How to get +/-15V from either 9V batteries or similar?

Discussion in 'The Projects Forum' started by midnightblack, Mar 30, 2012.

  1. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    I am working on a microphone preamp and I'm kinda stuck on how to make the power supply. Any help would be appreciated.

    I need +/-15V rails (@ max 15mA each) from a small battery source e.g. 9V battery.

    I have thought to get two 9V in series to get 18V then Voltage Regulator down to 15V but that leaves a negative 15V to deal with. How? :confused:

    I have looked at some switching circuits which if I understand correctly switches fast between + and - to create a virtual +/- supply from a single supply using some ICs. But I don't have the ICs or experience to do such a thing.

    I don't want to use more than 2x9V batteries as I want it to be portable and not so expensive. (Why are 9V batteries so expensive?)

    Is it practical to think of a power supply from the mains? And also was thinking of maybe using the USB port. But the max volts from there is 5V.

    Any ideas? I'm open to any..

    This is basic criteria:

    >Fit in a small box
    >Not expensive to maintain (e.g. so no buying 9V batteries every few days)
    >Will be used between a XLR dynamic mic and a line in to a computer
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Are you sure that +/- 9V won't work? Actually the batteries will quickly fade to about 7v, but even then I suspect your device might work fine?
     
  3. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    I am not sure entirely why but from what I've read so far, I do need +/- 15V supply.

    This is also a general question, not just application specific. For example there is another circuit which needs 48V. How would I power this using batteries?
     
  4. #12

    Expert

    Nov 30, 2010
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    My Shure 5885A might do 4 volts p-p into 1 million ohms if I scream at it, point blank, so I think your voltage range is a bit high and I'm sure you can make a preamp for less than 15 ma. Do that and batteries last a lot longer.

    "Line in" reminds me of 1 volt RMS or 2.8 V p-p (I'm sure somebody here can get that exact spec.) so you don't need to feed the input 25 or 30 volts p-p.

    I attached data for a 22.5V battery but I think I just proved you don't need 2 of them and they are feebler than the 9V batteries in the amp-hour department.

    I think, for your first attempt, you are trying to make an ampilfier that is way more than you need.

    Have you tried just plugging the microphone into the computer?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    +1
    Works fine for my guitar. Mic may have still lower output, but it wouldn't hurt to try. On a Mac, I've found that gain control can be software-limited. So my guitar signal seems anemic in some apps that aren't meant to amplify a guitar. In other words, experiment.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    At currents this low, you could use a resistor and Zener diode off each 9V battery.

    You may be able to use the batteries directly. Devices designed to run off +/-15V will typically tolerate +/-18V.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Most microphone preamps can operate on much less than ±15V. What circuit are you considering that requires such high voltages?

    If you want to operate from mains power just buy one or two wall worts of the desired voltage. Make sure they are the regulated type.
     
  8. bugsy0

    New Member

    Mar 31, 2012
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    +/- 15V is a 30v spread. A 9v battery might use a boost converter to generate the 30v. The circuit consists of a pulse generator (e.g. 555@30khz) and biasing ckts to set up a voltage increase across an inductor aka a "charge pump" or something like that. Search digikey for "boost converter" or "dc-dc converter" etc. Their operating principle is pretty interesting and you'll find lots of cool articles describing the theory.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    I agree, and reiterate my suggestion to just use two 9v batteries for ±7V or so. I'd be very amazed if it didn't work fine. Prove it doesn't before you worry about getting more voltage.
     
  10. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    Yup I've tried plugging it straight to computer. Its a macbook pro, and it barely, barely picks up any signal at all.

    The line in is just 2.8V? I mean thereabouts as you are unsure about exact figure?

    How did you manage to measure the output from your microphone? I connected it to a multimeter and can't see any voltage variation, the voltage is too fast for the multimeter I think. Or too low?

    Anyway- As all have said, I will try with 2x9V batteries and then post back soon as I've built the circuit.
     
  11. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    I didn't fully understand what you meant by this? Is this to get a negative voltage somehow from the same source?
     
  12. t06afre

    AAC Fanatic!

    May 11, 2009
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    Can you please tell us which circuits you plan to use. And perhaps post any relevant information. Like a schematics
     
  13. #12

    Expert

    Nov 30, 2010
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    I rummaged through my old band box, dragged out a Shure dynamic mic, connected it to my oscilloscope on the 1x channel, and screamed at it until I got the voltage range on the scope high enough for me to read the voltage.

    There is a spec on the Shure page I posted. Sensitivity. Seems to say, "1.85 millivolt at 94 db spl". I'm so uncertain that the math shows .52 volts peak to peak at 114 db that I just pulled out a mic and screamed at it. Much easier for lazy people like me.

    I can also see that if you aren't going to scream at your microphone like a rock star, you will need amplification. A normal conversation is about 70 db so you would be at less than 1% of 1.85 millivolts if you're just talking. Still, the computer isn't looking for 25 volts.

    Be sure to use a metal box. Gains like this are impossible to control with power line hum leaking in.

    Here's one I did. Needs about 1 ma. That's 440 hours on a 9V alkaline battery.
     
    Last edited: Apr 1, 2012
  14. wayneh

    Expert

    Sep 9, 2010
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    What software did you use? The system software limits gain, I think, whereas something like Guitar Rig or maybe even Garage Band will give more boost. It still may not be what you want, but worth a shot?
     
  15. wayneh

    Expert

    Sep 9, 2010
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    I believe he was confusing 2 batteries as giving +/- 18V, which might need to be reduced for your circuit. Of course 2 batteries gives only +/- 9V.
     
  16. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    Hey, thanks for the circuit. Its cool how you can create amplifier circuits to meet specification at will. I still am struggling to get to grips with transistors, mosfets and so on.

    But, I don't have many components here (just starting out), I managed to get hold of an INA217 chip from TI and I'm trying to make a preamp around that chip.
     
  17. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    Yes ofcourse, I should've done this at the first post actually. I've attached it:

    I'm building the circuit without the phantom power.
     
  18. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    Thanks for this tip, I wouldn't have thought to use one.
     
  19. midnightblack

    Thread Starter Member

    Feb 29, 2012
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    I tried garageband, If I scream, with the software amplification all the way up, it registers slightly which you can hear if you turn your speakers all the way up. Its not good enough for my application :/. Just out of interest, are there any plugins etc that can amplify further?
     
  20. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Here's the simple way: make a simple 9V to 30V boost converter using any one of a number of IC boost chips. You could stack two 15V zeners across the 30V and use the center point as circuit ground (the battery and boost circuit would be floating). That gives you +/-15V with respect to the center ground point.

    Here's an example of a 9V to 25V boost I designed a while back to run off a 9V battery. You can easily adjust resistors to make it 30V out.
     
    Last edited: Apr 2, 2012
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