sumsound wrote on Tue, 12 April 2005 20:40 |
David is there some kind of Capacitively coupled fuse set up that blows if High Frequency Harmonics are present from excessive clipping? I was Just wondering how that worked?
Sum
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Hi Sum,
The fuse is just an inline fuse. It is a 6 amp 3AG fuse, which is supposed to blow at 10A for 1 second. (See fuse time curve here
http://www.littelfuse.com/data/en/Time_Curve/312-318.tc.pdf ) It is a very rudimentary set-up and it doesn't guarantee the driver's survival, but in my experience it helps protect against accidental overloads quite well. In other words, when I have accidentally clipped the heck out of an amp and realized that the woofers were silent, I have been very glad to find that the fuses were out and the woofers survived. With regard to my assertion that clipping amps blew the fuses, that was based on the above experience and the following observations: I have used that exact amp/cabinet set up, (as have other customers,) and have never blown a fuse unless clipping was present. (And certainly not six at a time.) Also, through my conversations with these customers it was clear that they didn't know how to set up limiters or compressors, these amps were new to them, and when I visited them, on the system I saw, (which was not the same one,) it was not set up with limiting in place to prevent clipping. The person who actually bought the cabinets being referred to in the post, not the poster himself, told me he had not blown any woofers in many years of being in business. The truth of the matter is that he still hasn't. All the woofers I received back from him still worked. The fuses did their job.
Why the fuses in the first place? Most people will turn down the gain when they hear enough distortion to know that something's not right. The fuses were added because the distortion levels from our subwoofers are so much lower than what people are used to that they hardly ever reach the point where the operator thinks it sounds bad enough to turn them down. With our subs that is usually the point where the amp is clipping badly enough for it to be clearly audible, which is bad news for the woofers. The fuses hopefully save new users the cost of repairing drivers as they learn where the limits are, and also from the occasional slip-up.
The downside is that as the voice coil heats up and the impedance rises, the fuse becomes less effective. We are also looking for the best way to prevent long-term thermal failures. We've tried using the standard circuit breakers and light bulbs or resistors. Everything involves compromises. Some applications obviously need this type of protection where some users might reject it. Your mention of a capacitively coupled fuse led me to the thought of a capacitor and a low resistance resistor in series, wired in paralell with the woofer. If that's what you mean, we haven't done that, but that would tend to blow the fuse when higher frequencies were present. For certain markets that would be a very good thing. Looks like I've got some new testing to do.
Thanks.