Koss EQ50 3-Band Stereo Equalizer

My setup is Etymotic ER4P earphones (27ohms), Treo755p mp3 player running PocketTunes v4.0.3.
First I merely plugged my earphones into the EQ and did not connect it to the Treo yet. I turned on the EQ and immediately heard hiss and a bit of static. If I dropped all the sliders down to the bottom then the hiss and static where gone. If I moved all the sliders to the top then the hiss and static were amplified. I held the EQ in the air and moved my arm around... it picked up some slight feedback once in a while (thus it is poorly insulated).
I then connected the EQ to my Treo and played a few different songs in the genres of techno and rap. I began with all the sliders on the EQ at the bottom position and the same for the PocketTunes eq sliders.
When the song starts you can no longer notice the hiss/static at all with these slider settings, but the moment you move any sliders up then you get hiss/static and sometimes distortion in the song. This means you cannot adjust anything. Also in keeping all the sliders at the bottom position there is no volume amplification taking place.
After I disconnected the EQ and set it aside, I plugged my earphones directly into the Treo and selected one of my PocketTunes eq presets and appreciated just how wonderful the sound was before I ever got involved with the Koss EQ50!

UPDATE: I just bought the Boostaroo Revolution Amp... amazing! See my other review about it.

I am not sure how anyone can give this product more than 1 star. It is absolutely horrible. Extremely noisy. You hear hiss noise more than music you play. Worse thing is that the noise does not get less when you turn the volume down (and turn mp3 up instead) as someone said above. It also grabs all sorts of noise and interferances around including your hand when you grab it. It is bigger than many mp3 players with no clip on it. The equalizer part works however in general it is useless due to the noise. This product should be definetly taken out from the market.

Some headphones--particularly the in-ear type--benefit from extra EQ. Portable music players often provide a bass boost, but some situations--like noisy airplane cabins--dictate more volume in the mid-range.

The Koss portable EQ's provide this. I have owned (and just lost) the predecessor to the EQ50, and I was glad to find a replacement on amazon.com. I had not seen the portable EQ in a store for several years, and I assumed they had been discontinued.

The new EQ50 works well. The location of the on/off switch has been improved (the predecessor was easy to switch on inadvertently, thereby draining the batteries). I only wish this one also had the predecessor's balance slider.

If a portable EQ is what you want, this is it. Also, it is probably the only game in town.

As we all know, most portable electronic devices come equipped with substandard-sounding headphones. This little gadget greatly enhances the sound quality in a cheap set of headphones, so you can imagine what it can do for a good set. I've been using this EQ for over three years now and frankly, I can't listen to music without it. For the price, you just can't beat it! Yes, it has a slight hiss but once the music starts, it seems to disappear entirely. It's etremely light weight and it's built to last. I bought another one just in case my first one would break but it's still going strong! If you're a headphone buff like me and have a discerning taste for quality sound, this little EQ will satisfy you immensely! Highly recommended!

I use the Koss with my mp3 player and a small 2.1 speaker system at my desk at work and it's well worth the $20 price tag. The EQ does a great job of adding extra kick to the sound and the settings are quite responsive. There is a slight hum, but this can be eliminated by turning up the volume on your player and turning down the volume on the Koss EQ. I only have a couple of complaints. As said in another review, since it's designed to be used with portable players it really needs a belt clip. Also, there is no option to hook the Koss up to a wall adapter, so I must recommend using rechargeable batteries. The 20 hours that Koss says you get from a set of batteries, though, is lowballing it. I've gotten over twice that much time from one set of rechargeable AAAs before I had to recharge them. This may be due to the fact that I keep the volume on the EQ low in order to eliminate the hum. To make a long review short, you can't go wrong with this product.