A 9 volt battery is required to power the iRig Pre for 30 hours with dynamic microphones and 10 hours with condenser microphones using phantom power. There's no external power supply here, but it's unlikely you'd need one considering the usage you get from a single battery and the type of recording you're likely to do with this kind of accessory.
Output is a 40 cm (15.75") TRRS cable to connect to your iOS device as well as a headphone output to monitor what's coming directly out of the iRig Pre. Input is the XLR connector for a microphone. The specifics are as follows:
Frequency response: from 20 Hz to 20 kHz +/- 1.5dB
Noise: -98 dB fullband, phantom power ON
Maximum output level: 2 Vrms
Distortion: 0.025% THD
Like many of IK's iRig accessories, the quality of build is not what you would call high-spec, but it's sturdy enough in its own plastic way. It's not going to break in normal use, but if you were to drop it or get it squashed between two sturdier items, it feels like it's going to come out worse for wear.
I tested the iRg Pre with my Rode NT2000 condenser microphone through my iPhone using my Ultimate Ears headphones for monitoring. The first thing I noticed when I plugged the mic in and turned on the preamp was that there was a subtle, but definite and consistent hiss on the iRig Pre. With the gain on full, it was much more noticeable, but pulling the gain down a few notches did make it come down, although I couldn't create a situation where I could eradicate it. The hiss isn't overbearing, but if you're recording, it's always going to be there. And that's not what you want.
I did a comparison test with my desktop audio interface just to make sure my NT2000 wasn't playing up, but it wasn't. The NT2000 was hiss free through my desktop interface. Just like a good mic should be! However, my audio interface does cost significantly more than the iRig Pre.
Aside from the hiss, the iRig Pre did its job of powering the mic. I did a few test recordings using IK's Recorder app and then compared those with the iPhone's built in mic using the same app. It was surprising just how well the iPhone's built-in mic stood up to the NT2000 using the iRig Pre, but there was no doubt about it, the NT2000 had a fuller and more detailed sound. However, comparing those tests to the NT2000 through my desktop audio interface revealed that sound was significantly clearer, fuller and quieter, as you'd expect.
From there, I decided to see whether using a different app to record would yield different results. Using the iPhone's Voice Memos app, I couldn't get a monitoring signal. It would record audio, but not send a signal to my headphones for monitoring. However, using GarageBand with the monitoring turned on in the track settings gave me a monitor signal. Which is a relief because you can obviusly do so much more with GarageBand than with IK's Recorder app, or even IK's VocaLive.
By this point my ears may have begun to tire, but to me it sounded like the quality of the recording was better in GarageBand than in IK's Record app. I can't see how this can be unless it's all down to the programming of the apps. That, or my ears simply got tired.
At the end of the day, is the iRig Pre worth investing in, even at its low price point? Well, there is that hiss to contend with. If there was no noise from the iRig Pre, in many ways it would be a no brainer. But unfortunately, on my unit anyway, it was there. And even if you're just doing podcasts or demo recordings, it will detract from the result. Furthermore, what's the point of buying an expensive mic for more clarity and detail only to have that sound tainted by hiss. That takes me to my original point: you can have the best mic in the world, but if the preamp is no good there's just no point.
To sum up, I'd say the iRig Pre is worth getting if you happen to have a decent mic already, and your only form of recording is through an iOS device. At $39.99, you can't really go wrong.
However, if you've got an iDevice, but you haven't got a mic already, you're probably better off looking at a mic and preamp in one, like the Blue Mikey, Tascam IM2 or even the iRig Mic Cast or iRig Mic.
Update: IK Multimedia got in touch with me regarding the noise/hiss I experienced using the iRig Pre. They had this to say: "The noise is generated by the iOS device headset input. This input is great for general purpose usage or headset mic, but can exhibit some noise in excess when used for audio applications where noise is critical.For this reason iRig PRE has a GAIN control that let's you perfectly set the amount of signal that is going to the iOS device. Setting this to a proper level can minimize the evidence of the iOS device input noise on the recorded signal."
IK says the following is best practice when using the iRig Pre: "1. If present, set the input gain on the app you're using for recording at UNITY gain, typically shown as 0dB. 2. By looking at a recording level meter on the recording app, adjust the GAIN CONTROL on iRig PRE so that the recording level is as high as possible without distortion."