Stereo Preamp

Discussion in 'The Projects Forum' started by Wendy, Dec 6, 2015.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Just doodling with an eye towards building something like this. I would probably use the dead bug technique if I did. It would be used on my lappie or similar for microphones. Low power and small size is a must.

    So I'm throwing it out on the forum and see if it sticks.


    Stereo Preamp.png
     
    Last edited: Dec 6, 2015
  2. nerdegutta

    Moderator

    Dec 15, 2009
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    Nice!

    Do you have any particular Op Amp in mind?

    How would the power-part of the circuit look like, if you want to put it in a car, and use the cigarette-lighter as a power source? More caps?
     
  3. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Main application is for the laptop. I need to work on training my voice, and the several microphones I have for the use are too quiet.

    The NJM4565 is a good pick for this. I have several in my storage.
     
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  4. k7elp60

    Senior Member

    Nov 4, 2008
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    Why do you want to invert the audio?
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    Wendy
    A year ago I designed a mic preamp for my grandson. He wanted to record audio to his laptop but the mic level was too low. It runs off of 5 volts from the USB port. Here is the circuit.

    [​IMG]

    I made a PCB for it. Tested and works great.

    [​IMG]

    [​IMG]

    I have several spare boards. Interested? How is your surface mount soldering skills?
     
    Last edited: Dec 6, 2015
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  6. Johann

    Senior Member

    Nov 27, 2006
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  7. Johann

    Senior Member

    Nov 27, 2006
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    Perhaps you should add at least a volume control and maybe some tone adjustment?
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Note that Les's centerpoint ground is a) 100 times lower impedance; and b) AC decoupled. Without the decoupling there will be significant crosstalk between the two audio channels.

    ak
     
  9. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Not that want to, it just does not matter in the real world. I have two amps, I need two amps, so I used the minimum parts configuration.

    Because for this application I do not need them. Remember, size and low power is a requirement.

    I don't think cross talk in this configuration is going to be an issue. With a gain of 10 and audio frequencies, it won't be a problem.
     
  10. marcf

    Member

    Dec 29, 2014
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    Wendy:

    I assume that the 5, 3v cells are CR2025's or similar. Why not use 6 of them and put 'signal ground' between the 3rd and 4th one.
    Would also make the opamp 'happy' that it is working with a real split supply. ...+/- 9v.
     
  11. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Not a bad thought, but I generally like using virtual grounds. The power supply runs down for both sides equally.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The quiescent current of your op amp is 4mA. There are many op amps available with similar specs and much lower quiescent current (0.2mA in this one - giving 20x battery life - most of the time you are not likely testing your voice so current consumption is likely mostly based on quiescent time).

    This one is very close to your current op amps specs. A pretty good battery savings if using two op amps.

    I do like the idea of not adding an extra op amp to make a virtual ground and splitting the batteries to make your split supply.

    https://www.fairchildsemi.com/datasheets/FA/FAN4174.pdf
     
  13. Veracohr

    Well-Known Member

    Jan 3, 2011
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    A gain of 10 for a mic preamp is pretty low, is that really all you need?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    You may want to verify that there is no software solution. Here's my experience: I've played around with plugging my guitar directly into various computers, and in general it doesn't work because the signal is just too low. BUT, with software meant for this application, I can get pretty good results without any preamp. I believe the software can increase the gain of the computer's built in amp beyond what is normally available in the system settings. This experience is on a Mac, mind you, but might have a parallel on whatever you're using. Just something worth a look.
     
  15. marcf

    Member

    Dec 29, 2014
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    The virtual ground (signal ground) is developed by the two unbypassed 1meg resistors R1 and R2. U1 is shown for the power supply connections. And U1a and U1b are the same device, showing the signal connections.

    The op amp is developing a 'virtual ground' at pins 2 and 6

    Does any one else think the term 'virtual ground' is a little odd in this context?. I like 'signal ground' as it is just another ground, like 'digital ground'. There is really nothing 'virtual' about it. The "-" input of a op amp is at virtual ground.

    I think that the existing signal ground circuit, R1 and R2 is a little 'light' and at the very least should be bypassed.

    Also a split supply would require a double pole switch, which could add a little cost to the project.
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
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    The bias resistors (R8+R9) are there hold the Vcc/2 voltage on the 10uF cap. It is the cap that creates the low impedance virtual ground. If the cap was not there then the current from the op-amp feed back would drag the virtual ground around, causing distortion and odd gain instabilities. The cap is the important part (not the resistors) and should be sized based on the resistance of the feedback resistors and lowest frequency. Actually in my circuit the decoupling cap (10uF) is too small for a 20 Hz signal with a 412 ohm feed back resistance, but it wasn't designed for a sub-woofer.

    Edit: The above is true in a non-inverting op-amp design only. (The design I used.)
     
    Last edited: Dec 7, 2015
  17. marcf

    Member

    Dec 29, 2014
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    Sorry for the misunderstanding.

    I was remarking about the circuit in post 1.
     
  18. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Yes, the inverting input is called a virtual ground. You will find in electronics that many different configurations can be called the same things, just as a circuit can be used many different ways, each with its own name.

    Creating a Virtual Power Supply Ground

    Wendy's Index
     
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