GuitarJack works with Sonoma's FourTrack iOS recording app ($9.95) and is also compatible with other audio-based iPhone OS Apps--I have three guitar amp/stomp pedal modelers/tuners and they all work and sound great using GuitarJack. GuitarJack connects guitars and/or microphones via a 1/4-inch jack, any stereo instrument via a separate 1/8-inch mini jack and provides a stereo output from another 1/8-inch mini jack.

About the size of a small box of wood matches, GuitarJack is housed in an all-metal, crush-proof case. It has all metal jacks and has a certain amount of heft as compared to the other plastic-cased units I've tested. This interface has complete input routing and level controls and input monitoring when using a GuitarJack enhanced apps.

After putting my 3GS iPhone into Airplane mode, I connected GuitarJack's output to the iPod input jack on my ZT Club amp; a Gauge ECM-58 dynamic mic went into GJ's 1/8-inch input jack and my guitar to the unit's 1/4-inch jack. Not only does this hookup allow me to play through this very clean solid-state guitar amp using one of my amp modelers to get tone and effects but also later, for songwriting, I'll boot up the new FourTrack iOS app and start recording and overdubbing both guitar and vocals.

FourTrack allows for either recording the two inputs (as two separate tracks) together for a guitar/vocal demo session and then switch over to a single input for adding other tracks one at a time. If you're stuck for a studio mic to plug in, FourTrack also works with the iPhone's built-in microphone or an external headset mic.

I liked FourTrack's Song Tools page with metronome, AudioCopy for grabbing a copy of your song, AudioPaste to navigate to other iPhone apps to export that song file, and Song Import for pulling in audio files via your docked computer. WiFi sync allows FourTrack recordings to be transferred to any desktop computer either by launching Sonoma's RiffWorks recording software and/or by downloading the tracks using a browser.

FourTrack's main touch screen 4-track mixer GUI has individual track record arming and VU meters, transport controls and a Timeline indicator with jog wheel for navigating around a song quickly. It's easy to see where you are in any song at any time. FourTrack has been updated to include a GuitarJack control panel with an analog input control and up to 60dB of continuous gain for each input. There is switchable input impedance modes and both a normal and boost mode for the 1/8-inch mini jack input path.

I especially liked the TaylorEQ app. For each track I recorded, I could carve the sound wonderfully with this six-band, semi-parametric equalizer--you just drag around the overlapping band faders until it sounds good.