Current limiter lamp for horn driver

[Gtr_tech]

I'm installing some small 30w Selenium drivers in a friends crappy PA cabs in place of the piezo wannabe horn drivers. They will be amplified seperately via electronic crossover and Crown D60 amp. What I would like to do is put in some lamps to help save the drivers in the event of feedback from knuckleheads pointing a mic at a cabinet.

Never had to investigate this so not sure what type to use. The ones I've seen in passive crossovers look similar to automotive dome lamps...the old style tubular ones.

Whats the criteria for selecting the correct lame here?

[Enzo]

The ones that look like double dimension glass fuses - PV uses them and JBL among others - are described as SK-3 with end cap. They are also available with wire leads, no end cap. You can get them from PV or find them online, usually as a "JBL Part." I have never found them in the general light bulb market, I only see them as speaker parts, but that's just me.

PV part # 61-0063-0000 (clips p/n 63-0083-0000) If you have any of these drawings, you can see the circuit; In the SP series, SP2X, 3X, 3, 4, 4X, 5X. They don;t just use them alone.

But plenty of old Crate PA cabs and Yorkville used plain old auto tail lights, in the large bayonette base. Get those at Meijer or anywhere. Type 1156. I think they are like parking lights or backup lights. The 1157 is the dual brake light, not that.

I forget Crate model numkbers, Yorkvilles I mostly saw the Pulse series. They used the 1156 in some, but the 1141 in a lot more. Both 12v, the 1156 is a 24 watt bulb, the 1141 is 18 watt.

PV made their add-on SOundguard. It used an 1156 in series with a 5 ohm/50watt resistor. Across those was a polyswitch RXE040. All those SP series speakers use a polyswitch too. The bulb only enters the picture when the polyswitch warms up and opens. Mouser sells the poly, or PV does. Very simple circuit, and was designed to insert between Xover the driver anyway, so might be what you want.


There, more than you ever wanted to know.

[J M Fahey]

Agree and add:
Being outside USA, I just go to the local Service Station.
Tail position lights, labelled "12V 15W" or thereabouts have always worked for me.
Yes, they need bayonet sockets, cheap and easy to find.
I guess the "Pro" ones suggested by Enzo (which doubtlessly is the real thing) once started as some kind of standard car lamp too.
"12V 15W" means they will be happy passing 1 ampere and will try to limit higher currents.
Worst case, the lamps will burn saving the voice coil.
This is just a low cost alternative; of course if something more serious is available, go for it.

Although what you really need is a *very good* limiter before that HF power amp.

[cbarrow7625]

The real ones are HPCCR's (high positive current coefficient resistors). The SK-3 version is rated for 12 volts @1amp. There are other versions (like an SK-2) that are rated lower and better for lower power applications like hf drivers.

They were actually developed for the pro loudspeaker market, I have been told. The biggest difference between them and a light bulb is the linearity. They linearly change resistance as current increases through them. Auto bulbs react very little at low currents and then come more or less all at once when enough current goes through. In an audio application that is much more audible than an HPCCR coming on gradually. Community Professional Loudspeakers also sells them as replacement parts.

[soundguruman]

The DC 60 does not have enough headroom to drive tweeters.
The reason tweeters blow is that they amp is driven to clipping.
The first step in protecting a PA from blowing speakers is to use amps that never clip. Several times the required power.
This is where most PA designs fail. The amp is undersized, driven into square wave output, and the DC blows the speakers.

The second most important, is the crossover frequency.
JBL recommends 800 Hz for many midrange horns. Pure idiocy. NEVER follow the recommended frequencies.
The higher in frequency you can go, the more the horn is protected.
If your 12" speakers can reach 2500 Hz, then your horn should be crossed above 2000 Hz. This will make the horn last 20 times longer,
compared to the "recommended" frequency.

[Gtr_tech]

In this application (a basement jam spot) the d60 will be more than enough...

[soundguruman]

In this application (a basement jam spot) the d60 will be more than enough...

I really have to differ, I don't think there is an amp more likely to burn the high frequency drivers than a DC 60.
In the first place Crown DC amps are infamous for passing DC and frying speakers, when used in PA systems...
In the second place you have no headroom.
In the third place, what is the cross frequency?
And so instead of trying to stop tweets and horns from blowing by adding "more" protection,
then, look at what is "causing" them to blow in the first place.

[Gtr_tech]

You didn't read my post. The whole point of the lamp limiter is to protect the drivers in the event of strong feedback caused by someone pointing a mic at a speaker. The lamp should limit current long enough to kill the mic channel, or kill the dummy who caused it. This ain't a critical FOH application....

[J M Fahey]

Agree and add: a very powerful amp, in the situation described by the OP, will kill tweeters FASTER.
Best option is sill a properly set limiter, but cheap series lamps will certainly help avoid those OOPS!! moments.

[Enzo]

NEVER follow the recommended frequencies.

With advice like that, what is the point of even arguing with him?


Apparently he thinks all horn applications are the same. Apparently the needs of some small cabs and a 100 watt amp are the same as 1500 watt systems. And apparently the only thing we need to protect tweeters from is clipping, ignoring that CLEAN 100 watts is more than happy to burn up a 20 watt tweeter.

Also doesn;t seem to realize that manufacturers like JBL design and make compression drivers for tweeters AND mid-range drivers. One wonders how he might analyze a horn loaded woofer.

[Steve Conner]

It's a lose-lose situation. An underpowered amp can be overdriven to burn out the tweeters. An overpowered amp can simply burn out the tweeters.

Even if using an active crossover, you can still put a capacitor in series with the tweeter to protect it from DC. In theory, I don't know if it is done in practice.

I remember being in a jam session that suddenly degraded into a screaming wall of feedback. As we clutched our ears in agony, a ghostly light began to shine out of the floor wedges. Probably the tweeter protection lamps saving another set of tweeters.