I've been using iPads and iPhones in performances since 2011 and other performers often ask how I connect the iPads to the PA system. The easy answer is "just use the headphone output", but getting audio cables with a 3.5mm jack that are rugged enough to work on stage can be a bit of a problem.
I've actually gone through a few solutions for solving this problem so here's my ideas on the subject, including a few things to avoid!
3.5mm to stereo RCA cables
3.5mm to Stereo RCA (red and white plugs) are really common for connecting phones, tablets and laptops to hifi systems or TVs which use RCA inputs. A lot of DJ gear also uses RCA inputs and outputs and you often see them as the "Tape In / Tape Out" connectors on audio mixers. Unfortunately the 6.5mm (1/4") inputs on mixers are usually more useful, so I have a large collection of RCA to 1/4" adapters.
For my earliest iPad performances, I ordered four 6 meter 3.5mm to RCA cables from the european music store Thomann. These were cheap and were actually pretty awesome cables!
A year or so, and many performances, later and I found that one of the failure points of these long cables was the 3.5mm plug. Tripping over a cable or pulling the wrong way wouldn't break the iPad (phew), but it would break the tip of the 3.5mm plug.
So with a dwindling set of cables I tried buying another inexpensive version of the same thing from Ebay. I realised that these were a total fail as soon as I plugged them in because of the amount of noise in each channel with one of these cables. At first, I suspected dodgy sheiding in the cheaper looking plastic connectors, but soon enough, one of these cables broke and I realised that the audio signal was actually unshielded over the whole 5m length! Turns out interference is a real thing.
Another problem with all the RCA cables was that the RCA plugs were not designed to get connected and disconnected frequently and the metal sleeve that connects the ground got loose on all these cables and the 1/4" adapters started to fall out.
Smartphone 4-pole adapters
In 2013 I started performances with my iPad group, Ensemble Metatone, and a fresh set of iPad 4s with Apple Smart Covers. The problem was that all the chunky 3.5mm plugs didn't fit through the headphone hole on the cases. Eventually I found that you can get little smartphone headphone adapters with very fine 3.5mm 4-pole plugs and a bigger 3.5mm socket for the headphones (and sometimes another socket for a possible microphone). Not only did these go through the cases, but they can act as a strain relief and save the 3.5mm plug on the nicer cables from breaking in the iPad headphone socket.
Although these cables look like they have pretty poor shielding, it hasn't been an issue over just a few centimeters.
Homemade cables - 3.5mm TRS stereo to 1/4" TS mono
To streamline my four iPad setup, I actually made a few cables from scratch. All my performances were in mono with only the left channel used for each iPad so I made cables with a slimline 3.5mm plug that can fit through the Smart Case headphone hole where only the left channel is connected to a single 1/4" plug at the other end of a light audio cable.
These cables have been going strong for about a year and have travelled around the world for several performances. The cable is a bit inflexible even through it's so light but my main worry is that there isn't enough strain relief in the slimline 3.5mm plug.
If you want to try making these, here's the parts list from Jaycar (Australia):
Pro-audio cables - 3.5mm TRS stereo to dual 1/4" TS mono
Recently I've been doing gigs with even more iPads (up to 8) so I've needed a few extra cables and I thought it was time try the higher quality cables made for pro-audio applications. A common choice is the Hosa CMP159 cables with a 3.5mm jack on one end and two 1/4" mono jacks on the other, but it's tricky to get these in Australia.
Luckily I found a cable brand based in Canberra that has even better looking cables - the Swamp Mixer to PC cable. Just like the Hosa cable, these have a 3.5mm jack and one 1/4" mono jack for each channel, but the plugs are all good quality metal Yongsheng connectors that can be fixed or replaced if they ever break. Awesome. These are quite a bit more expensive, particularly for 5 meters, but a reasonably good investment for my ensemble.
Pro-audio cables - 3.5mm TRS stereo to mixed 1/4" TS mono
One other type of cable mixes the two channels of the stereo signal into a single mono channel and has just one 1/4" connector at the other end. These are typically sold for playing mp3s or backing tracks through a guitar amp with only mono inputs.
I bought two of these Mini 3.5mm to Mono 1/4" Jack cables from Swamp to use in my solo dual iPad rig. These are really great cables, but definitely a bit more expensive and tricky to get.
Handy Hint #1: Don't use 6.5mm TRS cables with stereo signals.
I've seen a lot of noobs using stereo 1/4" TRS cables on stage for their phone or laptop, often hacked together from a 3.5mm to 3.5mm stereo cable with a 1/4" headphone adapter. This will usually sort-of work with a guitar amp with a mono 1/4" input, but will have really weird results when connected to a mixer expecting a mono balanced signal. If your iPad sounds really quiet... AND THEN SUDDENLY REALLY LOUD through a PA system, this might be your problem.
Handy Hint #2: Bring your own DI to computer music gigs.
When I play gigs with my iPad ensemble we have four line level signals on stage that need to be distributed to the PA system. Each of these needs a DI box to connect to the mic level inputs on the mixer. Usually the engineer on deck doesn't have enough spare DIs so I got in the habit of bringing my own. I use the cheap Behringer DI20 which can handle two channels and works just fine. I often have to loan these to other performers who are stuck!