If what you really need is a typical TB303 sound in your iOS set up, then you may have just found exactly what you’re looking for. If it really isn’t anything that’s going to set your heart all a flutter then perhaps skip to something else for now. I know that for some the classic sound of the 303 is a big deal. Personally I found them awful to program and limited, so anything that anyone can do to make the sound without the issues of the original is a good idea in my book. Having said that, I’m not sure of too many situations where I’d want to use one right now.

However, if it’s what you want thenĀ 2XB303 could be just right. Here’s everything that the developer says about the app including all of the relevant iOS specs:

A creative bass synth sequencer for electro melodies or dirty phat & funky acid techno basslines. Look at the video tutorial for a how-to guide. Listen to what you can create easily with 2XB303. With sequence manipulation features, effects, external syncing & audio routing. Inspired by the acid house music scene. An IOS exclusive. Create sequences using standard step functions as seen on various sequencers like the Roland SH-101 or use one – button generation. Sound tweaking by familiar means: filter cut-off & decay Env-Mod & glide & accent etc for those classic sweet-spot acid growls.

Controls are per channel except the global/master effects (yellow ).

  • METRONOME: Toggle basic drum beats -better than cowbell or tick-tocks to accompany your basslines!
  • MEMORY: Next memory to load. If playing, it loads at the next loop else it will load immediately. Make sure you have saved your current sequence!
  • 1-32 steps
  • OFFSET +1 to +31
  • COPY, PASTE & CLEAR sequence
  • RANDOMIZE all or choose just NOTES (auto chooses notes if none chosen), SHUFFLE rearranges pattern, EXPAND repeats length to fill all 32 steps. PRESET loads pre-programmed sequences.
  • TRANSPOSE key changes the pitch of both sequences for cool melodies.
  • SWING A global level . Some step values like glide or rest are ignored with this


  • LFO auto controls FILTER CUT-OFF as well as PITCH. The pitch level can give a vintage effect subtly detuning pitch with low LFO SPEED. Or faster for chaotic random pitches.


  • Select WAVE from SAW, SAW + harmonics, SQUARE, SQUARE + harmonics & NOISE


  • PAN, BASS cut/boost, MUTE 1, 2, “I” (INPUT) & METRONOME. Input = mic/Audiobus filter etc
  • PHAT= pre low-pass filter via high pass returned to low pass mix. A nice fresh modern sound. The PHAT cut-off & resonance is synced to the main filter & env mod.


  • OVERDRIVE, COMPRESSION adjusts threshold, ratio, & gain. This is a global effect.
  • ECHO feedback & wetness. SPEED = tempo synced.
  • SAVE ALL. When not playing, save to memory.


  • Ableton Link syncs other apps or use without for precise timing. Press LINK button at the bottom for settings.
  • MIDI. External, virtual or Audiobus. You can sync with your favourite hardware using a MIDI interface or other apps. 2XB303 can be a master or slave. ( any simple class-compliant USB should work but some may need to be power assisted)
    Virtual MIDI Input is always on. If you send to 2XB303 virtual MIDI from another app AND have 2XB303 receive MIDI clock then you will be receiving both MIDI CLOCKS. Choose only one unless you want double tempo (which can be interesting).
  • While Ableton Link is enabled you can send MIDI out. This means you can use one app to control 2XB303 with your external hardware.


  • Record a fixed length, default to 32 steps. Can be changed in settings.
  • Arming the SAMPLER (flashing). Starts at counter 1 (ON).
  • PLAYBACK to hear sample.
  • Audio can be routed using inter-app audio into apps like Apple’s Garageband. Garageband will also sync your sequence for excellent multi-track recording.
  • Audiobus 3. Route 2XB303 to mix or record into other apps
  • Audiobus Filter. Audio from Audiobus 3 will travel through channel 1 filter & echo. Sound is gated by 2XB303s sequencer.
  • Mute Channel 1 to hear just incoming audio. Play your vocal loops or guitar riffs with the synced filter & delay effects set in 2XB303!


  • BACK ENABLE: If disabled, will shut down background processes to preserve CPU.

The acid sound can be close to a TB-303 but was first compared to a Cyclone Analogic TT-303 & then tweaked to extend sound shaping for further uses & not just bass. For instance, waves have an optional harmonic version, there is noise, LFO, glide speed & PHAT/ BASS levels to play with plus effects. 2XB303 is, of course, digital, & doesn’t contain a vital transistor. But, it is designed to give much more freedom for creativity and sound beyond the acid sound.

2XB303 costs $4.99 on the app store