digiZoid ZO2 Amplification and Control
There are two basic changes the digiZoid makes to your sound - it makes it louder and it can apply a measure of bass boost. From the manufacturer: The ZO2 is "actually not a headphone amp with bass boost, rather it's an analog sound enhancement device that happens to have high power headphone drivers at its outputs." This means that the output should stay constant as long as you have the ZO2 at the lowest level since the high power headphone drivers shouldn't change the sound - only increase it. This is very hard to test as the change in volume is enough to trick the ear into hearing a difference. At the same time, it is near impossible to equalize the volume quickly enough even if you didn't have to switch the headphone cable at the same time.
That being said, I did try to ascertain if the ZO2 modified the sound when set on the lowest bass boost level, which is supposed to be a flat level. I tried switching the headphone cable from the ZO2 to the player as quickly as possible while changing the volume. With a physical volume button on the side of my Galaxy Nexus, this was easier than it could have been. While non-scientific and certainly nothing that could be objectively verified, I could not hear any difference when when I used my most sensitive headphones - the Pioneer SE-MJ591.
But that's not to mean that I couldn't hear anything. When no content was playing, I did notice a slight hiss when using some of my headphones with some of my devices. Specifically with the Pioneer cans, there was often a slight hiss when using the digiZoid ZO2. It was an easy thing to ignore for the most part as it was completely inaudible if there was anything playing or a reasonable amount of background noise, but it was definitely there.
After extensive testing, it because clear that the hiss was coming from the source and not the ZO2. This points out an obvious problem with a device that amplifies everything. Any noise from the source is amplified as well. With my computer headphone output it was most noticeable. With my phone, for the most part, I didn't hear it unless there was nothing else being played back and I was in a quiet place. In all cases, it wasn't something that bothered me but it is something to consider. With harder to drive headphones like the Denon AH-D1000 I could only barely make out the hiss while the Arctic P402s covered the noise completely. It was only with the Pioneer headphones did I think that the hiss was loud enough that I would forgo the use of the ZO2 because of it. With most of the pairs of headphones, it was either inaudible or inaudible enough for me. What does this mean for you the consumer? Well, if you have a particularly revealing and easy to drive set of cans, the ZO2 may not be for you (if your source is not great). Otherwise, it will probably work just fine. Using the Line-out option may be a workaround as well.
The amplification aspect of the ZO2 should take much of the load off your portable device. This should result in increased battery life of your player. While I didn't test this, it does make sense. The ZO2 easily increased the volume by a factor of two or more (depending on the headphones I was using). This does mean that you will need to make sure your settings on your phone are appropriate for the new level or risk damaging your headphones or ears. Also, once you start using the ZO2, going without it requires you to change your settings back. An annoyance unless you have an app that stores different audio profiles.
The last thing I'll mention is about the user interface. The color-coded display is so hard to read as to be pointless. If the colors between the different levels were more distinct, or if there was some sort of audible or tactile indication that you had switched a level, it may have been useful. As it is, 32 gradations of color between green, yellow, orange, and red are impossible to read. I know that for some headphones I liked the bass boost completely off, with others I liked it around yellowish. Yellowish is not a setting. Since I'm not going to sit there and count my button presses, I don't know exactly what setting I liked for any given set of cans. Even if I did count the number of times I hit the button, without any user feedback, it is impossible to know that each press was registered once, twice, or at all.
I went through all the colors one click at a time and could more easily hear the difference than I could see it from the color. If you are okay with this sort of muddiness of control, don't worry about it. In actual practice, "yellowish" was good enough. But as a high-end device (they like to talk about how people pair their ZO2 with $300+ pairs of headphones), I'd think a little more feedback would be in order. With the color coding of the volume on the Line mode going between blue and purple, I can only imagine how confusing that would be.
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So I can get this one for a whole lot less and use my EQ settings to boost the bass:
FiiO E11 Headphone Amplifier 230-106