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Nov 29, 2021, 02:28 AM
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DIY Microphone Improvements


I came across a YT video on how to build a good quality microphone for low cost. He does a nice job on the mic and the circuit I figured could be updated for better performance .

Building a quality USB-C microphone (23 min 34 sec)


I thought of a few improvements:
Low noise is one of the keys to a good mic as you know. Many different paths for noise to enter and different ways to minimize the noise and some other improvements.

Filtering:
1. DCDC converter:
I don't see the need for the big RC filter (100Ω + 2200uF) with huge size electrolytic caps. The dcdc converter only has 33mA max output and the circuit current draw is very little. More importantly is low voltage ripple (from getting into the signal through the supply rails). The dcdc converter is running at 90KHz, that is the target freq needed to reduce ripple voltage. As a minimum a LC filter of 220uH and 1uF would be recommended. In effort to reduce voltage ripple even more, adding a second LC (making a PI filter) is one option. This should do much better for filtering noise from the switching boost converter and USB supply. For the input dcdc rail, low esr (X7R or X5R) 0.1uF and 22uF ceramic caps should be added.

2. IC Amp filtering:
I would add caps placed as close as possible to the IC power pins. A low esr (X7R or X5R) 0.1uF + 1uF ceramic caps on each rail (pin #4 and #7).

3. Max capacitance: The dcdc converter datasheet shows "the maximum recommended output capacitance is 10μF", having a 2200uF with 100Ω might be alright, but I think the LC and/or PI filter, plus the rail caps close to the amp IC is a better solution.

Signal path improvements: Keeping all input analog signals short can help reduce noise.

1. The number one most sensitive signal is output of the mic, so keeping this signal as short as possible so placing the transistor as close as possible helps like in the video and away from the power supply.

2. The second most sensitive signal is outputs of the transistor, in the video, these wires are quite long and sort of shielded a portion of the distance in the video. I really like the idea of very short wires from the transistor to the amp (THAT1512), this can help with lowering the noise from long wires.

DCDC preload / LED's: The dcdc needs preload resistors that help regulate the dcdc output voltage. A single 4.5K resistor will work on each rail. As a preload option, you can add two LED's (one for each voltage rail) if desired. Preload current should be a minimum 10% of the output max current (33mA). so a minimum of 3.3mA is desired for preload.

If using the leds as a preload, R5 and R6 with 3.2K should give minimum 3.3mA with any color LED.

Resistors: I would personally use 1% on all resistors keep the circuit balanced.

Volume improvement: (only if you dont like the stepped volume in version #1)

1. A nice quality Alps or Bourns pot. For more info see THAT Design Application Note 138 for GAIN control - (THAT dn138) has additional info.

PCB improvements: These can help keep noise low.

1. Ground plane: A dual layer pcb with mostly all ground on the bottom (and using a SMD AMP ic) with stitched (many vias) to the top ground are common in high preformance low noise pcb's.

2. Short signal paths and routed away from power voltage rails can help.

3. Terminate components close to amp IC. (all input and output components placed close to the amp)

4. Distance the dcdc converter away from all signals and the AMP (as much as possible). The dcdc has
toroidal magnetics so it may not be an issue, but it is just good practice. Another option is mount the
dcdc on the bottom of the board and use the ground plane as a shield.

5. Direct mount Mic to PCB: Another option is design the pcb to allow the mic tabs to be soldered directly to the pcb and having zero cable (and then having Q1 mounted on the pcb).

Mechanical: I like the look of the mechanical design in the video, but with my long mechanical racing education/experience/tuning, the long tight O-ring suspension I believe is not optimal. It's too stiff is my guess. Normal high end mic's use many soft O-ring suspension for more compliance to external disturbances and a heavy weight mic assembly to help stability. So more mic mass helps this issue and placing both amp pcb and usb pcb in the same enclosure helps two birds with one stone (less noise and more mass, exactly like the brass does in the video design).

I think if I designed a pcb, it would be small enough (without all the large, unnecessary large and wrong value output caps) by using all ceramic X7R or X5R SMD caps. The pcb would fit right behind the mic element (maybe around 35mm diam), then you would not have any long run wires of the tiny sensitive signals and not have any chance to pick up noise. Depending on the usb audio input module size, it might fit behind the mic/amp pcb too. With this setup I would use a nice gaming mouse braided usb cable for flexibility, aesthetics and better preformance.

----------------------------------------------------------------------------------------------------------------------------------
UPDATE:
I started playing with easyEDA free schematic capture and pcb layout about a month ago for the first time, its pretty fun.

After playing with the mic schematic, I added all the things I suggested above. Learned from reading the datasheets, I changed the dcdc output capacitance, added the LC filter, added a option to run led's front or back of the pcb for the required dcdc minimum loads, simple resistors or both. Use only LED1 or LED3, and LED2 or LED4. I decided to add the second LC filter for the dcdc as this is a audio application. Its easy to bypass this option by shorting the inductor pads (L1 to L4).

Soldering the mic to the pcb shortens the critical signal length, so the signal coming out of the amp is much less susceptible to noise.

Volume adjustment: I prefer to have a pot instead of a stepped volume adjustment. Reading the APM IC datasheet refers to the application note "THAT corporation Design Note 138" shows many options for a volume pot and how to implement. The basic config is to use pins 1 and 2 on the volume pads and no R9. and ~5Ω value for R10.
Using a quality and the correct audio taper pot is recommended.

The J-FET (Q1) comes in three packages, two with metal cans (these are the best), but they can have different lead pinout spacing. So I designed the pcb to accept both, and in addition a surface mount package that should also fit in the Q1 pads (without using the case connection). I suggest getting the 2n4416A version, as it has slightly higher voltage (35V) verse the non "A" version that is rated at 30V and with +/- 15V rails, thats 30V.

Mic mounting options: The dcdc is 10mm high, Q1 is 6mm high with the metal can package, this shifted the mic mounting up to clear the dcdc instead of being centered. Also, the mic can be mounted on the back side of the board.

DC input power: Standard usb voltage or any 5V supply between 4.5v and 5.5v is fine. Added 22uF and 0.1uF for high freq switching noise coming out of the dcdc. Added a second set of 5V pads for aux circuit if needed.

PCB:

I ended up going with 45x45mm square pcb two layers with mounting holes spaced apart at 35mm square. this should fit in a 50x50mm or 2"x2" project box. And can panelize the pcb with four up on 100x100mm pcb with JLCPCB, so quantity of 20 is around $2 plus S/H. (I have never ordered from JLSCPV yet)

Copper poured top and bottom for lower noise and separated the input to dcdc from the ground planes.

Added a couple ground pads next to the mounting hole pads (GND_1 and GND_2) that can be soldered jumpered to connect the pcb ground to the case if desired.

USB Sound card options: Another option to gain preformance, but at what cost,,,
1. The low cost USB audio input is probably 16bit/24KHz or maybe 48Khz and great bang for buck,
2. The soundblaster AE-5 is 24bit/96KHz and around $60 to $70 used,
3. Then the price starts to up up quickly, 24/192, 32/192, 32/384
Would be nice to test the difference between a few of these.

USB: USB3 is not necessary unless you need more than 5V 500mA and this circuit does not. USB2 has plenty of data rate for this application, is microphone will now work in a lot more hardware applications because USB3 is not everywhere.

Still thinking of improvements. Suggestions are welcome.

My current version of the schematic and pcb:
Will share all the files when I am finished.
Last edited by --Oz--; Nov 29, 2021 at 04:32 AM.
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Nov 29, 2021, 02:29 AM
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