I've had several requests for this - I usually ignore them, but I'm feeling magnaminous. Bolding is mine:
Saw Satelliteguy's post above and thought this might be a good time to talk about burn-in.
All equipment, tube or solid state, needs to burn-in when it's new before it reaches peak performance. Some gear takes longer than other. Of all the parts inside an audio component, capacitors are the primary item that need burned-in. Specifically, the film and the dielectric that make up the capacitors have to "seat" before the time constant of the cap becomes stable. This process is the result of using the cap, and cycling it on and off for extended periods of time. The best approach is 5 hours on, and 5 hours off, repeated at least 5 times. This brings the caps up to temperature and then allows them to return to room temperature. This expansion and contraction accelerates the process.
Some caps take longer than others, for example, polyester film caps take much less time than polyurethane film caps (found inside the Zenhead) but polyester film caps, while smooth and forgiving sonically, do not have the resolution or focus of good polyurethanes. As a basic rule of thumb, the better the capacitors used, the longer they take to burn-in, and the greater the sonic reward.
Once caps (and other parts) have burned-in, they stay that way. The initial burn-in process does not have to be repeated for the life of the unit. However, if you let an audio component sit unused in the closet for several months, you will find it takes at least a few hours to get with the program.
Frequency balance, dynamics, clarity, focus, and sound stage are all effected during the burn in process. Each of these aspects will go from good to bad on independent cycles during the process. This is why something brand new might sound great for about 30 minutes and then become confused sounding shortly thereafter.
When Satelliteguy used a pink noise disc to burn the unit in, he unknowingly trained the caps to favor pink noise, which is not music. So on music, the unit sounded less than stellar. There are many burn in CD's available these days that claim to accelerate the process, and while they do accelerate the initial stage of the process, they will actually delay or even prevent the final stage of the process from happening. There is no substitute for real music when burning in an audio component, even it takes a bit longer. Be sure if you use some magic burn-in tool, that you finish the process with real music.
The Zenhead needs at least a couple hundred hours to get through the burn-in process. So does almost any other audio component that I've ever owned. The reason it seems like a big deal with the Zenhead is because of the transparency of the design. By the simple fact that it IS so transparent sounding, the difference between being burned-in and not burned-in is DRAMATIC. An identical product that was not as well implemented and with half the transparency would logically have only half as much difference between being burned-in and not burned-in. Or appear to take only half as long to burn-in.
Hopefully this helps people understand why things need burned-in and why the results vary.
Steve Deckert / DECWARE