This is a fine headphone amplifier. The unusual shape makes it look like a hip flask, which is apparently intended to make it more comfortable to carry in a pocket or strap to an arm. I don't do either, so the shape is nothing more than an artistic statement.

The LEDs serve several purposes, including indicating battery/charging status and headphone impedance selection. There are four slide switches on the end opposite the volume control. These allow you to select headphone impedance (two settings,) gain control, bass compensation, and sound field. This is one of the few amps I have with effective, yet unobtrusive, signal processing. The unit seems to charge completely in about 6 to 8 hours and will function continuously at relatively high output for more than five hours on a charge. I haven't had the unit discharge completely yet, so I have no idea how long it actually will work on the internal batteries.

Perhaps the only irritating feature on this unit is the automatic shut-down in the absence of an audio input. I've actually had the unit shut down when I simply paused my player to use the bathroom. The problem is that you sometimes have to fiddle with the unit to get it to come back on. I've found that I have to turn off my player, turn off the amp, turn the player back on, and then turn the amp back on. If you just turn the amp on and off, it will play for just a fraction of a second and then shut down again.

This is a generic CMoy (Chu Moy) amplifier circuit built in a Red Penguin mints tin. For some reason, building headphone amps in mint tins, especially Altoids tins, is very popular. The basic CMoy circuit works just fine and sounds good, but the amplifier is just not anything exceptional. It uses a 9-volt battery and the battery is not rechargeable unless removed from the tin. The on-off toggle switch is on the side of the tin, where it is subject to accidental movement. Anytime I put this in a suitcase or laptop case, it ends up being turned on and the battery drains. The builder did a very nice job of protecting all the soldered joints from accidental contact with the metal of the tin. The power LED is connected with push-pull connectors, so it can be easily relocated. Overall, a nice job, but not my favorite amp to use or travel with.

It's a slippery slope. You get a decent set of headphones, plug them into your laptop/desktop/receiver headphone jack, and then you wonder why they don't sound like they did in the store. Where are the highs? Why is the imaging so poor? What happened to the dynamics? What's all that background noise and where did it come from? In the case of my Sony MDR-SA3000s, I wondered why they sounded so harsh and fatiguing. It doesn't take you long to figure out that (1) you need better source material, and (2) you need to improve the electronics and amplification between the source and the headphones.

Tube amplifiers are generally esteemed by audiophiles because of their "warm" sound, as they are free of most of the anomalies (usually non-linear performance) that originate from solid-state designs. (This is not to say that there aren't many high-quality solid-state amplifiers that perform exceptionally well.) The Sonys have exceptional definition, and in combination with most solid-state amplifiers they sound strident or harsh. (This is based on my listening trials at a local audio store.) Connected directly to my laptop or desktop, they sounded downright irritating.

This amplifier has a bit of a twist: no volume control. This is not uncommon in audiophile components, as it eliminates a non-linear component and a potential source of noise. I can't argue with this line of thought. This is, after all, the only amp I have for which I can vouch that there is absolutely NO noise when you adjust the volume. The amp works fine and sounds great. I did have one issue with it, though. Due to the impedance mismatch with my Cowon iAudio M5 player, I was getting some excessive noise from the data signal to the M5 remote display. I had a couple resistors added to the input of the amp and that improved the situation greatly, though it didn't eliminate it. (I later found that this happens with several amps, so I just changed the settings on my iAudio player so it doesn't scroll.) This amp uses a 9-volt battery and does not recharge the battery while it's in the amp. The rocker switch for power is located on the side and seems to be less vulnerable to accidental movement than a toggle switch would be. A nice, solid performer.

This Chinese-made amplifier was very cheap...and I found out why when I received it. Due to some miscalculation, the space for the battery is about 1/8" too shallow for a standard 9-volt battery. As you can see from the pictures, if you insert a battery, the cover plate cannot be screwed back on the end. With that aside, the amp works just great. The enclosure is aluminum and very durable. The controls operate smoothly and the graphics are subtle, but legible. The controls and jacks are soldered directly to the circuit board and their attachment to the end panel is the only thing that supports the circuit board inside. It is not mounted to the box at all.

I gave this away to a friend, who happens to have a lot of duct tape.

Very nice amp. Sometimes called the Twinstarr Black Apheared 47, this unit is constructed of top-notch materials and has some very attractive graphics on it. The audio performance is well above average and makes just about any headphones sound great. It uses a 9-volt battery that must be removed to be recharged. It is slightly larger than some of the other amps. The only thing I don't like about this amplifier is the blue power LED. It is unbelievably bright. The people sitting next to you on the plane will complain about it and will suspect it is a laser. I have actually used it to find things in a hotel room so I wouldn't have to turn on the light. It is not just bright enough to read your watch...it is bright enough to see all the way across the room.

Sound quality is exemplary, with good power output even using high-impedance phones.

Somebody put a lot of work into this amplifier. I have not seen any exactly like it in some years, but there were several for sale on eBay over a period of a couple months by ProAudio (I believe). This amplifier is much larger than the others and is constructed inside a very nice transparent blue plastic box. One reason it is so large is that it uses two 9-volt batteries, not just one. It also has a jack for the connection of a 24 VDC power supply (not included) to recharge the internal batteries, if they are NiCad or NiMH batteries. The main circuit board is over 2" by 1-1/2" in size. Other than the size and weight of this unit, the only downside is that the on-off toggle switch is on the bottom (or end) of the unit, opposite the volume control and jacks. This makes it very easy to turn on or off accidentally. If you put it in a pocket, it really needs to sit sideways. The output from this unit is pretty impressive and it will drive just about anything you want it to. Sound is very good, though I occasionally hear some noise in the volume control when I move it.

The toggle switch is definitely a big negative, and so this amp is most often relegated to bedroom duty with headphones connected to the satellite receiver. It works great for this purpose, and the end-mounted switch doesn't cause any problems. The same thing applies if you like listening to CDs or radio in bed.