Demagnetising

[Altec19]

Lets say we're going to apply 1 Watt of power to a speaker with a DC resistance of 8 Ohms.

According to a variation of the power formula:

V = square root (PR)

where

V is Voltage
P is power in Watts
R is DC resistance in Ohms

we have

V = square root (1 x 8)
V = 2.8 Volts

As another check let's say we apply a 9 Volt battery to an 8 Ohm speaker where:

P = V^2 / R

we have

P = 9^2 / 8
P = 10.125 Watts

Is 10 Watts too much? Personally, I wouldn't do it to a tweeter but I'm fine testing a woofer like that.

I would also make just a momentary connection, only enough to verify cone movement and then quickly disconnect.

[Altec Best]

Does driving them hard make them lose their charge?

Not in a home environment unless extremely hard for very long periods,I posed this very question to Bill at GPA a couple years back I thought my woofers were in need of re-charge 416-8B's as you are doing now.Mine were built Feb, '77 and he said my woofers in my model 19's if they were in a home environment for their entire life, they should never need re-charging.He said drivers in sound reinforcement-PA in clubs and venues were they are on tour so to speak are the drivers that typically need re-charging.I said but the woofers sound weak and he said to check the bypass cap in the X-over the 21 uF cap so that's when I rebuilt them.You know the rest

I would say that the bottom end on the 19's, after the recap, is superior to the LS now. More tuneful & more accurate...& certainly plenty of oomph.
From this I would say that they are OK.

Now this is the same exact result I had with mine. My Cornwall II's bass sounded deeper,louder,tighter after the 19 crossover rebuild not anymore "Complete Transformation" If you haven't heard a model 19 in quite sometime you got to hear one with the X-over upgrade,ground lug bypass, completely different pair of speakers performance wise. John if they have lost anything it will be minimal providing your details are accurate that they haven't been driven to extremes.Bill told me that I couldn't drive them hard enough,he said you wouldn't be able to sit in the room (Too Loud)and would turn the volume down way before they lost their magnetism.Now he could have made some money by me sending them in to recone,charge magnet etc.. :2thumbsup: But shouldn't need it. But if I insisted he would do it.

[Cal Weldon]

I would also make just a momentary connection, only enough to verify cone movement and then quickly disconnect.

I think that's the key. Remember you are running DC to it so you're not getting any back and forth to assist in the cooling. Also I think you have to take into account the available current available from the battery no?

[bfish]

I think that's the key. Remember you are running DC to it so you're not getting any back and forth to assist in the cooling. Also I think you have to take into account the available current available from the battery no?

Right, right, and right.

[Panomaniac]

Well we ain't leaving the 9V battery connected to the driver! :khan:

Just a touch will do. Yes, it jumps a lot farther with 9 volts, but it's no big deal. No matter the current in the battery, you'll only get a current flow of I = V/R. The DCR of the voice coil sets the current that can flow from the 9V battery. About 1.4A in this case, or ~13 watts (if DCR= 6.3 ohms).

[Altec19]

I don't recall what the sustained current capability of a 9V battery might be off hand but I'm pretty sure it won't do anything near 1 amp for more than just a few moments. That's not to say that it's safe to leave it connected to a speaker for any duration.

[SOOTSHE]

- ( FWIW ), the proper term here is "re-magnetizing" .
- Demagnetise ( or demagnetize ) , means ; to "lose magnetic charge" .


- Anyways, to answer your question, one first notices a loss of efficiency in the mid-band which actually has the effect of making the bass frequencies a bit more pronounced ( by comparison ) . FWIW, a lot of people may like this change of bass to mid re-balance .

- Additionally, I'd be somewhat cautious about having your alnico woofers remagged . Panos 416-8A dilemma should serve as a cautionary tale about what goes wrong when a tech re-mags alnico if the machine is set to the wrong polarity .

- As a "due-diligence" self check, I'd recommend that one should first verify ( with a 9 volt battery ) the polarity of ones' woofers before handing them over for re-magnetising . If they come back with the polarity switched then ( at least ) you have a good negotiating start point ( to remedy the situation, as was Panos case ) .

<> cheers

Thanks

[SOOTSHE]

Not in a home environment unless extremely hard for very long periods,I posed this very question to Bill at GPA a couple years back I thought my woofers were in need of re-charge 416-8B's as you are doing now.Mine were built Feb, '77 and he said my woofers in my model 19's if they were in a home environment for their entire life, they should never need re-charging.He said drivers in sound reinforcement-PA in clubs and venues were they are on tour so to speak are the drivers that typically need re-charging.I said but the woofers sound weak and he said to check the bypass cap in the X-over the 21 uF cap so that's when I rebuilt them.You know the rest

Now this is the same exact result I had with mine. My Cornwall II's bass sounded deeper,louder,tighter after the 19 crossover rebuild not anymore "Complete Transformation" If you haven't heard a model 19 in quite sometime you got to hear one with the X-over upgrade,ground lug bypass, completely different pair of speakers performance wise. John if they have lost anything it will be minimal providing your details are accurate that they haven't been driven to extremes.Bill told me that I couldn't drive them hard enough,he said you wouldn't be able to sit in the room (Too Loud)and would turn the volume down way before they lost their magnetism.Now he could have made some money by me sending them in to recone,charge magnet etc.. :2thumbsup: But shouldn't need it. But if I insisted he would do it.

No, they have had a pretty easy life. I might only run it up for maybe a minute or two at the most, just to hear that awesome crack....but these days it gets too loud for me. One of the best aspects of these speakers is that I can listen at fairly low levels & still get that big sound with all of the nuances still able to be heard.

From all the comments here & the sound that I'm getting, I'm sure they don't need zapping.

[bfish]

I don't recall what the sustained current capability of a 9V battery might be off hand but I'm pretty sure it won't do anything near 1 amp for more than just a few moments. That's not to say that it's safe to leave it connected to a speaker for any duration.

You made me curious, so I looked it up. At 1A, all the major brands drop instantly to ~7.5V, and can only sustain that output from 5-20 minutes before dying like a drumming bunny in a beagle's jaw.

I like the extra power of a 9V when doing a click test for critical damping.

But the most useful tip I can offer on the matter;

When you're done clicking/polarity checking/whatever, never inadvertantly drop that 9V into a pocketfull of loose change.

[Panomaniac]

:lol: I haven't done that one yet.

I did connect 2 9V batteries together not long ago. They are supposed to get hot and explode. They only just got warm. What a bummer. Maybe they were already dead.