Saturday, 23 April 2016

Little microphone preamp.

Now, I've not been very happy with the sound on my videos... you may have noticed...

What's needed is an external microphone, maybe a tie-clip or a lapel mic.

Now examining my new camera, it doesn't have a mic input. Damn... I google a few things, and, although there is a guy on a forum who's added a socket in to the camera, I didn't fancy taking my new camera apart. Other people with the same camera seem to prefer to record the sound separately, on a zoom recorder, or similar, and line it up in the video editing software. Good plan.

Now years ago I bought an Acer MP3-330 MP3 player, fitted with a mechanical 10GB hard disk... Now that has a record facility, but the in-built microphone isn't up to much. It does have a line-in socket, however. So we'll need to make a microphone pre-amplifier.

Left over from the Arduino beat detection and lighting controller project I had a number of small electret condenser microphones. One of these would work nicely as a discreet lapel mic.

The circuit is a simple op-amp, driven from a single rail 9V battery. R1 is used to supply the bias voltage to the microphone. It should be around 7.5 volts. C2 is used to block this DC. R2 and R3 put the incoming audio from the mic onto a half-rail supply, which is amplified by the op-amp, having it's gain set by R4 and R5 in the negative feedback loop. C2 gives us some DC stability, but ensuring that the DC gain of the amp is unity, otherwise any offset would be amplified also. C3 removes the amplified audio from the half-rail DC on the output. R5 is a level control. A snazzy blue led is driven at low current from the switched 9V supply.

I make the circuit up on a very small piece of veroboard and mount it, along with a couple of sockets for input and output into a small box, It works well.

I mount the microphone on a long length of screened cable, and enclose wiring side of it in heatshrink. A crocodile clip makes an excellent clip for it.

The finished unit. Watch this space for a demo!

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