Using fuses to protect loudspeakers?

July 8, 2005 5:07 PM

How can I use in-line fuses to protect the drivers in my studio monitors?

The primary speakers in my small recording studio are Yamaha NS-10M monitors. I've had to replace the treble drivers (tweeters) on these many, many times, and it's a thoroughly miserable (and costly) experience. It's even more miserable when, as is always the case, the damage is as a result of a careless co-worker or client, not observing kindergarten-level studio practices (i.e. turn that channel OFF before plugging something in).

Many times, I've heard tales of people wiring fuses inline with the drivers in order to protect them from the excessive signal that can blow the driver itself - the theory being that, if you use the correct rating, the fuse will blow before the driver does.

I've googled around, and the only reference I've found to this practice is a mysterious equation, regarding the rating that you should use :

I = sqrt(P/R)

where P is the speaker's power rating (watts), and R is its rated impedance.

If anyone here has any experience with this practice, and can possibly point me at suggested fuse ratings, or the kind of components I should be using to wire them in line, I'd be eternally grateful. I'm in the UK btw, should that have any bearing.

Thanks

posted by coach_mcguirk to Technology (8 answers total)

General thoughts about this : fuses take a (relatively) long time to blow - 1hr @ 50% overload is not uncommon. Even so-called "fast blow" fuses are often measured in minutes @ 50% overload.

From what you say, it's transient spikes that are damaging your speakers, either by burning out the coil directly or driving it beyond its travel. Fuses won't protect against this sort of damage.

I'd start looking at MOVs (Metal Oxide Varistors) or Polyswitches. MOVs would wire across the line, protecting the speakers by breaking down and shorting - but probably blowing your amp output stages by overload. Polyswitches would wire in series, going open-circuit to protect your speakers (and possibly destroying your output stage due to transients from the sudden loss of load).

Polyswitches seem to be the preferred method of protecting speakers these days, but some will argue they affect the sound (because they're in effect non-linear resistors, and non-linearity is bad; also, they affect the damping of the output stage to some - though probably minimal - degree).

Myself, I'd try suitably rated polyswitches and see if you think they affect the sound. They're cheap and simple [PDF link] to fit.
posted by Pinback at 9:04 PM on July 8, 2005

With regards to the equation I = (P/R)^1/2, it is a simple derivation from Ohm's law...

V (voltage, Volts) = I (current, Amps) * R (resistance, Ohms)

...and the definition of power...

P (power, Watts) = V * I

Thus:

1. P = VI
2. P = (IR)I
3. P = I² R
4. I² = P/R
5. I = (P/R)^1/2

posted by RichardP at 9:18 PM on July 8, 2005

What about a hard limiter? Properly adjusted it wouldn't affect the sound, right?
posted by Jack Karaoke at 11:27 PM on July 8, 2005

But it may add interference to the signal.

Don't know the answer but I do know that my old Rotel amp has glass fuses in each of the output channels; I have to change them every now and again when I turn the amp on with the volume turned up. Maybe you could find an amp with this feature?
posted by blag at 5:06 AM on July 9, 2005

As you say it is "always" operator error, for a quick fix try and change the behavior while you figure out the technology. Some thoughts:

A big red mute switch in a box, right on top of the console, in line with the speaker wires. Help reinforce the practice: mute, patch, unmute.

The equivalent of a "swear jar" where you put a quarter in each time you drop the f-bomb, but in this case a "blown tweeter jar" where you put in a $100 bill. Again, prominently displayed.

Get a second set of cheaper training monitors. Start out clients or engineers on them. Once they show good studio practices, let them graduate to the NSM-10M. If they mess up, demote them back to the cheaper ones.

The looming humiliation of blowing the tweeters by ignoring the mute switch and the tweeter jar, and suffering the shame of having to mix on an old pair of Alesis monitors for a month might buy you some time to figure out the fuses.

While we are on the topic, Rane has an interesting note about how tweeters are blown here (pdf file).
posted by sol at 5:07 AM on July 9, 2005

But it may add interference to the signal.

If it does, you've other problems. This is exactly the sort of problem a hard limiter was built to solve. Tweeter X cannot handle more that Y power for any amount of time. Hard limit it to Y-1 watts, and Tweeter X lives for a long time.

Given that fact that you have Joe Schmoe using your gear (and treating it as Somebody's Else's Gear), not having limiters is insane. Limit! Big brick walls! Slobs don't care about your gear, so you set it up so they have to really work to break it.

For you and your responsible clients, who know the drill, that's why we have bypass cords and patch panels, right? Set it up so that the limiters are normalled in, but if you need them out, and you can trust the engineer not to lurch the tweeters, you patch em out of the chain.
posted by eriko at 7:59 AM on July 9, 2005

Oh yeah -- something else to look at -- your crossovers. You may be killing your tweeters by sending them frequencies they can't handle, not because they're getting power they can't handle.

It's something to check. A limiter, fuse, or whatnot won't protect a tweeter that can't handle 1000Hz if you keep feeding it 1000hz.
posted by eriko at 8:02 AM on July 9, 2005

Would you like to wager that Yamaha made more money from the original sale of the NS10 series, or from the replacement tweeters and woofers, of which they sell copious amounts? That being said, why don't you hold the clients responsible for breakage and deduct the replacement costs, if any, from their deposits? This is a very common problem with the users of that speaker. Part of that is due to a belief that the woofer must display a "ripple" to show that you have the right EQ in your mix. We have replaced over 100 tweeters and 50 to 60 woofers over the years, and have always had 6 to 8 pairs on hand for immediate swapouts, rather than making the clients wait for replacements. Bottom line: the speaker is popular with mixers and not terribly robust. You pays your dollar, you takes your chances.

We used, for many years, a 400-W per channel amplifier and had fewer problems than when driving them with a 50- to 75-W per channel amp. It seems that they fare better on a high-powered amp, used sensibly, than on a low-powered amp driven to extremes. A hard limiter won't fix the problem because the mixer will hear something different than what's being stored. Putting anything inline that changes the sound is counterproductive and does not make sense. It works ok for sound reinforcement but not in a studio setting. A problem with a professional environment is the lack of "idiot-proofing." The clients must either have the necessary experience to use the equipment or bear the responsibility for replacing it. Good luck!
posted by Lynsey at 12:36 PM on July 9, 2005