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Combining multiple amplified outputs to feed one speaker only

Discussion in 'General Electronics Discussion' started by pb2525, Feb 28, 2015.

  1. pb2525

    pb2525

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    Feb 28, 2015
    Hi

    I am trying to combine four amplified (about 3W RMS into 8 Ohms) outputs to feed one speaker only. This is not a HiFi setup, one voice and 3 beeping inputs. I have found some schematics, but they generally deal with line level sources. This is not possible in this case as I cannot get to the line level signals. I am looking for the simplest unpowered solution if it exists. One of the most important considerations is not to damage the originating equipment.

    Any help greatly appreciated.

    Paul
     
  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Perhaps you could passively sum the four 8 Ω outputs by connecting each to a 27 Ω, 5 W, resistor. Connect the other end of each of the four resistors together to an 8 Ω speaker. The original output signals will each be attenuated by a factor of about 0.12 so the gain of each amplified channel would have to be increased to produce the same loudness as a single channel driving a single speaker.

    Alternatively, four 2:1 step-down audio transformers could be connected, one to each source, and the secondaries connected in series to drive an 8 Ω speaker. The 2:1 turns ratio provides a 4:1 impedance transformation ratio, so each 8 Ω source appears as a 2 Ω source on the secondary, and four of these in series appear as an 8 Ω source to the speaker.
     
  3. pb2525

    pb2525

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    Feb 28, 2015
    Thank you hevans1944

    Since I have posted my query I have came across this post post on the sparkelectronic forum
    "My guess is that the speaker output - terminal is not ground, but is driven to some AC voltage out of phase with the + terminal. It's how most automotive amps (and other low $ items) work to maximize power out, given a single +DC voltage supply like you'd have with a battery (vs a + and - DC voltage supply that would be made from the mains AC line) . The speaker + and - outputs should not be connected to anything other than the speaker itself."
    I suspect that the the avionics radio I have has an amplifier as described in the quote, They say not to connect the speaker -ve to a common ground. So I suspect I better not pursue this project. Dual voice coil speaker is still a possibility though.

    Cheers

    Paul
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    You didn't mention in your original post that this was an avionics application! I wouldn't touch that with the proverbial ten-foot pole. Only a qualified, perhaps licensed, avionics technician should work on avionics (aviation electronics). It's not like you can pull an airplane in flight over to curb to work on it if some avionic component fails. So, regarding my two proposed solutions to your problem: Never mind!
     
  5. pb2525

    pb2525

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    Feb 28, 2015
    Hi

    It is not as bad as that. The installation is in a glider and involves combining the audio output of two varios, and the radio. If you lose the vario sound, it may be inconvenient, but no threat to flight safety, similarly with the radio. I do not fly in a congested airspace, so no problem either.Besides, one can always land in a field.

    Thanks for the thought though.

    Cheers

    Paul
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Well, they're your radio and your acoustic variometers aren't they? Gliding is sooo personal! Have you ever been to Albuquerque NM to see them hang gliding off a Sandia mountain top? There is a aerial tram that takes the gliders to the top from whence they jump off and ride the thermals coming up from the desert below. Some stay up for hours as the sun goes down. There is also a huge hot-air balloon international festival in the fall there every year, lift off occurring in the morning on several successive days, weather permitting.

    The transformer idea would probably work since the primaries will "look like" speakers. It is probably not too important to match impedances exactly, but if you use four 2:1 step-down transformers the numbers work out right. Unused channels could have an 8 Ω resistor connected to that transformer's primary in lieu of connection to the vario's output.

    I know you would like to have an unpowered solution, but this may not be practical. Since you obviously have access to the speakers, you could use three or four unity-gain differential amplifier circuits to isolate and buffer their outputs; then sum the buffered outputs in an op-amp adder that drives your output speaker or earphones. The whole rig would occupy about as much space as a match box, or with a 9V battery for power, an Altoids tin. Power consumption should be a few milliwatts into earphones of reasonable quality.

    Gliders are pretty quiet, so I doubt you need three watts of power in your ears. Still, that is doable too, but you may need a larger battery, depending on how long you stay aloft. Are you interested in pursuing this "powered" approach? Yeah, I know weight is important with gliders, but no one flies naked... or do they? What type of glider do you fly?
     
  7. pb2525

    pb2525

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    Feb 28, 2015
    Thanks, I do not fly hang gliders or paraglider, which I assume you are referring to. I fly the gliders that looks like conventional aeroplanes with long wings. So, I do have power (12V) and small amount of weight is not that much of a problem. What I wanted to do was to reduce complexity, by reducing the number of speakers required and I did not want to have a powered solution for that reason. Whilst I have some rudimentary understanding of circuits(high school long time ago), clearly it is not good enough to figure out this, hence my request for help, as a lot of searching found precious little about this and what I found generally said do not do it :(.
    Anyway, since my last post I have found out that the amplifier in my radio puts some potential on the ground terminal, apparently this is done in some amplifiers to increase efficiency and hence it cannot be connected to anything but the speaker.
    So I have decided to cut my losses and go with the conventional solution.

    Once again, thank you for your assistance.

    Paul
     
  8. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    You could try 10R in line with a diode from each of the tones and connect the speaker directly to the audio amp.
     
  9. pb2525

    pb2525

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    Feb 28, 2015
    Hi Colin

    I am not particularly versed in circuit design (read - do not have a clue), so I would need a bit more info. I assume the 10R means a resistor of 10 Ohms. I also assume that the diode would be there to allow the current to flow in one direction only. So, are you saying to connect the radio directly to the speaker and then connect the resistor and the diode in series on the negative line of each of the other outputs?

    Cheers

    Paul
     
  10. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    I would connect all the 0v lines together and put the 10R and diode on the "active"or "signal" line.
     
  11. BobK

    BobK

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    Jan 5, 2010
    Colin:

    If you had read all of the thread, you would know that the output of the radio is bridged and there is therefore no common.

    Bob
     
  12. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Put a 1u (non-polar) on each of the tone lines and connect them across the speaker.
     
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