Need a little advice with audio mixer project

Discussion in 'The Projects Forum' started by Lambda, Feb 28, 2011.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    I just registered here on this forum to ask this question. I have some experience in electronics, just not in audio.
    The situation is this: I am building a circuit that mixes two audio signals. That seems simple, and indeed it is in the big picture. But the devil, as they say, is in the details.
    I have the circuit diagram that I am going to use; it is pictured here.

    [​IMG]

    The first thing that I need some advice on is the output. This is meant to drive a 100k ohm line input to a radio transmitter. Will the output from the op-amp do this, or must I incorporate a separate audio amplifier?

    In regard to the inputs: One input will be from a main "house mix", taken from an unbalanced output from a equalizer. Is there anything that I should be aware of here? Anything that I have to do other than just connect it to R1?

    The other input will be from a headset system, and I have no idea how I'm going to derive an output yet, the system not being designed to output to anything other than the headsets. This is not a big deal, worst comes to worst I'll just use a spare headset and derive the output from that, designed to drive a small speaker.
    But it's the house mix that worries me. Make note that this is NOT an amplified output, i'm taking it before it goes to the power amplifiers.

    And this mixer WILL be free of crosstalk, correct? That's the whole reason why I went with a virtual-earth mixer.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The unbalanced output from the mixer will be in need of a higher input impedance than the headphone output(which is amplified already)

    The use of two seperate preamps before the mixer might be better in this case, each one would need different gain level. The idea is to get the signals into the same 'ballpark' of voltage and current BEFORE you try to mix them.

    Also, crosstalk can be caused by inductive coupling between conductors, so without knowing the layout and shielding you are using it is impossible to verify your expectations on this point.
     
  3. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Thank you for your reply. Do you have any recommendations as for pre-amps? I believe that I have some audio amplifier IC's, would those be the best things to use here?
     
  4. bribri

    Member

    Feb 20, 2011
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    what kind of IC's?
    TL07x is good for audio pre-amps.
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
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    I see that you are using an inverting configuration. Why?
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    What kind of power soruce will you use for the opplication. Will it be a single or dual supply source. This will be important for your selection of opamp. And also your final circuit
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    He said he used the virtual-earth mixer circuit (an inverting opamp) because it avoids crosstalk between the inputs. The headsets might be communication for the stage crew and the "house mix" is the program.
     
    cumesoftware likes this.
  8. lespaul

    Active Member

    Jan 30, 2008
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    Will this be the only mixing device between your inputs and the amps/mains? Just curious, is there another higher end mixing console being used? If so, coming out of an aux. send or group/matrix output can give you better gain control.

    I will take a look at this in a few hours...speaking of mixing, I have to spec a sound system for large corporate event. Back in a few.
     
  9. bertus

    Administrator

    Apr 5, 2008
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  10. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Wow, thank you for all your replies.
    Answering your questions in order here:

    I already have two LM386 audio amplifiers, I thought I might use those.

    For the power source, I was going the use a regulated DC power supply, and to obtain the negative voltage the op-amp requires, I planned on using an ICL7660 voltage converter. I may be wrong here, I have never worked with op-amps or negative voltage.

    Perhaps I should have elaborated. Yes, headsets are communication for stage crew. When I say "house mix", I mean the sound that is fed into the main power amplifiers, what the audience is supposed to hear. It is important for there to be no crosstalk, because I don't think the audience wants to hear stage directions.

    I should have been more specific about this. The output from this is not even going to the main amplifiers. It is going to an old auditory assistance transmitter, which was originally meant to help the hard of hearing to hear the show, but was never set up. They want to give the receivers to the "runners" (who can't wear the wired headsets) and transmit stage directions as well as the show mix (so they know where in the show we are when they are out of the area) directly to them. So the only purpose of this mixer is for the transmitter.
    The transmitter has an input impedance of 100kΩ, and I need to figure out how to drive this.

    I'm taking the house mix from an output on a graphic equalizer in the equipment rack. The show is mixed in the control booth (on a Mackie SR-32, if you must know), fed through an equalizer up there, and then through this one. Why two? I have no idea. I can't take the output from that mixer itself, if I could I would use one of the post-fader aux sends. It is a long way from the transmitter, and I'm not about to run, or buy, a hundred feet of cable.
    The equalizer in question has two outputs on it: a balanced XLR, which goes to the power amplifiers, and an unbalanced 1/4 in TS jack which I will use.
     
  11. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    I didn't see your post at first; I was writing my response. Thank you, that looks like a very informative site and it looks like it's exactly what I'm looking for.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    An LM386 is not a preamp, it is a little power amplifier that is used in cheap toys although some people use it to feed low impedance headphones.
    It has built-in negative feedback so it does not have a virtual earth input for a mixer circuit.
    It has hissss.

    A TL07x opamp is low noise and is used in preamps like you need.
     
  13. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Thanks, I'd never actually used any of the LM386's, I don't remember where i acquired them.
    I'll get myself a couple of those TL07x.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    A TL071 is a single opamp with 1 opamp.
    A TL072 is a dual opamp with 2 opamps.
    A TL074 is a quad opamp with 4 opamps.
     
  15. cumesoftware

    Senior Member

    Apr 27, 2007
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    Silly me. Thanks!
     
  16. lespaul

    Active Member

    Jan 30, 2008
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    Yeah, ok I see now why you are coming out of the EQ for your send/output. Depending on the overall gain needed and location for power, I have used the Rolls mini-mixer in a pinch. You can get one for around $45 ...not as fun as building one, though.

    http://www.rolls.com/product.php?pid=MX42
     
  17. lespaul

    Active Member

    Jan 30, 2008
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  18. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Thanks lespaul, it would have been a lot easier to just buy a device. But I have been given a budget by the department of exactly $0.00. Anything i need to spend on this project would have to come out of my pocket, and I can definitely build it for less than the cost of the Rolls. That, and if i just bought one I wouldn't learn anything more about audio circuits.

    One more thing I have to make sure of: Will the final op-amp have enough power to drive the transmitter's input? The impedance is 100kΩ.
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    Look at the datasheet of your favourite opamp. Most are spec'd to drive a 2k ohm load and a 10k ohm load. A 100k ohm load is almost nothing. It can probably drive up to 50 of them.
     
  20. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    OK then, thank you.
     
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