| Off the top of my head, an intermittent buzz could be caused by a power transformer in another piece of gear mounted above or below the API in your rack. It could also be caused by a ground loop, but that gets a little trickier to understand how it affects just one channel of a transformer-balanced preamp. It's possible, but not probable. Preamps are very high-gain devices, so it's actually rather difficult to keep all the noise sources out. But that's why you paid all that money, so you should get 4 identically noise-free channels. Just make sure it's not caused by your studio before you try returning it again. |