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Microphone (the DumbbellMic)

Active Microphone (pre-amp circuit plus housing) for camera or other devices

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I wanted to explore what it would take to build a good microphone for video (using a Sony A6300 camera). As it turned out I learned a lot and I'm happy with the end result. Although I think the pre-amp is a bit overkill for this particular setup. Could come in handy though for other projects. The sound is very full with all frequencies well balanced although perhaps low bass sounds could be suppressed a little (they can sound a little too full...). This can easy be edited when needed.

*** board now available on Tindie ***
https://www.tindie.com/products/nsdesign/dumbbellmic-microphone-amp-board/

The housing is 3D printed with self casted rubber pads for muffling vibrations or other mechanical sounds. Inside is the preamp PCB with low noise components and two good electret condenser microphone capsules. The preamp circuit is powered by a 9V battery mounted on the hot shoe of the camera. The preamp features a power saving circuit which senses the phantom power of the camera (or device the microphone is attached to) and switches off the 9V power supply completely when not sensing a voltage from the camera (when the camera is switched off). There's also an indicator circuit which turns on a low current LED for a short time (indicating switch-on with a full battery) or long time indicating (warning for) an almost empty battery.

There are two switches; one for the Gain (G) which can be High (H) or Low (L) and one for Power save (P) with a setting for Save (S) and None (N). The gain switch switches R14 in parallel with R2 to regulate the gain and the power save switch just bypasses the current sensing circuit for devices that do not have phantom power.

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  • 2 × R1, R10 10K R0805
  • 4 × R3, R5, R6, R9 100K R0805
  • 2 × Q2, Q3 MMBT2222ALT1G TRANSISTOR, NPN, 40V, SOT-23
  • 1 × U1 OPA172IDBVT OP-AMP, 10MHZ, 200UV, SOT-23-5
  • 4 × C1, C3, C5, C6 2.2uF C0805

View all 15 components

  • Finding the right amplification

    M. Ploegmakers05/04/2020 at 20:21 0 comments

    The build went well. But at the end during the first few tests it turned out that the initial values for R2 and R14 were much too high. The gain was too high for the Sony A6300. Tests on a PC with Audacity were fine because I could set the Mic-in sensitivity with Audacity. But on the camera with the lowest setting of 1 the amplification still was way too high and the sound was distorted. So I had to do many iterations for finding the right values for R2 and R14 (not knowing what the camera could handle).

    The microphones I used have a sensitivity of -42±3dB therefor the volts per Pascal of air pressure is:


            -42dB
           ---------
              20
    10                 = 7.9 mV/Pa

    The impedance is 2.2K so the current per Pascal is 7.9 / 2k2 = 3.59uA/Pa but I have two in parallel. So I assume that the calculation for the current of the mics for 2 Pa would be (2 * 3.59uA)/Pa * 2Pa = 7.18uA

    For a typical line level of 1.228Vrms this would result in 1.228V / 7.18uA = 179kOhm. for R2 (or R2 in parallel with R14)

    This seemed a little high when looking at the amplification that would give (which would be 1 + R2/R1 --> 1 + 180 / 10 = 19 !!)

    So I started out with just 75k but even this proved way too high for the camera. So maybe my thinking was wrong here or the camera is just too sensitive. Anyway, I ended up with acceptable sound levels with an R2 = 10k and R14 = 3k9.

    This gives an amplification of just 2 on the HIGH gain setting (without R14) and 1.28 with LOW gain setting (R14 parallel). And for the Sony camera the LOW gain setting is enough to get an acceptable level with the camera sensitivity set to 1. So this turns off the internal amplifier mostly I assume.

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  • 1
    The microphone body (housing)

    I included the STL files and an animation for reference but I think the housing can be done in many ways. I didn't spend much time on calculating the right acoustic shape of the capsule or the way I placed the slots or anything. So the polar chart of this particular design could be okay or just plain wrong. I did try to avoid hard joints with bridges of silicone pads (casted with plain construction silicone), some velt pads and a vibration construction for the holder. I think this is a must to avoid clicks and sounds from the camera. That did work out fine I think.

    The decals for the switches I made from very thin pvc sheets which I printed on a laser printer and after placing them I did put on several layers of clear coating.

    For the wire mesh I used a mesh fabric used for fuel filters.

    The only thing left todo is a dead cat for outside use. I'm planning to use some fake fur for that.

  • 2
    PCB and parts

    The Gerber files and parts used are included. I still have several PCB's left and also some parts. Maybe I will finish a run for these and advertise them later on on Tindie as assembled boards.

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