Nostalgia mixer

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Fig.1: The mixer.

What is it about?

Everyone started somehow. After a chat with michai and nomead, sharing our beginings in the electronics field I decided to share one of my first projects - a four channels stereo audio mixer.

It is hard to keep all the "piece of wood + nails + DC motor + 9V battery" kind of projects, but this one I always keep from throwing away. It reminds me of the good old "back in the days" technique I was using to manufacture PCBs, as well as of the times I liked to mix music tracks. How much better is a tape recorded without quiet spaces splitting otherwise perfect collection of tracks? That's right - it's a day VS night comparison. So apart from this one being a good learning base, it actually also worked and was being used from time to time. What else should one want from a project.

Needless to say, a tear rolls in my eye as I am writing about this project... pure nostalgia!

How did I do it?

The project itself

Well as most of starters, I didn't invent this whole mixer. I've got the schematic and the PCB layout base from an AVT self-build projects sheet printed out in 1998. I was leaving primary school at the time and just about to gain some interest in electronics. But i got the set of notebooks a bit later, in year 2000. World didn't explode and I managed to join my secondary school class on an electronics-fair trip to Poznan that year. Got these projects' sets for free as an "giveaway outdated stuff" promotion. I am not sure but I think the build was taking place in the year 2000 as well. Oh and did I write "layout base"? Yes! I actually made some mods from the original project here... ;-)

What I lacked

Apart from soldering skills and money to buy quality audio mixer?

  • First thing: The "save some bits of cash" sense triggered and I decided to shrink the PCB a bit. As you can see on the PCB layout scan below, Most of the PCB was serving as a potmeters mount/support only. Why waste this much of laminate on such an unimportant task... Few cuts here and there didn't introduce much chaos into my first good PCB manufacturing technique back then. All was still good.
  • Second thing: I remember it as if it was yesterday - no proper potmeters available/affordable for me. Linear characteristics, supersmall short-scale ones were used here... And not even enough of them to build a complete four channels device. Only two audio sources here.

PCB manufacturing technique

The main part didn't change at all with years in here. I used a laminate covered with plain copper layer and an acid to burn off unneded parts of it - leaving usable traces for electronical signals. The magic part was actually applying the PCB layout onto the board. No cash to buy a photo-resistive laquer spray, no PC to print out the layout for termo-transfer... Well every problem has a solution:

  1. I covered whole copper side of the laminate with electric insulation tape stripes.
  2. I drew paths with a marker.
  3. I cut out the unneeded empty spaces of the tape-layer, leaving the copper exposed where it shouldn't remain in the end.
  4. I put the so-fetched PCB into an acid-bath (overnight bath, no heat-up/mix system).
  5. I drilled holes and soldered elements onto the board.
  6. Life is perfect.

The gallery

Here are some pics of the ready device and some scans from the original project.


Good times, good times, good... I always take this board into my hands when I somehow accidentaly find it when searching for something else in my electronics-junk box. I didn't even have to remove any dust of it to take pics of it today. A milestone in electronics here... possibly the first time the project wasn't just a "learn something from the experiment" kind of deal, it actually served a purpose. What I've learned back then? That with just a tiny bit of clue and a handfull of cheap components one can build up his dream-device on his own desk. What's better about electronics in general. No question mark intended here...

Yours truly,

pit, 2012-08-16