This equalizer is extremely useful if you are trying to listen to -- say -- tapes of old time radio, shortwave broadcasts, or noisy AM signals. You can reduce harshness, improve bass response, and reduce noise effects.

However, it does have some limitations, related to its low price. At $20 retail price, such a helpful and flexible gadget will not have perfect audio performance, and it doesn't. There is a SLIGHT hiss audible at all times, apparently intrinsic in the audio chip used for the headphone amp. The hiss is introduced after the volume control; it's there all the time. Depending on the input voltage of the device to which it's connected, the hiss can be as loud as -30 dB, or as soft as -50 dB or lower; and it is predominantly a very bright hissing. So extremely efficient, bright headphones may exaggerate it. I hope Koss improves this device in a future model; but for low fidelity source material, and with phones that are not extremely bright, the hiss introduced by this device is MUCH, much lower than the hiss intrinsic in a tape, radio broadcast, or old recording and will quickly be ignored. If, however, you are hooking this to a CD player to listen with excellent phones to wideband digital recordings, you'll notice the hiss -- and be annoyed, depending on your sensibilities.

I am glad I purchased one. My intended use was to listen to cassettes of old time radio shows. Many of the commercial tapes are not well corrected and sound extremely harsh and thin; this equalizer fixes that quite nicely. The bands are well chosen to have an aural effect where poor recordings have flaws.

For background: I am a retired audio engineer with three decades in broadcasting and recording, and an enthusiast of OTR and recordings of old classic performances of music from the fifties back to the twenties, many of which NEED help that this equalizer can provide.

Hiss is more than slight (Rating: 2.00)
Review : Granted the product is very cheap, but I decided to try it out. It is pretty noisy, about like a old cassette tape with no dolby. If you listen to rock and pop, you probably won't notice it that much. The 3 eq bands do what they should and help warm up the the smaller ear buds and phones.

horrible product DO NOT BUY!! (Rating: 1.00)
Review : This e-band stereo equalizer is horrible. There is a heavy hissing sound whenever you try to listen to music through it. For example, you can use it to enhance the bass on an mp3 player, but all you get is a loud hissing which totally ruines its purpose. Don't waste your money! It's just not worth it.

Not bad for its price. (Rating: 4.00)
Review : Overall it is pretty good for what it costs and what it does. Granted, it isn't audiophile quality, but it does make a good "beater" headphone amplifier/equalizer. It adds enough power to push some higher resistance headphones to decent levels, although it probably wouldn't be very satisfying with any headphone that costs over 50$. That being said, I give it a 4 because it does a good job for its cost, and makes a good headphone amp/eq for general purpose and active use.