powering Audio circuit through USB

Discussion in 'The Projects Forum' started by wesleykonrad, Feb 12, 2011.

  1. wesleykonrad

    Thread Starter New Member

    Feb 12, 2011
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    0
    I am still somewhat new at electronics but I could use some help.

    I have a video capture box that for some reason has very poor gain on the audio input. I want to add a pre-amp to increase the gain.

    I purchased a Velleman Super Stereo Ear kit MK136 (http://www.vellemanusa.com/us/enu/product/view/?id=351278) and did a simple modification of changing the two microphones into a 1/8" stereo input jack. I have tested the circuit and it works fine for what I need.

    Since I will only be using this with my capture box, I would like to power it by the USB port instead of batteries that I'll have to keep replacing.

    The circuit is measuring at 4.8 V which I figured would be close enough to the 5V that USB provides that it should be fine. I wired it up to a usb port and it worked but I got horrible line noise.

    Just to see if the voltage did make a difference, I hooked it up to a 4.5 V power supply at 500mA and still got line noise.

    I plugged my multimeter into the circuit and it seems to be drawing just under 10 mA. My understanding is that USB gives about 500mA so my conclusion is that the amps are too high.

    Does this sound right?

    I could add a resister to lower the amps but won't that effect the volts as well? I know that V=IR but I have never been really clear on how that relationship works when V and I are dynamic (which seems to be the case).

    any assistance would be appreciated.

    Wesley
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Cant say much without having the circuit
    On the website under specifications section it specifies that it runs only with 3 1.5V AA battery,their is no detail about the amplifier i.e.. not much about the circuit so cant say much.But their are some high gain low power amplifiers which could only be powered by batteries properly,may be this one is one of them.

    Try using battery and check if the noise still exist.

    Good Luck
     
  3. wesleykonrad

    Thread Starter New Member

    Feb 12, 2011
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  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This circuit appears to bias the amplifier inputs and the microphone input directly from the power supply. It is therefore likely to be very sensitive to supply noise. Given the fact that the supply from a USB port probably won't be all that clean, it might be easiest just to stick with the battery. You could try adding extra filtering in the feed to the power input, but I can't say how much would be needed. An alternative would be to try using a regulated power supply, preferably not switched-mode.

    The following explanation is added in an attempt to help overcome some misconceptions: You seem to think that power supply having a greater current capacity is the root of the problem"my conclusion is that the amps are too high." This is really not right.
    Actually, adding a resistor of a few tens of ohms in the positive line might possibly help, but you need to understand why. The aim would not be "to lower the amps", although the current might reduce somewhat. The resistor would help filter the supply, in combination with the internal supply capacitor in the preamp. An additional capacitor of say 1000uF added across the amplifier power input might help. Adding resistance in the supply line would also affect the supply voltage, dropping it by an amount depending on the supply current V=IR.

    That said, you might find that adding such a resistor would cause other problems. The current drawn by the amplifier is likely to change with the signal level, and therefore the voltage dropped in the series resistor may vary. This could affect the sound quality, or even lead to oscillations. If in doubt, don't.
     
    Johann likes this.
  5. wayneh

    Expert

    Sep 9, 2010
    12,090
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    Here's the schematic.

    You need a low-pass filter on the supply. Could be as simple as a big cap across the poles. I mean biggER than shown.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I can't see it. Do attachments take time to come through, or is there a problem?

    Edit: Sorry, I see it OK now.
     
    Last edited: Feb 12, 2011
  7. wayneh

    Expert

    Sep 9, 2010
    12,090
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    I think you're right, that this could be a challenge to fix if the "more-is-better" approach with the filter cap doesn't work. I guess there's always a chance the current 470µF cap is bad or installed backwards.
     
  8. PatM

    Active Member

    Dec 31, 2010
    81
    72
    The 500ma is the maximum that the supply will provide.
    If the circuit only requires 10ma, that is all that it will draw.
    There is no need for a resistor.
    You have a supply that will supply a greater current, but in your case it doesn't have that much of a draw, and will run cooler with almost no load.
    PatM
     
  9. wesleykonrad

    Thread Starter New Member

    Feb 12, 2011
    3
    0
    Ok folks, you have answered some questions and drew up some more.

    One suggestion is to try using a regulated power supply. So that would mean adding a 5v voltage regulator like a LM2937 with the appropriate capacitors and resistors?

    As for adding a low pass filter, this is where my ignorance will start to show. In terms of sound, a low pass filter only lets lower frequency sounds through. So there are frequencies coming from the power supply? Sorry, but when I think of frequency in terms of power, I thought that only applied to alternating current.

    When you are talking about installing a bigger cap you mean at C6 right?

    I am also still a bit unclear on how voltage and current interact with each other. I understand that if I have say 10v at .5 Amps and I add a 500 ohm resister which is going to change, the voltage or the current or both. If both, how do I calculate to get to a target voltage because the equation then has two unknowns?

    wesley
     
    Last edited: Feb 12, 2011
  10. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Only if you have a high enough input voltage, probably over 6v. (I haven't read the datasheet for that IC)
    Yup! It's lovingly referred to as 60 cycle hum.
    Yes exactly.

    We need to be talking about the same circuit to make any specifics meaningful. But in general, Ohm's law works. Period. If you need to make successive estimates to narrow in on a solution, so be it. Ohms law works whether we know the math needed to apply it or not.
     
    Johann likes this.
  11. Erik Jochums

    New Member

    Nov 10, 2016
    1
    0
    Use a DSL filter. Off the shelf, works with the wattage range involved, and cuts the electronic noise. Tested this at varying volume levels of the MK 136 and it works great.
     
  12. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    A good study of Ohm's Law will reveal a lot of mysteries!
     
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