RCF ART300A speaker

The horn is DC coupled ? Why the hell would they do that ? Sometimes I wond er what kind of drugs these enginers are on.

I was on audiokarma the other day and someone mentioned a protect circuit t hat wa powered by the DC from the amp. Not hard to figure out how it works, it is a matter of finding a relay, normally closed of course, that will ac tivate at a low enough voltage and current. Couple resistors going to a bri dge and a filter should do it.

I commented about finding a suitable relay, someone replied that it has bee n done and I might be abe to find the link if anyone wants it.

Another thing was about the magnets on the sides of the relay, remember tha t thread ? you said that it was to draw the arc away from betweent the poin t so they would be less likely to weld themselves together due to the induc tive CEMF.

I posted that a cap, like a 10 uF or whatever, across the cntacts should wo rk just fine. The guy on AK said it might still pass some sound and I repli ed in the who cares ? you only care about that DC. If the amp is still putt ing out sound with a shitload of DC on the output, well isn't that just dan dy. So what ?

My Phase Linear 400 Series 2 has no relay, no nothing. My buddy's Adcom GFA

545-2 is the same. Both of these amps have plenty of current to smoke thing s. an Ampzilla is the same way. Depends on a fuse. Shit really.

At any rate, with the right relay there could be a decent addon protector f or this.

I also happen to like SCR crowbar protection. I like the SOUND of "crowbar protection" actually, and how it works. I designed a bridged car amp a loin g time ago used that. Of course must short out the drive to the outputs whe n it is active, which it did. If any fusibles blow, then something is short ed for sure.

I said at the time "This baby is more than idiot proof, you hook up the spe aker outputs to the car battery it will burn the wires off.

I like cool shit like that. The UL or CSA or whatever might not, but I do.

** Most are like that and a real lot have LM3886 amps driving the horn dir ect.

that wa powered by the DC from the amp. Not hard to figure out how it work s, it is a matter of finding a relay, normally closed of course, that will activate at a low enough voltage and current. Couple resistors going to a b ridge and a filter should do it.

** The relay MUST short the speaker to protect it and that means the contac ts are normally open. Since the coil is not polarity sensitive, there is no need for bridge - just some RC filtering and a fuse on the amp side.

hat thread ? you said that it was to draw the arc away from betweent the po int so they would be less likely to weld themselves together due to the ind uctive CEMF.

** Nope, it is to help break the arc that forms when contact open under hea vy DC current and voltage. 40V and 5 amps are the limit for most small AC r ated relays - adding a suitable magnet might increase that to 50V and 10 am ps.

work just fine.

** No it wont.

... Phil

the feed to the -15V supply did the job. The amps test just fine.

iac crow bar would have saved the woofer. A couple of dollars worth of comp onents.

** I have now added a 20uF, 250VAC cap to the horn feed and an active rela y board to the woofer amp. The NC contacts short the speaker while unpowere d and open about 2 seconds after AC power is applied, the NO contacts then close and the amp is connected. If more than +/- 9VDC is detected the relay drops back to it's rest condition. There is a 7Amp fuse in series with the relay contacts in case of an arc developing to ground from the 80V rail.

... Phil

Not what I would've done but sounds good.

Now how do you write tyhe bill ?

Really, when you modify something there is no price schedule. I now I only charged rtwenty bucks to wrap a flyback for an HK short in the CRT, but tha t was after a couple of basic charges that brought it up to a livable level .

Time and materials ? Alot of machne shop shit is judged that way. Not all, they have a couple different ways of doing it.

Here's a freaky one. How to figure out what to charge for a modification :

In the GTE Sylvania E 4X series chassis of color TVs there was a chip witht he part number of 15-43301. this was custom or made by them and only for t hemselves. I supported a countdown circuit for line sync and frequency.

Well a bunch of them even brand new werr coming out with no sync. You got t he picture but it was floating out there, and nothing really fixed it. Well I hade a look at three of the pins and with the resistor, cap and whatever thee, determined that I could probably just put a transistor there to dete ct the sync. It worked.

Then I figured that this is what Sylvania had done,to simply put that trans istor inside the package with the IC. I know people who lost a shitload of monet for not being able to get good ICs like this, if I had done this a fe w years earlier I could have made bucks and kept the landfills a bit cleane r.

** Yep - always tricky to put a price on designing a custom modification that fixes a nasty problem the maker could not be bothered fixing. I simply had to fit the mods cos the dumb fault that fed full rail DC to the speaker voice coils just might return.

The owner has now seen the work and was happy and he has another 11 of them in constant use.

In this case, my bill is well less than the combined bills for re-coning the woofer and a new horn diaphragm - which I estimate will exceed A$400.

... Phil

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Price I guess. I don't work on all that many music amplifiers, but I don't recall ever seeing one with speaker protection relays installed. In the c ase of home audio, they do provide protection because outboard speakers can be pricy (much more expensive than the amp), and folks wouldn't be thrille d to see their amp take out their speakers. In the case of a music amp, th ey're self contained so I guess manufacturers figure they can sell new driv ers if they blow.

As for mods, I've done my share over the years. Technically, we're not supp osed to make any modifications unless the manufacturer provides a procedure in a bulletin. From what I heard years ago, the manufacturer is supposed to submit the mod to U.L. (in the States) for approval before it's allowed, but I don't know if that's a fact.