Need help making a summing amp

Discussion in 'The Projects Forum' started by dumle29, Sep 10, 2011.

  1. dumle29

    Thread Starter Member

    Jun 26, 2011
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    okay so i need to make a little box that can mix a mic signal with a line-in signal, and throw it out to a headset.

    So i have looked asked around and have come this far:

    • This mic
    • Two summing amplifiers (stereo)
    • (Possibly some buffing amps)
    • And then two pot´s, proberbly 10k as voltage divider volume controlls.

    now im having a hard time figureing out these things:

    • Should i add a pot for gain?
    • What resistor´s should i use?
    • would a 5v supply work (USB)?
    • Can i use two L165 op-amps?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    1. What is the impedance & power of the headset?
    2. What is ur power source?
    3. Or what kind of power is feasible for the application, meaning is USB 5V easier for you or to use a separate adapter ?

    {ed}
    Ur mic needs more than 5V, so I presume a separate adapter is in order
     
    Last edited: Sep 10, 2011
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You could use an ICL7660 or 7660S or similar capacitive boost IC powered from a USB port to supply your mic. They are good for 10mA current.

    As far as mixing/volume control, you could feed the end of 10k audio taper pots, take the signal level from the wiper, feed it through buffer amps, then mix the two using 10k or 20k resistors, and feed that output to a 2:1 gain opamp. Not very complicated.

    If you really needed adjustable output gain, I suppose you could add a pot in the 2:1 feedback path.
     
  4. dumle29

    Thread Starter Member

    Jun 26, 2011
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    okay, so thanks for the lightning quick responses, just saw that with the mic too :/ guess im going for external supply.

    i have no idea of how to find the impedience of my headset, though messured with an ohm meter the resistance is 36Ω

    soo, if im fine to leave gain alone (no pot) ill go for that :)

    i have a hard time finding a stereo log pot around, couldnt i use a lin?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Of course u can use lin pots. Just measure the resistance of ur head phone or show it to us
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Linear pots won't work very well.
    Dind't know you wanted to drive headphones. That'll require more than an opamp.
     
  7. dumle29

    Thread Starter Member

    Jun 26, 2011
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    hmm okay now im confused? i have no experince in audio stuff. I was told that i should use a op-amp (summing)

    the resistance is as before said, 36 ohm messure with an ohm meter

    its a sony xb700 headset
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U need an op amp as a summing amp and a power amp to drive the headset
     
  9. dumle29

    Thread Starter Member

    Jun 26, 2011
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    maybe i could use a cmoy after the summer?
     
  10. yourownfree

    Active Member

    Jul 16, 2008
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    an LM386 is powerful enough to drive the headphones and be all you need. All you do is sum the input to the op amp, the LM386. The output is mono of course so tie the left and right headphone line together and connect to the output of the op amp. The op amp amp has a gain of 200 if you need that much. The diagram that comes with the op amp or data sheet on the device is all you need for the diagram. here is the url for the data sheet. http://www.national.com/profile/snip.cgi/openDS=LM386
    here is an op am mixer. you could use this circuit and add the lm386 to boost the power or just put the lm386 in place of U1 and call it good. http://www.aaroncake.net/circuits/mixer2.asp
     
    Last edited: Sep 11, 2011
  11. dumle29

    Thread Starter Member

    Jun 26, 2011
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    what about the cmoy? looks really simple, now just for the summing amp. what about a LM 358 dual channel op-amp?

    an mono is a no go, ill tell you why.

    im going to a LAN 20 days from now or so, and when i play one my ps3, i have no volume controll and cant hear whats going on around me, so im gonna need this box for both.

    i will need stereo since im gonna use it for gaming :)
     
  12. dumle29

    Thread Starter Member

    Jun 26, 2011
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    what about this design (know its confusing as hell, but i dont know how to make it simpler)

    [​IMG]
     
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you want to make circuits without lots of mistakes, you need to learn to draw them less confusingly - the effort will be well worthwhile in the end.

    Your circuit appears to be meant to have unity gain both for the microphone and for the line. I would doubt that this is nearly enough, especially for the microphone unless this has a very substantial internal amplifier. Note also that there is no provision for power to the mic.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    What kind of mic are you using? If it's an electret mic, you'll need to power and amplify it as Adjuster implies.

    Here's a power source and amplifier for an electret mic:
    [​IMG]
     
  15. dumle29

    Thread Starter Member

    Jun 26, 2011
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    okay, the mic has a built in amp. it uses a 6-12 volt supply, plugged into a barrel connetor, (forgot that)

    so i made a block diagram thing

    [​IMG]
     
  16. vrainom

    Member

    Sep 8, 2011
    109
    19
    As an output you could use two lm386 (one for each channel) it is way more powerful than needed to drive headphones. Trust me, I use one of these as a personal musical instrument amplifier.

    Here's a pretty common configuration as seen in http://www.minidisc.org/headbanger.html

    [​IMG]
     
  17. dumle29

    Thread Starter Member

    Jun 26, 2011
    45
    0
    thanks, i think ill go for that as the amp after the summing´s.

    just a question more. how would i go about making the summings? i have the basic layout figured out. Im however not sure how to connect the mic out to both summings. Is it okay to connect it like this:

    line in L---Resistor------------ summing
    ............................./
    mic ------Resistor---
    .............................\
    line in R---Resistor------------ summing

    wouldnt there be a problem with connecting the two summing´s?
     
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