preamp + phantom power setup

Discussion in 'The Projects Forum' started by MattP, May 16, 2013.

  1. MattP

    Thread Starter Member

    May 21, 2012
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    I'm going to build my own low noise preamp as the prices for ready made stuff is astronomical ($300 would be 'cheap').

    My plan is to build this low noise balanced pre, and power it with a 22v lipo split into two +/- 11v rails (the circuit can work down to 9v).

    The lipo will also be directly powering the microphones (which work on P24), with a protection circuit to prevent the current from hitting the preamp (the 2nd circuit on this page: http://sound.westhost.com/project96.htm).

    I have basically two questions.

    1) will I need to connect the 'virtual ground' of a rail splitter (top one here: http://sound.westhost.com/project43.htm) to the 'gnd' pin?

    2) the phantom power protection circuit is designed for 48v, so will I need to change the resistors to get it working with 22v?

    I apologise if these questions appear really, uh, dumb... but that's probably because they are. ;)
     
    Last edited: May 17, 2013
  2. Wendy

    Moderator

    Mar 24, 2008
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    There is the equalization issue, you need it?
     
  3. MattP

    Thread Starter Member

    May 21, 2012
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    By equalization, do you mean the phase inversion going on to mix the two balanced signals?
     
  4. MattP

    Thread Starter Member

    May 21, 2012
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    Figured out the answers to two of my four questions (edited them out of the first post). Still wondering about the other two that are still there, though.:)
     
  5. patricktoday

    Member

    Feb 12, 2013
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    Hi Matt,

    On (1), definitely. The circuit requires a split supply so it needs a zero volt reference point and that goes to "GND." On (2), for your typical condenser mic, the phantom power needs to actually be 48V. Changing the 6.8K resistors won't help the situation. The internal mic circuitry will complete the circuit your phantom power supply has started eventually passing the supplied current back to your ground; it's expecting a 48V reference followed by a 6.8K resistor. Some mics are spec'd for less but, if not, it probably means you're going to distort the signal or reduce the headroom, degrading sound quality.
     
  6. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Thanks for the help! The mic I'm going to be using is specifically designed to work on both 48V and 24v, both of which have a +/-4v threshold, so should still work with my lipo (which has a charged voltage range of 24v-20v).

    In this case, will I need to change the values of those resistors, or are they okay as they are? :)
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you are using lipo batterries for a power supply, just use 2 ~ 12 volt batteries in series and use the mid point as the gnd.

    The phantom supply is and must be referenced from the same gnd as the mic pre, so you would only have 11-12 volts available. If your mic is spec'd to work down to +/- 4 volts it may be ok. But I would think about using a separate supply for phantom. Otherwise you will be drawing a bit more current from 1/2 the supply which will discharge the batteries unevenly.

    As patricktoday mentioned, don't change the 6.8k resistors.
     
    Last edited: May 18, 2013
  8. MattP

    Thread Starter Member

    May 21, 2012
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    2
    Ah, that throws a spanner in the works. I was hoping to have a single battery for this phantom/pre unit, for charging purposes. Is there any way I can get this thing to run on just one battery?
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, I get it. To use a single battery, I would use the buffered virtual gnd circuit.
    Should be fine current-wise with a single mic preamp. Just need to make sure the mic operates well on the lower phantom
    voltage.

    Oop's I had posted about your mic spec'd down to +/- 4 volts, phantom power is not +/-, just +... sorry for any confusion :)
     
    Last edited: May 18, 2013
  10. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    That's the problem - the mic is only specified for 24v and 48v, and I doubt it will run on 11v.

    How long is a 22v 1000mah battery likely to last powering just one mic? If it can for many hours, then it might be worth just going with two 22v batteries (one with a splitter for the preamp, and the other for phantom power).
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How long depends on the current drawn by the mic and preamp. Normally this would be in the ma's range.

    What model is the Mic?
     
  12. Shagas

    Active Member

    May 13, 2013
    802
    74
    It is possible/practcal to make an inverter fed into a voltage multiplier to generate about 50 volts and then rectify and filter & regulate it to give 48 volts?
     
  13. patricktoday

    Member

    Feb 12, 2013
    157
    42
    The current draw for the mic is very small. If you just take 22V in series with a 6.8K resistor and pretend the mic connects it directly to ground (it won't, more resistance will be added) the maximum current it could possibly use would be 3.23mA, so it will be less than that. If it draws 1mA, you would have 1000 hours.

    These are the two options of taking 2 22V batteries and using a voltage splitter on one battery:

    A) -22V ....................... 0V ... +11V(split) ... +22V

    or here:

    B) -22V ... -11V(split) ... 0V ........................ +22V

    You might be able to use option B and use -22V and 0V as your pre amp rails, -11V as your common ground for both circuits and use +22V as the source supply for phantom power. This would give you 33V as a supply to your phantom power supply.

    However, if you use a intermediate circuit as in figure 2 on the phantom power supply (http://sound.westhost.com/project96.htm), I don't believe that the mic's GND has to be the same GND as the preamp's (the -11V point), because the signals are DC blocked by capacitors.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    Here is a really simple mono preamp I built for a wireless mike to stereo. Equalization is used for many things, such as record players.

    [​IMG]

    The op amp is a NJM6545. R4 was replace with a 100Ω. You do not need a split power supply, as shown.

    Creating a Virtual Power Supply Ground
     
  15. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    I really need a balanced mic pre, Bill, so I'm not sure if that's suitable. I specifically need absolutely the lowest noise levels possible, which is why I can't afford the 6db drop in going from balanced to unbalanced.


    Tubeguy, the mics I'll be using are NT1-A's (because of their super noise floor).

    Shagas, I was thinking the same thing, roughly. Some converters 'float' the ground and voltage rails, making them separate from the input voltage/ground rails. I was wondering whether that would be a potential solution, but I'm not sure about whether it would be suitable to have a totally different ground connected. Anyone have any thoughts on this?

    Would a voltage doubler help at all, to isolate the ground?

    [​IMG]


    So, that means that it should be okay to run the mic off the 22v battery directly, with the pre running off the virtual ground and +/- rails generated by the splitter, right? Those 22uF caps aren't polarised, if that makes a difference.

    Hmm, this is really puzzling (for my brain, at least!).
     
  16. tubeguy

    Well-Known Member

    Nov 3, 2012
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    So, very low noise is the goal.

    An inverter to step up voltage is interesting but I would think noisy. Out of my comfort zone as far as using it for a phantom supply.

    The voltage doubler needs an AC supply, so probably not. :)

    Batteries are great because they provide quiet DC.

    The preamps output has a 100 ohm series resistor and is intended for a split supply in order to provide 0 volts out at idle, referenced to its gnd.
    What are you connecting this preamp to ?
     
    Last edited: May 18, 2013
  17. patricktoday

    Member

    Feb 12, 2013
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    Yeah, it looks that way to me. Here's another phantom power circuit:
    http://www.eetimes.com/design/autom...tom-microphone-power-the-ghost-in-the-machine

    There's no DC connection between the phantom power supply and the signal outputs. The microphone just needs a DC voltage to power itself.

    So you could go:
    0 ...... 11V(split) ...... 22V

    and your rails would be 0 and 22V, preamp ground would be 11V; phantom power circuit would use 0 as the ground and 22V as the positive.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Why do you assume it is not suitable, did you even look up the specs on the chip I names. Doing preamps is one of several of its listed functions.

    The other issue is balanced vs. unbalanced. If you are using a RCA phone input it is not balanced, most preamps and audio stuff isn't. You are making some major assumptions there.

    Most op amps have fair rejections specs, the real bug a boo is the cross over distortion, which is why the model number of the chip is important.

    As far as the output goes, for the rest of the world it is direct coupled, and swings ± directly (no coupling capacitor needed).

    But that is OK, I can suggest solutions, you are free to reject them, it just seems you did not put much thought into it.
     
  19. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Don't take it that way, Bill. I am using balanced mics, as I previously mentioned, and I bought the components for the balanced mic preamp on Friday, so I'm not particularly keen on choosing a different circuit.

    I dismissed your circuit because of those reasons, not because I can't be bothered to look into it. I need that extra 6db the balanced pre gives me from my balanced mic.

    I'll be connecting it to a 3.5mm consumer line-input, like ones that you find on most of the smaller portable audio recorders, or on PCs. I initially wondered whether the virtual ground would cause issues... is there anything to be concerned about?

    That is good news!!

    So I'd just have to not connect the phantom power circuit's ground to the preamp's ground? Doing so would be linking the 11v(split) to 0v, so I'm guessing so? I'm used to using unbalanced pres, where the signal wouldn't be received unless ground is connected too, so it seems like an odd concept.
     
  20. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, good suggestion patricktoday. I have to say at first glance I wasn't sure it would be ok but seems like it should work. :rolleyes:

    MattP, you would need to add a coupling cap to the output of the preamp, because its signal will be centered at 11 volts. 10-22uf for example.
    Also, make sure to include the proper protection diodes in the phantom supply.
    Because without those, if the mic is accidentally connected to the pre with the power on, large, destructive voltage spikes can be sent into the preamp.
     
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