you'll need loudspeakers and an amp
I got what originally was the foot of a computer monitor with built-in speakers for one or two Euros at a garage sale. It expects 9 Volts, runs fine at 6 Volts too, so I feed it six AA batteries. For its size, it has a quite balanced sound and doesn't turn the bass into a tin drum as quickly as other, comparatively small speakers.
Of course, you are free to use one of those newfangled cell phone "boom boxes" that are for sale for a few dollars in every asian electronics store, which are rechargeable via USB and have bluetooth and whatnot. It'll save you the trouble of coming up with a punchy bass line.
you'll need a mixer
I have opted for a passive mixer, which I soldered into a candy can. It basically consists of five 3.5mm stereo input jacks and one output jack for the sum, with potentiometers in-between. To avoid crosstalk and prevent the output of one source from destroying the amp of another source, each of the ten input channels is separated from the sum with a 4.7kΩ resistor, so that even with an open potentiometer, the signal will be a little less loud.
The only trouble with this thing is that audio engineers are reluctant to hook it up to their 30,000€-PAs. "Do you mind if I put a limiter in-between?" No, sure, go ahead.
you may need an amp and a recording device
At ~200,-€, the BOSS microBR might be one of the most expensive devices I ever bought to make music, and it has proven to be worth every pennycent. You can tell from the wear of the finish it has suffered, and survived, a lot. It is a complete 4-channel recording studio fitting into your pocket, with mastering effects, guitar amp, drum pattern sequencing, distortion effects and a built-in microphone. I have recorded jams, live sessions and mixed and mastered entire songs with it. It is to this day my go-to choice to record my GameBoy tracks, simply because it is so easy to write the WAV or MP3 to SD card and read it into the PC.
I once plugged in, by accident, the 12V power supply of a tube amp, instead of the 4.5 Volts it expects. It gave a little spark, a very silent "bang", smelled like a blown-up condenser and never worked with external power supply again. It still works with 2 AA batteries though. This is the kind of hardware I'll praise to heavens, even if I don't get a penny for it - if all manufacturers continue to design their gear like that, it will be reward enough.
I usually put it behind the mixer for make-up gain, recording and usually, very carefully, activate the compressor. I'm not a fan of the loudness wars, but if I find the FOH is afraid of the neighbors calling in the police, I reserve the right to counteract their cowardice a little.
time to bring out the keys
Everyone knows how to use these and will instantly begin to play the Flea Waltz.
Believe it or not, but this originally was a Casio SA-1 keyboard. 100 sound programs, a bunch of quite useful rhythm presets, tempo control, and now beautified with Lego applications and green spray paint. Oh, and I almost forgot, I circuit bended it too, so it now has a few additional knobs that... do stuff.
The one in the upper left regulates the power the processor gets, and the less it gets, the more often it stumbles, creating a distorted sound. The two on the lower left - fine and coarse - feed the audio signal back into the processor, which again confuses the poor thing and creates even more distortion goodness. Both together cause utter mayhem and it takes a while to become familiar with how the electronics will react to your inputs. Don't ask me about the other knob, it's an armageddon device or something.
For comparison, this how well-behaved it looked like when I brought it to one of my first gigs to play the bass ostinato from Pachelbel's Canon in D and before I took the Dremel to it:
time for some loops
My old friend Tony T., who lived next door back then, bought the Korg Kaossilator from his first money earned with Shine On, made a few minimal beats with it, got bored and gave it to me and I did not exactly protest.
This track for example "happened" when Tony T, Metaphysics (Söhne Mannheims) and I chilled at "Café Z" in Wuppertal with only the Kaossilator and the microBR. You can even hear the glasses at the bar clinking in the background at ~1:12. It was the first time I ever seriously tried the microBR too, so I did not even do any of the wizardry I know today I can do with it.
Anyways, back to the Kaossilator. 100 programs, 10 of which are drum loops, x on the touchpad controls pitch of the instrument and y a parameter specific to it (volume, cutoff, arp speed, attack, stuff like that). Even unmusical persons will instantly fall in love with the ease with which they can record a simple loop to bang your heads to.
let the least musical person control the effects
Korg's Kaossilator has 100 effects, the parameters of which are controlled with the touchpad. Set it to a low-pass filter and leave it to someone to create some wub-wub cutoff FX over any instrument...
the gameboy, for example
Haha, just kidding. Wait, no, I'm not. My GameBoy is a musical instruments thanks to the GB USB Smart Card (~40$, alternatively, you may want to check out the Drag'n'Derp which is ~100$, but simpler to use), which connects via USB to the PC and can be fed any GB ROM. Preferably, that ROM should be a musical program. There are plenty of these, but the mightiest by far is Johan Kotlinski's Little Sound DJ, or just LSDJ. Others to try are Shitwave, Carillon, nanoloop, pounder and last but not least the good old Gameboy Camera (Trippy-H). For a well-curated list, visit Tronimal's homepage, who I consider one of the most knowledgeable people on the planet regarding GameBoy music. He also installed the backlight into my "work" gameboy (you have seen my "show" gameboy in my introduction post, it is on the right of the original Casio SA-1 on the picture a few paragraphs above also).
And yes, you can sync two or more GameBoys via MIDI or the link cable, if four channels are not enough for your symphony!
The amount of music software for the Nintendo DS is legion. Not only official software like Korg DS-10, Korg DS-10+ or Elektroplankton are at your disposal - with an R4 card, you can also download open source homebrew software like bliptracker, groovestep, cellsDS (!), nitrotracker (XM-tracker) or glitchDS.
Usually, I only use it to play a short voice sample from the movies or some "pause noise" between two tracks, while my GameBoys are loading.
Please be not disappointed that Pocket Music (GameBoy Advance) is not mentioned here - it is simply unusable. You may disagree, but then hats off to you for your patience!
finally, a calculator
This time I'm kidding! Wait... this Texas Instruments TI-82 does have a 2.5mm jack after all... and a Z80 CPU...
...and irrlicht project's (aka utz) wrote a complete, native stereo tracker for it!!! It is called Houston Tracker and now is in v2.2. Check out the latest bugs, errr, features here:
So this is, by and large, my "rucksack orchestra", so called because it fits into a medium-sized rucksack with plenty of room to spare for fresh underwear and a towel in case a gig leads me to a place where I'll have to spend a night or two.
And batteries, LOTS of batteries :) I prefer the rechargeable ones for composition work at home or in the train (the Eneloop you see in the second picture have quite convinced me, my oldest set is over 10 years old and still works) and only buy fresh, cheap ones for shows. Stereo jack ⇔ stereo jack and stereo jack ⇔ cinch cables are wearing parts in comparison, I always buy plenty of them where they are cheap - no use buying the expensive ones, they break just as easily, and all this humming, warbling, sizzling, crackling, circuit bent 8-bit noisy fuzzyness is not exactly addressed at audiophiles who insist on golden connectors either :) And always have two cinch ⇒ TRS adapters with you so you can connect to the PA of the venue!
Next time, I'll tell you about the Horsinator and why I never found the courage to record a track with my theremin.