Combining electrect mike output

Discussion in 'The Projects Forum' started by izon, May 13, 2016.

  1. izon

    Thread Starter Member

    Mar 17, 2013
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  2. #12

    Expert

    Nov 30, 2010
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    Put a resistor in series with each one and connect the other ends of the resistors together. If there is a DC voltage on the output, add a capacitor to each output to keep the DC levels from fighting each other.
     
  3. izon

    Thread Starter Member

    Mar 17, 2013
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    What size resistor are you thinking? Around 10K ? Suppose that is going to reduce the output level also?
    Is there a value that would drop the level very little yet provide good isolation?
    Thanks very much.
     
  4. #12

    Expert

    Nov 30, 2010
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    You have given me no information about what kind of circuit you will be connecting to or the impedance of the microphones. If it's a TL071 op-amp you want to use, you might use as high as 1 million ohms. If it's a one transistor amplifier, the resistance must be less, possibly only a few thousand ohms. I can't just guess why you want a passive connection and what it's going to connect to, and then calculate the proper resistance.
     
  5. izon

    Thread Starter Member

    Mar 17, 2013
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    Well I may be using all 4 of these for picking up sound (singing and verbal responses) in a church for recording. They would be placed two on a side spaced to get a general coverage. Currently two standard magnetic mikes on stands are placed at the front but the output is too weak on recordings.
    So the mikes I'm talking about using would be more sensitive and placed much closer to the sound sources
    thereby resulting in a stronger level in the recording.

    The output of these mikes would connect to the mike input of a Bogen PA amplifier. It's output goes to the recording unit. I'm trying to use the least expensive way to get acceptable results so don't want to invest into a multiple input mixer if I can manage with a lower cost way of accomplishing this. Sorry the specs on these mikes don't give the output impedance.
     
  6. #12

    Expert

    Nov 30, 2010
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    I see some difficulties here. I mic'ed a church choir with 2 mics and the recordings were so good you could listen to them through headphones and know where the preacher was as he walked around the stage. More open microphones cause more noise, and what you want is good signal to noise ratio. Can I interest you in a summing junction? You can attach as many mics as you want to the input by adding one resistor per mic and use the feedback resistor to get some voltage gain (if necessary). In this case, a TL-071 amplifier chip will allow input resistors in the range of 1K to 100k and be very good about noise. I'm dead sure you can find a good resistor value for the mics in that range, still get some voltage gain, and have a low noise mixer for about $3 worth of parts. (Needs a power supply.) You could run this with a 12 volt wall wart, or maybe the DC supply the mics need.

    Or you can just put a 10k resistor in series with each mic and plug them into the Bogen PA.
     
  7. Sensacell

    Well-Known Member

    Jun 19, 2012
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    I think you will find adding more microphones provides diminishing returns, the noise increases as fast as the signal - not much overall improvement.
    The pros seldom use more than one mic. (the exception being multi-track recording)

    Try some simple tests with a mixing board before venturing to build something fancy.

    It's more about getting one good mic located in the right spot.
     
  8. izon

    Thread Starter Member

    Mar 17, 2013
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    Thanks for the ideas, fellas. Since you can't know how many people will attend on a given Sunday or where they will be sitting, the idea of a good place for just one mike is not practical.

    The point is to have a few mikes placed in the several possible places and that will put them close enough to wherever the most sound will be. This automatically reduces the signal to noise ratio by being able to keep the gain down that otherwise is needed to overcome the longer distance where the mikes were usually placed.

    Just for the record, these particular mikes already have a built in preamp so the output signal is relatively strong.
    This mike is also available with attached RCA connector for the signal and a coaxial power connector for the supply.
    If not mistaken, I've used this type in a couple other projects and the RCA connector went right into the aux. input on a regular amplifier with good results. I've chosen this mike for its sensitivity and tiny size. The size will make it inconspicuous and possible to direct wire with no bulky connectors. Anyone care to comment on whether I could use CAT cable hoping the twisted wires will handle any possible "noise" induced that a shielded mike cable would normally take care of.

    I hope to have one mike on the left wall and one on the right and then "sum" these two. I would do the same with two others at the front of the.church for times when children sing or have a program. So I would have two mikes summed for the two areas and then those inserted to two separate inputs of the main amp at aux level.

    Your thoughts......? Thanks.
     
  9. #12

    Expert

    Nov 30, 2010
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    A low impedance on either end of the cable will keep noise down. That's why low impedance mics (600 ohms) were invented.
    I think the twisted pairs in an ethernet cable should work with powered mics.
    The last time I saw a drawing of this kind of mic, the impedance was 2200 ohms. That suggests you could try 2200 ohm resistors to isolate them from each other.
     
  10. izon

    Thread Starter Member

    Mar 17, 2013
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    Thanks again, I will have to find some time to do some experimenting... If I get a chance
    I will try to respond with results found.
    Best regards
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, it doesn't. If the noise at each mic is uncorrelated or incoherent (truly random), then it sums as the square root of the number of microphones, while the correlated audio signal sums almost linearly. Even with the mics all picking up the same room noise, their different locations introduce enough variation for there to be some benefit. Of course, if the speaker is very near only one mic and the other three are picking up mostly noise, than any statistical benefit is lost.

    ak
     
  12. Sensacell

    Well-Known Member

    Jun 19, 2012
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    At one time in my career, I had to engineer a system of multiple video cameras, microphones and motion detectors that would produce an "automated TV show" from any activity within the 2000 square foot space. (90's DOTCOM madness...)

    We had all the audio going into a matrix switcher and mixer / EQ.

    We experimented with many different concepts of combining audio, in the end, the single microphone nearest to the action was always the best, Whenever we tried to add more mics, the sound was notably noisier and speech recognition diminished.
     
  13. izon

    Thread Starter Member

    Mar 17, 2013
    163
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    So am I supposed to wait until everyone is seated, then run over to the area with the
    most people and hold the mike there or set it on a stand in the middle of the group.
    Besides that, people become mic shy and if they see one near them they tend to speak
    and sing softer or not at all for fear they will hear themselves or others will not like
    their voice.

    It just isn't practical. If it is such a bad idea to have 2 or more mics, why do I see
    professional installations, for example, for a stage where a choir is singing or
    a group of actors are performing, that they use these overhead suspended mics.
    Often there will be at least 4 to 6 mics. If one mic was the best why do the
    professionals do it differently?
     
  14. Sensacell

    Well-Known Member

    Jun 19, 2012
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    No magic bullets here...

    Getting good sound in that situation is difficult.
     
  15. #12

    Expert

    Nov 30, 2010
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    I am a professional. I used 2 mics to create a stereo recording in a church. I used several mics for performance groups where each person needed to talk or sing at close range to a mic. The, "other" pros with 6 or 8 mics hanging above a stage used a proper mixing board.
     
    Last edited: May 18, 2016
  16. izon

    Thread Starter Member

    Mar 17, 2013
    163
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    I've worked with several church sound systems. All were very happy with the results I was able to achieve except this current issue was always a stickler. I've successfully used Sound Craft professional mixers.
    The results you get when trying to pick up the congregation singing and responses recorded with video onto a DVD can vary greatly depending on the type of service. Those where the members sing loudly and are flamboyant in their praise and worship come through loud and clear. However congregations where singing is subdued and verbal responses are soft and reverent are still going to be difficult to pick up.

    No mixer can compensate for an input level that is too weak. You can only raise the gain to max and if it is still weak.... then what. I've tried several types of mics but even the best was still not strong enough. Complaints come when folks are viewing a DVD of the service and hear the pastor and other speakers loud and clear but when it goes to a hymn being sung or the liturgy being read.... you can barely hear anything.

    At one location I ended up using a "shotgun" mic with fair results but yet not the greatest.

    On my last job I added an RDL (Radio Design Labs) device to boost the sound before the mixer and this channel ONLY was directed only to the recording unit and not out through the regular sanctuary speakers This worked fine since what goes onto the recording is most important. Playback was much better and everyone was satisfied.

    Since I'm dealing with small churches, they do not have the resources to put in the ultimate sound processing equipment so one has to come up with the most economical alternative that will still leave them pleased with the results.

    I do thank you for your input and explanation of how things should be done. It is just a difficult scenario and hard to solve simply.
    Your professionalism is very much appreciated... so thanks again for your help.
     
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