Apps similar to Carla include LMMS, which is free and open source. Other options are GarageBand, Zrythm, Yadaw and Giada. There's no shortage of Music Production Apps like Carla, with more than 25 alternatives for Windows, Mac, Linux, iPhone and iPad.
You might use a complete DAW to record your music, but when you are in front of your audience, it may be easier to use something dedicated to live situations. And during live situations, the access to your instruments and effects must be fast and intuitive.
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Blue Cat's PatchWork is a universal plug-ins patchbay that can host up to 64 VST, VST3, Audio Unit or built-in plug-ins into any Digital Audio Workstation (DAW) in one instance.
Rack Performer is a high-level modular environment hosting VST plugins and internal modules (over 100 and counting), with MIDI support, multi-channel audio I/O and internal processing, complete automatic patch-delay compensation, sample accurate sequencing, low jitter and...
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LiveProfessor is a live oriented plugin host for Windows and OSX. LiveProfessor makes it easy to create a powerful, portable and flexible effect rack using VST or AU plugins.
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LinuxSampler - free, open source pure software audio sampler with professional grade features.
MainStage lets you take your Mac to the stage with a full-screen interface optimized for live performance, flexible hardware control, and a massive collection of plug-ins and sounds that are fully compatible with Logic Pro
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STANDALONE is a lightweight AU and AUv3 plugin host for Mac designed specifically for guitarists and bassists who want to run amp sim and effects plugins without opening a DAW.
A minimalistic VST host. Holds up to 4 VSTs. Allows for multiple instances (see how on GitHub). If you try to load an unregistered plugin, it may fail. Register somewhere else, then load it. If the sound clicks a lot, increase the buffer size or switch to Direct Sound in Audio...
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Vienna Ensemble Pro has become the definitive cross-platform network solution in studios and production facilities around the world. This mixing and hosting application enables you to set up a multiple computer network with both Macs and PCs.
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A VST (plus AudioUnit on OSX) plugin host designed for live use. Based on the JUCE 'audio plugin host' code, heavily modified to better suit my purposes.
NanoHost is not a synthesizer. It is a free, minimalist host application for Windows that can load VST-synths. Furthermore, it can act as a wrapper which converts VST-synthesizers into standalone applications.
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