I’ve been meaning to write this up for several years now, but I’ve finally got a version of this project that’s worth writing up.

The project would also work well for an ordinary (6-string) electric or electro-acoustic guitar, but you might want a preamp with EQ, FX send-return etc. Simplicity, and the need for for bass to have higher power output drove this project direction.

Requirements

• Run time of several hours - long enough for multiple gigs between charges
• Uses easily available Li-Ion battery packs
• Based on off-the-shelf Class D power amp module
• Goes loud enough for medium-sized gigs

Nice-to-haves

• Drives 8 ohm speaker loads (at ~70 watts) or 4 ohm speaker loads (at ~140 watts)
• Runs cool enough that the fan hardly comes on

At the end I’ve included notes on previous iterations, which were lower powered with less-capable modules.

An extremely simple yet capable compressor circuit is published by Elliott Sound Products,here (link updated 31 Dec 2023).

Compressor Mk I

I built the circuit exactly as-is (in an Altoids tin).

Care was taken to optically couple the bulb to the photocell (LDR), using a perspex rod, wrapped in smooth tin-foil.  The perspex came from eBay: http://stores.ebay.co.uk/thewholesaleposco2009

as did the miniature “Grain of Wheat” bulbs:  http://stores.ebay.co.uk/GnA-MODELPARTS?_trksid=p2047675.l2563

This was put together, and sealed in heatshrink tubing, to make it fairly lightproof from external light sources.  The rest of the components came from http://cpc.farnell.com (free postage) or from my junk box.

I have an Ashdown Little Giant amp.  Several reviewers have mentioned that this amp is not at loud as it should be (e.g. here).

It all depends on whether the output levels coming from your bass are enough to drive the output stage fully.  I have an active bass, but even with the input set on max, to get a decent output level I need the output volume control set to around 3 o’clock.