I get a lot of questions about which MIDI interfaces work well with Patch Base, so I will work on getting a list on this page.
The bottom line is that any MIDI interface that correctly supports sysex data should work with Patch Base. However, I highly recommend using a wired interface (as opposed to wireless, such as Bluetooth) if you are going to be sending or backing up banks of patches for your synths. Wireless interfaces can lose MIDI data during transfer, meaning that you won't get a complete bank transfer sometimes when using them. This is just a limitation of wireless MIDI. With a wired interface, this doesn't happen.
These days, the best way to go for iPad MIDI is to use a USB adapter with your iPad, and a USB-MIDI interface of some sort. There are good iPad-specific MIDI interfaces out there (see below), but they seem to be getting phased out of production now. So to use a USB-MIDI interface, the first thing you need is a USB adapter. For iPads with a Lightning port, you'll want Apple's Lightning to USB camera adapter (also available here on Amazon).
With that, you can now use a wide array of USB-MIDI interfaces that are available. A great, simple, high-quality USB-MIDI interface I recommend is the iConnect mio. I've always been a fan of the iConnectMIDI1, which is the iOS-specific interface they make. If you have the Apple USB adapter though, this is a great way to go.
I own and regularly use the iConnectMIDI1 from iConnectivity. It plugs directly into your iPad or iPhone and doesn't require any external power, so it's about the simplest MIDI interface setup you can have. Unfortunately at this point it looks like the iConnectMIDI1 has been discontinued, so it's harder to find online. But some online stores still carry it.
The iRig MIDI 2 is also a good option. It plugs directly into your iPad as well, but does not have built-in MIDI cables attached, so you have to have those yourself. These also seem harder to come by these days.
If you have an older iPad (with the 30-pin connector instead of the newer lightning connector), the Griffin MIDIConnect will plug directly into your iPad.
I also have the Quicco mi.1, which is a small bluetooth MIDI interface that's easy to use as well. It's powered by the MIDI port of the device you plug it into, so it's a great, portable solution for connecting to your iPad. With the latest firmware update (2.2.1), it establishes a solid MIDI connection that supports sysex and works well with Patch Base.
The Yamaha MD-BT01 is also another good quality interface that many users have had success with.
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