I started my first electronics project and am in the process of building a USB powered portable microphone pre-amplifier for PC-style electret microphones as they are commonly used in consumer style headsets.
PC style electret microphones are powered by a bias voltage of +5.0V. This voltage is usually supplied by the soundcard on the tip and/or ring of the TRS 3.5mm jack. A good in depth explanation of the different wirings can be found in this article on powering PC microphones. I measured roughly a dozen soundcards (USB and PCI, low end and high end) but unfortunately none of them delivered the full 5V but most only deliver a meager 2.3 to 3V to the mic. This results in low signal levels which then requires amplification (either analog or digital) but that also amplifies noise and sounds bad. Conveniently USB supplies 5V at 200mA which should be enough for an electret mic. My goal is to build a little box that takes USB power and provides a PC style microphone input port on one side and a line-out signal on the other side.
Since I'm an electronics noob I decided to go with breakout boards and chose a MAX4466 based board that is available from a couple of vendors. My idea was to de-solder the electret mic capsule and replace it with a cable connection to a 3.5mm jack port.
I expected that USB power is noisy. So I put a LT3042 linear voltage regulator (to be precise, a chinese CJMCU-3042) board in front the of the MAX4466 and hoped that it would smooth and filter out the noise from the USB power supply. I had to figure out the board layout and desolder one the of SMD resistors to put in a 48kOhm resistor so that is set to 4.8V instead of the factory 3.3V. To my surprise I got it right and it worked. I will probably make a seperate post with schematics when the project is done. For now, this roughly what I have put together.
+--------+ +---------+---------4.8V---> mic in | | | | USB 5V-------+ LT3042 +----4.8V--+ MAX4466 | | | | | +--------+ +---------+---------2.4V----||---> line out 100uF
Surprisingly this whole thing works. I used a TerraTec Aureon 7.1 USB to record the audio because it has a dedicated line in port. This is what I saw on my first recording attempt in Audacity:
For a cheap 8 EUR microphone on a USB soundcard with a few electronics thrown in, this an acceptably low noise floor with a nice and strong voice signal.
I then kept playing around with all the cables on the desk and for some reason switched the USB power supply from a totally overpowered 4.5A charger to an older 0.7A one. At first I though I had broken something. With that power supply there was an awful noise floor. Then I tried the PC USB3 port and it was even worse. There are rhythmic patterns, hissing and beeping:
I then tested a few more USB charger style power supplies and there are great differences between them. Generally if the USB power supply output was rated above 2.0A, the noise floor was low and clean. USB power supplies below that output rating were much worse.
Keep this in mind when you power RF equipment like WiFi or Bluetooth devices. I subsequently fixed an annoying range problem with a Bluetooth audio dongle by replacing the USB power supply.
Since I want this to work with any USB power, I have to filter out the USB noise and plan to add an LC filter in front of the LDO to remove the high-frequency noise. If this works, I will post updates afterwards.