Basic pre-amp for DMX controller

Discussion in 'The Projects Forum' started by crazyman, Jul 3, 2011.

  1. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    Hi,
    I may be asking questions in he wrong forum so please direct me in the right way thanks.
    Here is my problem: I have a DMX controller which is sound activated unfortunately the inbuilt mic is next to useless and has no control over the sensitivity. It does however have a audio input (RCA) which is labeled 0.01/1 VP-P. I have used this to connect to rca line out from mixer. I also put a potentiometer in line to adjust the sensitivity. This works sortof ok. But I have better ideas[​IMG] I have a diagram which hopefully explains what i would like to achieve
    [​IMG]
    From what I have learnt the mini mic does not provide enough volts so a pre amp is required. (Note: Audio quality is not important! I hope this makes it a much easier to build.) I then want to adjust the sensitivity of the mic signal depending on the music volume. Also I would like to control what frequencies are let through. Eg I want to have the drums activate the DMX lights so high-cut strings etc, but low-cut the deep bass. Hope that makes sense!
    I was hoping I could get a maybe a schematic diagram and parts list for this little project from you experienced people and of course some good advice or ideas.
    I am a newbie at this so if If have not provided enough info, not clear or something else please let me know.
    Thanks for reading through anyway.
    Dane
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I've done something similar(sound activated input/music program) and was successful by using a graphic EQ coming from a soundboard output. By cutting bass and boosting treble, or vice versa, the lights would react to that portion of the music. If you have access to one(a friend maybe) you could try it and see it works in your application.
     
  3. #12

    Expert

    Nov 30, 2010
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    Start by choosing a microphone. The crux of the biscuit is in adapting what you have to get you where you want to go.

    Once the mic is defined, changing the frequency response and amplitude is a minor detail. Right now, I'm thinking in terms of an op-amp because putting frequency modifying feedback loops on it is, as I said, a minor detail. Are you prepared to build an amplifier stage with the DC voltage supplies and such as that?
     
  4. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    Kermit2, I have no doubt that what you suggest would work, unfortunatly I am trying to simplify/minimise equipment. I would like to make something the size of a mobile phone which I can mount inside my DMX controller with 3 dials (bass,treble,Volume) and a mini mic sticking out the back. I cant always get a feed from the music player :( so i need the independant microphone and controls.
    Dane
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Your want requires WORK on your part. Are prepared to do the work AND spend money to accomplish this?
     
  6. #12

    Expert

    Nov 30, 2010
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    It's doable. Name a microphone.

    It would be very helpful if you provided a spec sheet on the mic, too.
     
  7. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    I have two of these mics http://www.jaycar.com.au/productView.asp?ID=AM4011&keywords=microphone&form=KEYWORD
    "prepared to build":eek: yes definatly, capable....will see:confused:
    I knew there had to be voltage added into the circuit somewhere, be it a battery or connected to the existing controllers power which would be much better but probably more difficult.
    I have done a bit of soldering and as long as it doesnt get overly complicated I reckon I can do it.
     
    Last edited: Jul 3, 2011
  8. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    Work...I would like to look at it as an interesting learning exercise, no challange! Are we looking at under $50 for parts?
     
  9. #12

    Expert

    Nov 30, 2010
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    page 6, figure 3.3 is enough information to get this started. Some of their circuit is not needed.

    Take a peek and we'll start negotiating about what voltages you have available, etc.
     
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  10. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    Great! So I would be starting with the first 2 stages (Electric mic, Amplifier)
    The other stages in that diagram is not needed if I understand correctly because all I would be looking for is an amplified signal/voltage. The controller already does the rest of the work??
     
    Last edited: Jul 4, 2011
  11. #12

    Expert

    Nov 30, 2010
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    There are a few things to change.
    First, take out D1. The original circuit is a peak detector. You want an audio amplifier.
    Replace D1 with a capacitor calculated to cut low frequencies
    Second, ask again if you can provide higher voltages, like +/- 12 or 15 volts.
    Third change to a modern amplifier chip, like a TL072.
    4th, eliminate R4
    5th calculate R2 and C1 to cut off the lowest frequencies.
    6th turn R7 into a volume control.
    7th eliminate C2
    8Th duplicate R5 and R6 on the second stage.
    9th Add a capacitor across R5 to cut high frequencies.
    or do you need full functioning tone controls?
    Can you narrow down the frequency cut offs?
     
  12. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    #12, At this stage it looks like I have only +9v@2A to play with if I use the existing power source. I would like to avoid getting another transformer. How critical is the extra voltage? Is it going to complicate the circuit a lot?
    I want to have adjustable frequency cut-off controls. However some basic passive bass and treble pots should work. like this [​IMG]
    Will that work ok?
     
  13. #12

    Expert

    Nov 30, 2010
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    I like it! This tone control can go between stage one and stage two. (You're being very helpful!)

    The extra voltage is a convenience. It makes clipping less of a problem. The TL07x can only make 4 or 5 volts peak to peak with a 9 volt supply (page 5). The 324 can maybe do 6.5 V p-p. I feel sure that another opamp can be found that will do better, but not by a lot.

    I am not sure how much voltage the mic can make, therefore I do not know how much gain must be applied. Fortunately, changing the gain of an opamp is as simple as changing one resistor. Perhaps a 100k pot for R5. After you find the right setting, it can be replaced with a standard resistor.

    I am sure that the first stage will be needed to buffer the mic and drive the tone stage. In this case, the second stage is called a "tone recovery" stage. (All passive tone controls cost voltage of the signal.) You will have to build the circuit and find out how much gain is required to boost the mic signal in the conditions you will be experiencing, but not go into clipping. Whatever that is, will go into the tone circuit and the output of the tone circuit will go to the second stage of opamp gain. You can add a volume control after the second gain stage or you can use a pot in the R5 position of the second stage as a volume control. (I've done it this way, it works.)

    Are you ready to begin assembling?
     
  14. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    Sorry I haven't posted a reply for a while, not because I didnt want to, but was giving myself a crash course in electronics:eek: amounst other things. Anyway the result of this is I know more but really nowhere near enough to competantly design the circuit.:( Thats where you guys come to the party and back me up:D Thanks
    Ok I have read some other post regarding guitar amp/tone circuits (http://forum.allaboutcircuits.com/showthread.php?t=31235) and have drawn/copied a wiring diagram from that thread that apparently worked. I figured starting with one that worked would be a good idea.
    [​IMG]
    Here is the data sheet for mic http://i585.photobucket.com/albums/ss298/Daneo_photos/Audio/AM4011.jpg I have the feeling testing in real life will be the only way to see if it will work?
    R15,R16,C8 were put in for extra gain. Do you think I will need it?
    Where should I put the main volume control?
    I could use a trimpot to set the level without clipping instead of replacing resistor?
    Please feel free to give me some criticism, preferable constructive! haha
    Dane
     
  15. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Good morning. (Please excuse the grumpy until the coffee kicks in.)
    First impression: #$@* you're good! 'specially for a beginner :)
    Second look...
    You didn't give the mic any DC current. There's an amplifier inside the mic. You have to put it back the way it was in the TA4 drawing.
    The tone control is capacitively coupled and single ended. You have it DC coupled to the first opamp and too many connections from the first amp to the second. Watchu doin' Willis? Did you find a feedback loop tone control and not tell me?

    R15,16,C8 are the gain setting parts, but they aren't causing extra gain. They are stopping the gain from being infinity. Without negative feedback for gain control, an opamp just maxes out.

    Last...you can turn R11 into a volume control.
     
  16. crazyman

    Thread Starter New Member

    Jul 3, 2011
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    The more I look into it the more I see how little I know!:(
    Thanks for hanging in there for me:)
    Ok now I am bringing it right back to the basics (for me anyway) and if it works I can always add to the circuit. Yep just a basic mic preamp. Circuit and parts list attached. Looking at going ahead an buying the parts....but thats not that simple for a first timer either.
    Firstly cant find a 470K pot, can I use 500k pot. (or look harder!)
    Secondly, is there preference to type of caps?
    Thirdly. How important are the tolerances (eg gold=5%) will 20% be ok?
    Appreciated your help
    Dane
     
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