I bought myself a pink MSI Wind when they were first released last year with the intention of using it for university and as a perfect travelling laptop. The Wind is great in many aspects, but the stock inbuilt microphone is severely lacking. It is noisy and not sensitive enough for general use. You have to have your mouth 10cm away from the mic for it to be useable.

As I didn't want to carry around an external microphone with me on my travels, I figured I'd crack open my Wind and install a preamplifier to boost sound levels and improve the sensitivity of the microphone.

The Circuit

The Wind uses an ALC888S codec chip for its analog-digital conversion. The datasheet can be found on Realtek's website. After a brief flick through and a poke around the area of the motherboard where the chip is mounted, I found that the stock Wind microphone actually connects to both channels of a stereo microphone input on the chip. Seeing as most opamp chips nowadays come with at least two opamps on them, I decided to install another microphone.

The circuit schematic.

The circuit is a very simple inverting amplifier based on the LM358. It has a gain of about 1000 (60dB). The power is supplied from the +5V lines that run all through the motherboard of the Wind. The electret microphones require a bias voltage for their inbuilt FETs, which is provided by a 3.9K pullup resistor. The DC bias is removed from the audio signal by a 1uF capacitor. The opamp amplifies the signal in a no-frills inverting configuration and the output is passed through another capacitor to the audio chip.

A SPICE simulation of the circuit results in frequency response that is suitable for the typical human voice range, 300Hz to 3000kHz.

A SPICE simulation of the preamp circuit.

The Microphones

The microphones used were two electret condensers. They were mounted behind the Wind's screen bezel. The left Wifi antenna is an ideal mounting point for the second microphone. As the second microphone is spaced around 18cm from the right microphone, a slight binaural effect is produced in all recordings. It is not truly binaural, however, as there is no barrier between the microphones to simulate head shadowing.

The electret microphones were attached to their mounting points with double sided tape, and wired up with headphone wire. A new hole for the left microphone was drilled through the bezel, and the right hole was enlarged a little.

The mounting points for the microphones.

The headphone wire was routed down through the screen hinges in the same way as the stock microphone.

The PCA

The printed circuit assembly was made using the standard toner transfer method and 0603 surface mount components. The board size was around 20 x 15mm. Photos of the board production can be found in the article gallery.

The completed PCA.

Mounting

The preamp PCA conveniently fit in the gap directly below the Wind's WiFi card. Blu-Tack was used to secure the board into place, still allowing for removal at a later date. Wires for power and the audio signal were easily run between the motherboard and preamp.

The mounting point for the preamp board.

Results

With the Wind back together, the modification was barely noticeable. The only visual clue of any modification was the second hole drilled through the screen bezel.

The completed modification.

The functionality of the new microphones is far better than the stock microphones, but unfortunately still not perfect. Sensitivity is greatly improved, and the microphones can now be used without having to move closer to the Wind. However, there is still a large issue with noise.

Noise is generated both from the +5V rail of the Wind, and from the fact that the microphone wires run right over the WiFi antennae. To improve the noise levels, the opamp should be used in a differential amplifier configuration and the DC bias to the electret should be isolated as much as possible from the Wind's power rails.

More photos of the modification can be found in the article gallery. If anyone has any queries, don't hesitate to leave a comment below.