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self powered speakers

Discussion in 'Electronic Design' started by H.Johnston, Feb 27, 2010.

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  1. H.Johnston

    H.Johnston Guest

    I am trying to decide what wire size to use for 10 parallel
    connections of an array of self powered speakers each with an
    amplifier incorporated in it.
    ..
    They are 5 mounted on each parallel line making a total of 50
    speakers.
    I measured the input resistance of the speaker and it is about 1Kohm .
    Power use is 25 W and I have a 50Amp 48Vdc power source to give power
    to the speakers.

    Any suggestions on how to improve design , calculate wire size.

    The total lengths of the wiring provoding is 70meters.

    Thanks all for the help.

    H.Johnston
     
  2. pimpom

    pimpom Guest

    H.Johnston wrote:

    I hope you won't take this amiss, but I thought it would be best
    to first clarify what you wrote.
    Are you talking about the power connection, the audio signal or
    both?
    Are the amplifier-speakers designed to be driven from a
    microphone or from the line level output of a preamp/mixer? 1K is
    quite low for the line-level input of a power amplifier. It's
    more in the range of an input for a low impedance mike. Or did
    you measure the power supply input?

    Sorry if you know your stuff about these things and what you said
    are what they appear to be. But I thought it would be best to
    make sure.
    Is 25W the power consumed from the power supply or is it the
    output power rating?
    Are the amplifiers specced to be powered from an external 48Vdc
    source?
    More details about how the speakers are to be located with
    respect to each other will also help.
     
  3. H.Johnston

    H.Johnston Guest

    Well the 1 K is for the transformer which connects to the speaker
    inputs. You can change the transformer settings though to vary the
    output audio power.
    Consumed by speaker and I would guess the amplifier unit though the
    data sheet does give sufficient detail on that.
    Yes the 50A source mentioned above
    Thanks.

    H.Johnston
     
  4. Are you sure that you are talking about active speakers and not about
    remote speakers intended for 70 V or 100 V line operation ?

    Your descriptions fits quite well to remote loudspeakers as found in
    schools and other large buildings for PA applications.

    These contain an ordinary 4-16 ohm speaker, bit if it is driven by
    long lines directly, most power would be lost in the cabling due to
    the high current. To avoid this, each speaker contains a transformer,
    which is intended to fed from the nominally 100 V audio line. The
    speaker unit usually contains a step attenuator implemented by
    selecting the appropriate winding tap from the transformer.

    A 1000 ohm impedance on the 100 V line would draw 0,1 A, thus the
    speaker could deliver up to 10 W. Thus for 50 units, a single power
    amplifier would be required with 500 W output power. If the current
    really is 0,1 A for each speaker, the total current would be 5 A,
    possibly distributed along several branches, so wires less than 1 mm
    thick should be sufficient.
     
  5. H.Johnston

    H.Johnston Guest

    Maybe I wasn't clear enough.
    No I'm familair with the units you're talking about and we've set up a
    lot of those at 70V /100V and as a matter of fact for PA systems they
    are the most used and the literature is full of advice on setting
    those up
    Self powered speakers or active units are not as common in PA , at
    least for me, which is why I was asking . The attenuating unit on the
    output of the inbuilt amplifier is a transformer or inductor is what
    the manufacturer is using to reduce the voltage instead of a resistor.
    The data sheet shows a combination of 6 dip switch settings that are
    set on this unit to vary the output power on the loudspeaker from
    values ranging at 0.76W to 25W. The datasheet gives the speaker
    impedance as 16Ohms, I measured it during audio reproduction and it
    varies from 1 Ohm to 25Ohms depending on frequency.

    The loudspeaker+amp unit have practically speaking 5 wires entering
    into them 2 for the 48v source which powers the internal amplifier
    unit
    2 for the audio signal + 1 wire for the shield .

    The input impedance to the amplifier , which I think is more important
    then the figure I gave earlier as the audio signal passes through
    here, is 11Kohms before getting to the speakers.

    Thanks again.

    Johnston
     
  6. pimpom

    pimpom Guest

    For the input signals, a shielded microphone cable to each
    speaker will do.

    The power supply cable is more involved as we have to work out
    the best way to group, separate and run the wires from the common
    power supply to the speakers. We need to know the distances
    involved as this is crucial to calculating the size of wires
    needed. It's vital to know how far the speakers are spaced from
    each other and from the common power supply. We have to consider
    voltage drops over the wires, not just the current carrying
    capacity.
     
  7. pimpom

    pimpom Guest

    And that is assuming all 50 speakers are bunched close together,
    powered from a common supply 70 meters away. I'm not sure that's
    what the OP is trying to do. The speakers or groups of speakers
    may be scattered over a sizeable area.
     
  8. H.Johnston

    H.Johnston Guest

    Thanks there for the insight on calculating the power cable size
    though one thing I didn't mention was that the amplifiers were class D
    which should give a higher efficiency figure . Well still the exact
    power is still unknown because before the
    signal goes to the amplifier it passes through a mixer and attenuator
    unit (there is one in an audio rack cabinet ... whatever the power
    consumption is here is unknown to me ... I'll see if I can find the
    documentation somewhere. The attenuator units (who knows why its
    called so ) from the documents I got are supposed to amplify the
    signal to sufficient levels as it comes from a microphone so as to get
    to the amplifier + loudspeaker units sufficiently intact. But
    actually what goes on in these units is still unknown to me at the
    moment.

    I was thinking of for the sake of simplicity using a 4 wire cable (2
    twisted pairs, each sized 10AWG if the 500W power consumption
    assumption holds) with a shield for both power and audio.

    H. Johnston
     
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