Circuit Breakers


2005-05-17 4:44 pm
I am going to be using 5 "TangBand TB-871S" in my surround sound setup with a Panasonic XR-50S receiver and a Lightning Audio P2.10.4 subwoofer. Because the receiver emits 100 WPC (watts per channel), and the rms power is 12W and the max power is 25W, on big loud scenes, I don't wanna go blowing the drivers. I was wondering how to calculate the RATING IN AMPS that I need on the breaker to keep power under x watts.
I have seen commercial (sound reinforcement) speakers that use automotive light bulbs (i.e. 1157) in series with the speaker (I think on the far side of the cross-over), to prevent them from being overdrive by the amp.

If I understand correctly how this works, a 23W lightbulb would limit the power to... 23W.

You can also look inside these speakers to get an idea of how close they are to their operating limit. The more they're lighting up, the more power is running through them. Full brightness would be when they are limiting the current.

Of course, I might not fully understand how those boxes were built. I saw them a LONG time ago.

Caveat: If you use this method, make sure that your cab holds spares. Finding spares when they blow at 11pm in a small town can be challenging!

the advantage of the light bulb is that when cold the resistance of the filament is low and as they heat up the resistance rises substantially.
When you run at sensible volume they have almost no effect and suddenly limit when you overdrive due to the resistance increasing and causing more self heating effect.
You will have to try different wattages and voltages until you find the best combination of sound quality and protection level. Try 12V 5W and 12V 21W to start with. 24V truck bulbs might be an option and there are plenty of halogen bulbs from 10W to 100W all at 12V (old 60W/55W headlight bulbs with blown dip filament cost nothing).
Andrew -- that's a neat observation.

I just realized -- the light bulb doesn't limit the power output to the same value as the bulb wattage; it effectively functions as a clipping detector/limiter right?

As the audio signal gets more compressed and shifted due to clipping, the light bulb will glow more, and it's resistance will go up.

The corollary to this is that fast, high-energy transients will get let through without any significant resistance at all. Quite suitable for music!

Do I understand correctly?

For what it's worth, I searched my memory and I'm pretty sure that the bulbs used in the speakers I'm thinking of (HUGE speakers, two filled a 2,000 sqft room to very loud levels) were 1156s. IIRC these are 23W 12V automotive bulbs.

For the home designer, the 1157 socket style is probably a better [single] choice, because you can use dual-filament bulbs, providing a much wider selection of wattages. As you pointed out, there are 5W bulbs available in this form factor, but I can't remember the p/n... 2397, maybe?


If I use a 23W bulb in series with, say, a 100W driver... I would be able to pass through normal unclipped music signals, because the bulb is cold and allows fast transients (e.g. kick drum).. but if the total energy starts to get "constant" (as in the case where clipping starts to compress the signal), the bulb lights up, and we limit the power to 23W?

Basically, yes. The bulb will limit to a bit less than 23W, but it't not worth trying to calculate exactly what. The point where the bulb starts showing increased resistance but only dimly glowing will be a factor as well, it might be that it impinges too much. Try it and see is the only way I'm afraid. But beware this might cost you a set of drivers!