So, next project!

I'm not certain if this will do what I need but with any luck it just might. Ah, yes, the task? Well I have microphones. Unfortunately they need to be amplified before being suitable for use with most consumer-grade gear. They don't need anything beefy for this though - what they need is a pre-amp to bring them up to the right level.

The idea being that one could go out to interview someone with a couple of microphones and some cables. These could then be wired to a phone or MP3 player for recording. The gear to conduct the interview would fit easily in something as small as a handbag.

When recording multiple people one tends to find that they speak at different volumes. Not a problem in person (usually...) but to the audio gear they'll sound vastly different. The basic answer to this is going to be keeping the two sound channels separate. A 3.5mm stereo jack will be used for both input and output. Traditionally one would use 1/4" (6.35mm) audio jacks or even XLR plugs (to match the microphones). No dice for this job though as what they're going into is a Läckerol tin I got from Scandinavia. There's room for a 9 volt battery and a bit more. With any luck I'll have the space to get the whole thing to fit!



Pretty small, as you can see. Here's it without most of the parts:


It's going to be tight!

So what's going in side?
Well the plan is for a slightly modified version of this project.


Well I don't know about you but some of those symbols aren't ones I initially recognise (I think they're ANSI symbols) so I did my best to redraw the diagram into something I'd stand a better chance of understanding at a glance:


Hopefully I didn't balls that up in some way. What you can hopefully see is that there's only one potentiometer in the circuit diagram (although there will also be a trimmer replacing that circled 100K resistor). There's two on the tin though...
Yeah... I need to get the circuit to fit twice! Stereo, y'see?

Also not pictured are the two 3.5mm audio jacks that are part of the design. Worst case scenario I could instead wire those to actual wires that hang out of the case but ideally I want it all to be internal. It's not all that many parts, thankfully. The ICs are LM741s that are fairly tiny, a few capacitors and resistors, and the rather small trimmers (potentiometers that aren't expected to change after being set).

So, things that need to be done:

  • Missing parts: 2012 me ordered the parts and missed out the 47K resistors I need. He also ordered linear potentiometers rather than logarithmic ones.
  • Wire. I could really do with a colour other than white!
  • Insulation. I'm not sure if it's required but it might be a good idea to lay down some plastic inside to keep the parts isolated from the case. On the plus side the casing should help filter out RF interference!
  • Knobs. It might be fun to add some cool knobs to the volume control pots.

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