The correct stress test for this is: what if the left and right signal
    is quite different? For instance, complete silence in one channel and
    loud music in the other. In this situation, the output device with
    signal swings sees the silent one as a low impedance. This is like
    asking it to drive a near-zero-ohm speaker. (Basic electronics: a
    voltage source is a low impedance, and a perfect voltage source
    is a zero impedance!)

    So, in essence the common mode signal should mix to the speaker nicely,
    but any differential signal faces a short circuit between the two
    different voltages.

    Even if this doesn't damage the output device, are you okay not hearing
    much of the differential signal out of the speaker?
    It's not a bad idea just from the consideration of the possible low
    impedance of the speaker compared to headphones.

    Both outputs should be separated from each other by at least the minimum
    required load resistance so that means idnvidually coupling them to the
    speaker through resistors which are at least half that resistance.

    But remember since R will shunt any differential signal, but not common
    mode signal, your differential signal is attenuated. The common mode
    signal goes through R and the speaker, but the differential signal goes
    through R, and then another R in parallel with the speaker.
    This will work best if R is significantly larger than the speaker load,
    in which case you take a lot of loss of volume across R.

    The best approach is to feed the channels to another device which can
    properly mix them, and re-amplify to a single speaker.
  4. Rich Grise

    Rich Grise Guest

    Why not get two pillow speakers, and just set them next to each other?

    Good Luck!
  5. Kaz Kylheku

    Kaz Kylheku Guest

    The output clearly has no power to drive these speakers. The power
    dissipation may be there on paper, but what you need is efficiency: to
    move air. That speaker is not converting enough of the electrical energy
    into sound decibels. Headphones also don't, but they are right next
    to your ear!
    That's silly; why don't you get two pillow speakers and have stereo.

    Don't there exist powered pillow speakers?

    There are all kinds of power speakers that are form-factored to work off
    1/8" line out jacks.

    Don't you have some old wall-wart-powered computer speakers lying
    around? The might not fit under a pillow very well, but some
    positioning arrangement could be negotiated with them.
    It's very likely that there is a separate amplifier for that speaker.

    Also, according to the specs on the website, the speaker is a neodymium
    driver. A stronger magnet translates to a more sensitive speaker.
    No static? Sure, if you don't count compression artifacts as "static".

    Most sites will not provide quality, high bit rate streams, unless you pay for a
    subscription, or you are willing to listen to tons of embedded ads.
  6. Jasen Betts

    Jasen Betts Guest

    sometimes. Sometimes they take feedback from the output terminal
    and if you tie the two channels together you'll get distortion
    do you feel lucky.
  7. amdx

    amdx Guest

    Nope, I wasn't lucky, not enough power to drive an 8 ohm speaker
    at (quite low) pillow speaker level.

    I'll need to open it up and add a switched 1/8" mini plug jack
    to the existing speaker circuit.
    In the process voiding my 2 year parts and labor warranty. :)
    Would be nice to be able to shut the display light off too.
    It is menu adjustable, but even the lowest level is more light
    than I want, during my beauty rest.
  8. amdx

    amdx Guest

    Thanks Rich, but one speaker is enough to deal with, 99% of what
    I listen to at night is talk radio, so a stereo signal is not real
  9. amdx

    amdx Guest

    I'm used to listening to AM stations at night with static and fading
    as a constant, varying with the season. This audio is beautiful,
    to me!
    Regarding ads, so far everything I have programed in are commercial
    broadcast stations, so yes tons of embedded ads, ideal bathroom breaks.
  10. Kaz Kylheku

    Kaz Kylheku Guest

    Okay, let's compare digital with ... AM. :)
  11. Rich Grise

    Rich Grise Guest

    Oh, heck! If it's just talk, then just use one channel and ignore the
    other one - no parts, done. (Use a stereo plug, and just connect up
    one of the active lines. The other one will have no load, so draw only
    standby current.)

  12. Rich Grise

    Rich Grise Guest

    Masking tape? ;-)

    Have Fun!
  13. amdx

    amdx Guest

    I made a cardboard flap with a magnet embedded between layers.
    The magnet holds the flap to the speaker grill on the top of the radio,
    and it hangs down over the display. I can lift and slide it a bit when I
    want use the display. Not the the first one I've made, did this for my
    Icom R-71A also. Hmm.. also a Chrysler car radio, is it silly to have
    three radios at your bedside? :) Don't ask my wife!
    I have now installed a (switched) speaker output jack, on the internet
    radio (Quattro Tangent) and all is good.
    I see a lack of motivation to build AM Band antennas in my future :-(
  14. Rich Grise

    Rich Grise Guest

    Tune one to Myron Floren music, the second one to Dick Contino music,
    and the third to Florian ZaBach music; play them simultaneously while
    making love. =:-O

    Have Fun!
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