Shop OBEX P1 Docs P2 Docs Learn Events
Microphone Preamp — Parallax Forums

Microphone Preamp

ercoerco Posts: 20,256
in General Discussion
1) Anybody need a $4 "professional" microphone stocking stuffer for their rockstar kids? http://www.walmart.com/ip/Pyle-Professional-Moving-Coil-Dynamic-Handheld-Microphone-Silver/20659775

2) Anybody have a favorite one-transistor preamp for such a dynamic (moving coil) mike? I need to feed two such mikes into the AUX input (stereo, one on left, one on right) of a car stereo.

This is one I found so far: http://www.epanorama.net/circuits/micamp.html

Comments

  • Tracy AllenTracy Allen Posts: 6,664
    Favorite, I don't know, but the one in that article wouldn't be it. (Have to get the best quality sound from the rockstar!)

    In the referenced circuit, drop R1, C1 and C2, which are meant only for an electret mic, also drop the red led ("to show that the circuit operates"? You'll know!). I'd add another resistor of say 39kΩ from the base of the transistor to ground. That will raise the DC voltage on the collector of the transistor closer to the middle of the 9V power supply range. The circuit bias will be around 2mA. And as another step, break the direct connection of the transistor emitter to ground and fill the break with a 100Ω resistor in parallel with a capacitor of 10 to 100µF. The two extra resistors stabilize the operating point and improve the linearity to make it sound better, and the capacitor restores the high gain to make it punch through. Bring on the rockstar kids and dad!
  • ercoerco Posts: 20,256
    edited 2015-12-17 06:19
    That's my man! Thanks, will do. Is a 2N2222A ok?
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2015-12-17 16:56
    The 2N2222A will work in the circuit. It not usually thought of as a small signal amplifier though. For example, it usually does not list a noise figure on its data sheet. It is roughly comparable to a 2N3904, which does list one around 5dB. A better audio choice would be something like the 2N5088 or the BC547B specified in the article, which are designed for low level like audio with a noise figure around 2dB. All said, the rockers are not going to bring their most discriminating ear, am I right?

    If you have a bag of 2N2222A's, I suggest you take out your ebay component tester and find a pair of transistors for your stereo amplifier that are fairly well matched in terms of Vbe and gain, and pass up on ones that have an exceptionally low gain. You will find quite a spread. The gain of this circuit we are talking about with the emitter grounded will depend very much on the gain (Hfe, beta) of the transistor itself. You want a voltage gain of around 40dB, which is 100x. The transistor needs an Hfe above that.

    Here is a nice hobbiest explanation of transistor characteristics...
    http://www.aoc.nrao.edu/~pharden/hobby/HG_DS1.pdf
  • Beau SchwabeBeau Schwabe Posts: 6,566
    Try this one for simple ... This is of my own design using an Emitter follower configuration and a self biasing technique to place the transistor in it's most sensitive linear region (exactly where you want it for an Audio pre-amp). The idea is that the 10uF Capacitor trickle charges through the 10k and the 47k until it reaches a point that the transistor turns ON. By turning ON, the trickle power is quenched causing the transistor to turn OFF. A balance is established at the "sweet spot" of the transistor where any perturbation on the Emitter of the transistor is amplified at the Collector of the transistor and in phase with the Emitter.

    I have used this circuit with Electret mics with pleasing results. I have not tried it with dynamic mic, however in a similar circuit I have used a magnetic pickup coil in the same location where you would place the dynamic mic. The dynamic mic should work in the same place in the circuit as an Electret mic.

    PREAMP.JPG
  • ercoerco Posts: 20,256
    Thanks Beau, will try before Christmas. Santa needs to"wrap" this project and get it under the tree.
  • ercoerco Posts: 20,256
    Yay, both circuits worked, I breadboarded them both up tonight (Christmas project #1, heater). I couldn't hear any difference in my low-volume tests (kiddies sleeping, can't wake 'em up) so I went with Beau's circuit for simplicity & parts count. It can use both dynamic and electret mikes (I bought a variety of microphones for the twins for Christmas). I have two 1/8" jacks for electret and two 1/4" jacks for dynamic, wired in parallel pairs, one to each channel (one type of mike on each channel). Quick homemade plastic box needs some finish work but it works a treat. Thanks to Tracy and Beau for their helpful input on input!
    CAM02431.jpg
    1539 x 1280 - 254K
  • Beau SchwabeBeau Schwabe Posts: 6,566
    Glad that worked erco!! ... and seems to be universal between a dynamic and Electret ... Now we just need someone to do a fancy frequency sweep, and see what the response curve is compared to other pre-amp methods.
  • ercoerco Posts: 20,256
    Heeeeeeeeeeeeeeeere's Amy...

  • kwinnkwinn Posts: 8,697
    My goodness they're growing up fast. Seems like only a few weeks since you posted baby pictures.
  • PublisonPublison Posts: 12,366
    Very cool! Maybe a little autotune? :)

  • ercoerco Posts: 20,256
    Publison wrote: »
    Maybe a little autotune? :)

    Can Tracy and Beau save Christmas with a simple one-transistor autotune circuit? Dig deep, guys!
  • PublisonPublison Posts: 12,366
    erco wrote: »
    Publison wrote: »
    Maybe a little autotune? :)

    Can Tracy and Beau save Christmas with a simple one-transistor autotune circuit? Dig deep, guys!
    Chip can probably throw one together for the propeller.
  • ercoerco Posts: 20,256
    I think I can code a vanilla BS2 to do SFT.

    SLOOOOOOOOOOOW Fourier Transform.
  • Heater.Heater. Posts: 21,230
    erco,

    When I first ever saw an FFT it was written in BASIC for the Atari ST.

    It was SLOOOOOOOOOOOW....

    I think the BS2 might need many more "OOO" in there :)
  • PublisonPublison Posts: 12,366
    Heater. wrote: »
    erco,

    When I first ever saw an FFT it was written in BASIC for the Atari ST.

    It was SLOOOOOOOOOOOW....

    I think the BS2 might need many more "OOO" in there :)
    First time I saw FFT was on a NorthStar 8080. Probably BASIC.
  • Heater.Heater. Posts: 21,230
    Wow, someone who remembers the NorthStar. Happy days.

  • Tracy AllenTracy Allen Posts: 6,664
    Erco, how endearing, Amy's rendition of "Let It Go"! My granddaughter is 8 yrs. old and drove us crazy with Frozen for months after it came out in the theaters. Now, thankfully, she covers a wider mix along with mainly Taylor Swift. It's a grow-up-fast world, isn't it, yet she still believes in Annabelle, her elf-on-the-shelf.

    The common base configuration for audio preamp circuit is neat, vintage Beau. The current gain is less than one (beta/(beta+1)), so the voltage gain will depend strongly on how much current the mic can support into the emitter of the transistor. The frequency response is probably good, a characteristic of common base.



  • ercoerco Posts: 20,256
    edited 2015-12-30 18:57
    After I bought 2 mikes.
    After I built the preamp.
    After I went to Fry's yesterday to buy a new HD.

    Today only, this complete 2-wireless mike system is on sale for $12 at Fry's: https://shop1.frys.com/product/6709545

    Trying. To. Resist. Going. Back. To. Fry's.

    Weakening.

    Failing.
  • abecedarianabecedarian Posts: 312
    erco wrote: »
    Heeeeeeeeeeeeeeeere's Amy...
    Cute!

    My 10 yo nephew prefers Oingo Boingo.

  • XNORXNOR Posts: 74
    Beau Schwabe wrote: »
    Try this one for simple ... This is of my own design using an Emitter follower configuration and a self biasing technique to place the transistor in it's most sensitive linear region (exactly where you want it for an Audio pre-amp). The idea is that the 10uF Capacitor trickle charges through the 10k and the 47k until it reaches a point that the transistor turns ON. By turning ON, the trickle power is quenched causing the transistor to turn OFF. A balance is established at the "sweet spot" of the transistor where any perturbation on the Emitter of the transistor is amplified at the Collector of the transistor and in phase with the Emitter.

    I have used this circuit with Electret mics with pleasing results. I have not tried it with dynamic mic, however in a similar circuit I have used a magnetic pickup coil in the same location where you would place the dynamic mic. The dynamic mic should work in the same place in the circuit as an Electret mic.

    PREAMP.JPG

    Hi,

    Sorry for resurrecting an old thread:

    I was wondering where one would add a potentiometer for a volume control to this.

    Would it be at the input between the mic and the transistor, or would it be at the output
    after the 1uF capacitor?

    I assume the potentiometer would have a different value depending on where one places it?

    Thanks for the help
Sign In or Register to comment.