Rock&Roll, baby!

As an aspiring musician, I always wanted to play the electric guitar without lugging around a bulky amplifier. As an electronic engineer, I could not resist the temptation to create my own mini-amplifier.

It was when @maxvoltar pointed me to an article about the mini-Marshall today, that I realised I never written a blogpost on this pet project of mine. It all started when I first saw the Smokey Amp, a mini-amplifier built into a pack of cigarettes.

When I set out to build my own copy of the Smokey Amp, I found the very simple schematic that was used:

The people at www.runoffgroove.com baptised it the "Little Gem" and soon variations on the circuit started to appear. I did build most of them on a prototype board and experimented with different sounds.

In the end I settled for a couple of important design changes:

• use 2 batteries instead of 1 to create 18V to achieve more headroom.
• use a buffering transistor in the input stage like with the Ruby
• provide a larger airmass behind the speaker
• use a more efficient speaker
• include a "bass reflex" port to increase bass response tuned at 80 Hz
• omit any potentiometers, since you can just use your guitar volume control

To achieve the larger airmass, I tried different solutions. I ended up using 2 pots of hairgel (empty, of course) glued back to back. Most of the lids is cut away and then they are glued together. By uncrewing the lid, you can access the batteries for replacement / recharging. There is 1 switch to activate the power. The bass reflex port is a 3 cm piece of ballpoint inserted into a whole in the bottom pot.

The end result was astounishing. It sounded a lot better and louder then a Smokey Amp, more along the line of the Marshall mini-amp that Maxvoltar directed me to. But the difference is in the coolness factor: You can just strap your guitar around your neck, attach my portable amp to it and play whilst walking around. And yes, you can also show off the brand of hairgel you use!