So here’s the story of how to connect your electric guitar or bass to an iPhone, iPod touch, or iPad via the headset interface.
We’ve been considering this issue since as far back as 2008: GuitarToolkit customers have long been asking for a way to tune their electric guitars and basses without needing to power up an amp. We looked into what it would take and started building prototypes in 2009. At around the same time, the lightbulb went off: “Wow, if we could connect an electric guitar as input to an iPhone, and still have headphone audio coming out, we could do some really interesting things …” and this eventually led to our forthcoming AmpKit app.
Today, after a full 10 generations of prototypes, we’re thrilled to be collaborating with Peavey Electronics on the commercial realization of our explorations: AmpKit LiNK, a guitar and bass interface that, paired with AmpKit, bring full-scale amp and effects simulation to iPhone, iPod touch, and iPad users. (AmpKit LiNK is available for preorder here.)
OK, so let’s look at the challenges of interfacing a guitar through the iPhone’s headset connection. One *could* simply rig up an adapter cable that would connect a guitar signal to the iPhone headset input, and a headphone jack to the iPhone headphone output; our first prototypes did just this, and there’s even a commercial product made this way. But you would encounter a number of problems.
1- The iPhone will (possibly intermittently) fail to detect that an audio device has been plugged in to the headset jack. The iPhone is looking for the electrical signature of its earbud headset, which includes an electret microphone. A guitar signal directly connected to the mic input is very different electrically; in our experience, a directly-connected guitar signal will “kind of work sometimes,” which can be a highly frustrating experience.
2- The incoming guitar signal will suffer severe attenuation of its high frequencies, resulting in an awful, muddy sound. This is again due to the different electrical characteristics of the guitar signal versus the electret microphone the iPhone expects.
3- You’ll get really nasty feedback if you try to use headphones with a high-gain guitar amp simulation. The issue is that, in the cable between the iPhone and the interface, the faint, low-current guitar signal runs right alongside the comparatively high-current headphone signal, using a common ground. The result is that the outgoing headphone signal bleeds onto the incoming microphone signal, a phenomenon known as crosstalk. Now, if you happen to be simulating an amplifier on your iPhone, especially a high-gain amp like a Peavey ValveKing® or 6505® Plus, this crosstalk turns into the ugly screech of feedback.
Over the course of our 10 prototypes, we grappled with each of these problems in turn. Here’s how we solved them in the AmpKit LiNK interface:
#1: The failure-to-detect problem requires a bit of circuitry such that the iPhone sees the electrical characteristics it expects with the earbud headset’s electret mic. Result: immediate and consistent detection of the connected interface by the iPhone OS.
#2: The muddy sound issue involves a more significant set of circuitry, which transforms the guitar signal such that it has the signal characteristics of an electret mic to the iPhone, while at the same time preserving the full frequency spectrum of the guitar signal. The result: a very nice, rich guitar signal which sounds great when run through an amp simulator.
#3: The toughest nut to crack was the feedback issue. We found that using a well-shielded cable between the interface and the iPhone was not by itself enough. Instead, we had to tackle the heart of the problem: the high current levels needed to drive headphones. We incorporated a tiny but powerful headphone amplifier inside the AmpKit LiNK interface itself, which enabled a radical reduction in the current flowing between the iPhone and the LiNK. Between great cable shielding and the current-reducing benefits of moving the headphone amp inside the interface, feedback on high-gain amps is virtually eliminated. You can still cause feedback if you crank a super high-gain amp up to max volume, and do the same with your iPhone hardware volume control. But this is actually fairly realistic: a real amp would have feedback under similar conditions.
AmpKit LiNK is powered by two AAA batteries. Buyers of guitar interfaces should bear in mind that, if the headset guitar interface you’re considering isn’t powered (with batteries or an external power supply), you’re almost certain to have feedback problems when simulating high-gain amplification over headphones.
Also note — anyone who owns an iPad should consider cable length. AmpKit LiNK’s cable is long enough to connect to a docked iPad with the LiNK laying flat on the table. A shorter cable would leave the interface dangling off the side of the iPad, which is likely to result in an iPad crash of the physical variety.