I'd been using a Korg Pandora PX4 headphone amp for a few years when traveling.  It was a nice little device and also included some very basic backing tracks, guitar effects and built-in tuner.  Not bad for something the size of a pack of cigarettes.  But the sound quality was never that great, especially if you wanted to lay down a rhythm guitar track on top of some drums. Whatever effects you applied to the guitar (reverb, distortion) would also get applied to the drums.  Lame!  Also, because of how the power switch is located on the side, I would often take it out of my travel guitar accessories case only to find that it got jostled to an on position and there was no battery power left.  

At any rate, since it had been a few years, I figured there was bound to be some improvements in this category and I decided to take a look and see what would fit my needs.  Here are the key capabilities I was looking for:

• Small & light enough to fit with my travel guitar while on the road
• Runs on batteries
• Built-in effects (chorus, reverb, distortion)
• Built-in tuner
• Basic rhythm tracks with bass & drums
• Good quality headphone output, since that's how I'd mostly use it
• Easy to use, so I could focus on playing rather than futzing around

There are lots of other features you can find in portable headphone amps, but these were the most important to me.  The key point being that playing with a rhythm track is essential for improving your timing.  

Since portability was essential, I ruled out the nice looking Line 6 JM4 Looper, which weighs a couple of pounds and doesn't run on batteries.  Roland's also got a comparable new eBand JS-8 product coming in November that looks very promising.  Since I wanted backing tracks, this precluded the Line 6 Pod family and their portable Pocket Pod and Pocket Pod Express.  All of these are great looking products, but they just didn't have the combination of features I was looking for.

Ultimately, that left three different devices for me to look at: a recently updated Korg Pandora PX5D, the Zoom Z1-Z2 etc, and the Boss Micro BR, a very cool looking portable device focused on recording.  Any of these could fit the bill.  Here's a quick summary of all three:

KORG Pandora PX5D
The Pandora is the most expensive of the three, with a street price of around $250.  It's a good upgrade from the PX4 with more rhythm tracks to choose from, higher quality sound and a USB interface that you can use for recording.  Although recording is not my intended purpose, the USB interface also gives you the ability to easily customize the sounds and "chains" of rhythms (e.g. songs) from a Mac or PC.  It's about the size of an 1980's cassette walkman and runs on 2 AA batteries or USB power.  

BOSS Micro-BR
Boss has a well deserved reputation for some of the best guitar effects pedals on the market.  The Micro-BR is a small device that is focused on multi-track recording and also includes effects, tuner and rhythm tracks.  There's a significant community around it trading tips, settings and the like.  If you're mostly looking at recording without a PC, this could be the right choice, but there's a bit of a learning curve here.  

Zoom G2
While the Zoom G2 is the cheapest of the bunch at around $100, its still got the basic features: guitar effects, rhythm tracks and tuner, all built in.  Zoom also makes more advanced versions with a built-in expression pedal (think "wah wah") and USB recording ability.  If you're on a budget, or have used their earlier products, this could be a good way to go.  But again, some have found it hard to use since the display only gives you a number, no other information.

Ultimately, I decided to go with the KORG Pandora PX5D, partly because I was already familiar with the earlier version and based on reviews, it seemed to be the easiest to use.  In addition to improved sound, the PX5D adds some new features like the USB interface that could make it more flexible for customizing sounds and backing tracks.  

I'll post a full review of the Pandora PX5D in the coming weeks.  If you're looking to improve your rhythm playing with a headphone amp that includes rhythm tracks, any of these gadgets could work well.  If you've used one of these products, let me know what you think by posting a comment below.

• Amazon: KORG Pandora PX5D, BOSS Micro-BR, Zoom G2, Line 6 JM4 Looper
• GuitarVibe: G-DEC Combo Amp, RiffWorks
• Korg:  Main site, Pandora PX5d, PX5D Review, Forums, 
• Roland Boss: Main site, Micro BR, eBand JS-8, User Group
• Zoom: Main site, Zoom G2, 
• Line 6: Main site, JM4 Looper, Pocket Pod, Pocket Pod Express