Speaker Protection

[ambulancevoice]

can someone recommend some speaker protection?
ive seen (like on the on the mac ruby, and zvex nano amp) to eletro caps across the + - terminals
plus, ive seen these things called polyswitches, im not sure how to hook them up, but they look like thin ceramic caps and in the catalog there in, it says they act as speaker protection http://www.jaycar.com.au/productView.asp?ID=RN3460&CATID=&keywords=speaker+protection&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

[runmikeyrun]

I would assume the polyswitches are in series with the speaker somewhere, but you'll have some math to do to find out what kind of current your amp is producing and exactly how much you need to shut down at.  It looks like those are pretty low wattage and meant for tweeters, so maybe someone else sells bigger ones, i don't know.  If they do i have a feeling they'll get expensive as you get up into the 50-100 watt range.

If you use a cap in parallel across your + and - terminals you're basically bypassing AC signal instead of it going through the speaker.  The bigger the value you use the more high end you will shunt.  Too big a cap and it will bypass everything.  If you're worried about excess low end killing your speakers, then you can put a cap in series w/ the speaker somewhere and it will act as a 6db/octave high pass filter and block more or less low end depending on the value.  An L-C network, depending on the complexity, will increase the cut to 12, 18 or more dB/octave.  There are many many online calculators if you google LC crossover or LC network.  (LC uses a capacitor and inductor).

Some cab manufacturers have used incandescent light bulbs inside the cabs (not sure if series or parallel w/ speaker load) but that only pulls down RMS power, not the peaks that kill speakers, especially tweeters.  So not sure of your application, maybe a light bulb will work.  It looks neat if you have a ported cab or speakers with foam surrounds, on a dark stage you can see a light pulsing from around the edges of the cones or inside the port as you play.   

[ambulancevoice]

ok thanks man for the help

[R.G.]

Protection has always been a racket. (Sorry - the O. Henry Pun-off is held in Austin every year.  )

To protection something, you have to know what you're protecting against, and how close the unusual event which you wish to protect from is to normal operation. If the normal operation is nearly the same as the death-threat to the device, then splitting the margin between normal and overload is tricky design work.  If the two evens are widely separated, it's easy.

What kills a lot of speakers as opposed to tweeters is DC failures in the power amp driving them. If the power amp turns from an AC source to a DC source, most large speakers turn into smoke generators. For this purpose, a lot of pro gear has detector circuits that sense DC levels over X and turn the power off to the power amp, hoping to protect an expensive speaker since the amp is presumably already in need of repair. There are various ways to do this, and I would need about thirty minutes of typing to list them all and the pros and cons.

Fuses, light bulbs, PTCs, relays, and all kinds of things including having Icelandish virgins throw salt over their left shoulders at midnight at a crossroads have been tried.

I personally like my own variant. I use one of the variety of DC sensing circuits, and have that turn off the large power MOSFETs which are in both power supply rails to the power amps. This gets both sides of the power supply turned off at the same time (which is GOOD!) and does it much faster than a relay can disconnect speakers. There's a write up on GEO, as there is for most things.

NOTHING will protect a speaker against an honest signal that's just too powerful for it. That's how tweeters die with distorting amps. The distortion makes huge amounts of treble, which is then - properly - routed to the tweeters, and kills them.