Compression Driver Questions

[Shadrock2]

I used to run sound for several working bands in the "80's and early 90's.
I quit that for awhile and pursued my own music.
Recently I started running sound for another working band and decided that I could use my vintage pair of Altech 908 drivers attached to the 511b segmented horn with their PA. But alas one of the drivers blew a diaphragm on the first gig I tried them. It did not fail because it over heated but the winding on the diaphragm came unglued on a short section of the former and suffered metal fatigue and opened up. It had received heavy weekly or even daily usage for about 5-6 years and then occasional use in my basement and had worked fine for a long while.

I have always use a simple series capacitor as a crossover.

So genuine replacement diaphragms do not seem to be available and even the third party ones say from Radian are rather pricey.

Has anyone ever successfully rewound a diaphragm voice coil?
It almost looks like it would be possible.
The old diaphragm used edge wound aluminum wire that was copper clad.
Is it fantasy to think that round copper wire could work in its place?
It is only 28 - 30 turns depending on the accuracy of my turn counting.
The ribbon leads are still intact and I might be able to solder to them.
What kind of glue would one use to fasten and seal the winding.

I have a little mini lathe that I could use to turn the old diaphragm.

When installing new diaphragms I see that one should attempt to align it.
To do this I read somewhere that you would run signal to the diaphragm and move it around in the slot such that there is no extra buzzing. Is this feasable for a normal human being. There are no locating pins on my driver but the mountin screws are a very close fit and don't allow for much lateral movement.

I'll have more questions later and I am not in any particular hurry to resolve this right away.
Any insight that someone might have on this subject is appreciated.

[Enzo]

In my own opinion, I'd call it fantasy.

[The Dude]

Pricey?

Recone Kits - Search

or this with free shipping

Altec 8 Ohm aft Diaphragm for 604 802 806 808 902 908 Drivers Free Shipping | eBay

[Shadrock2]

Thanx Dude!
That is certainly better than the $91.00 ea. from Parts Express.

You are probably right Enzo. but there are still some potential issues for installing a new diaphragm.

[The Dude]

Usually on those, there is enough slop in the gap to allow for no locating holes. Still, my usual method of installing a new diaphragm is this.

1) Remove old diaphragm.
2) Fold masking tape in half and insert into the gap and clean out any debris. Then use canned air to blow out any remaining junk. Inspect with a light to make sure there is no debris left. If you leave anything, you'll ruin a new diaphragm.
3) If the diaphragm has ferrofluid and you removed it via cleaning, "refill" gap as necessary. (not the case in your instance)
4) Carefully install the new diaphragm being careful not to damage the coil or coil form, but DO NOT TIGHTEN THE SCREWS ALL THEWAY.
5) Apply an audio source to the driver and listen while rotating/wiggling the diaphragm on the magnet structure until you hear no rubbing or distortion.
6) Tighten the screws.

[J M Fahey]

I used to run sound for several working bands in the "80's and early 90's.
........
Recently I started running sound for another working band and decided that I could use my vintage pair of Altech 908 drivers attached to the 511b segmented horn with their PA. But alas one of the drivers blew a diaphragm on the first gig I tried them.

I told you
No, I didn't , but would have if you had asked.
I also come from an earlier age, and power levels involved simply were WAY lower.
An Altec A7 , original or clone, would have been driven by a 60W amp ... which if tube would have been closer to 50W undistorted; ..... today? ..... easily 200W RMS or worse.

It did not fail because it over heated but the winding on the diaphragm came unglued on a short section of the former and suffered metal fatigue and opened up.

Agree: a simple capacitor is way too "slow" , as in not fast enough cutoff, allows a lot of undesirable lower frequencies through, no need to reach 50Hz, just a bit too much @300Hz will overflex it and cause mechanical damage ... or fatigue, which is a given on aluminum parts.

Replace it with a proper crossover, at least 12dB/oct and preferrably 18dB/oct AND also add a lamp bulb current limiter in series with the diaphragm itself, a car tail or courtesy lamp 12V 15W is fine, your choice of base and socket.

Also, if possible choose a 50% higher crossover frequency, your driver will love you, as in 500>800Hz, or 800>1200 or 1000>1500Hz , you get the idea.

Has anyone ever successfully rewound a diaphragm voice coil?
It almost looks like it would be possible.
The old diaphragm used edge wound aluminum wire that was copper clad.
Is it fantasy to think that round copper wire could work in its place?
It is only 28 - 30 turns depending on the accuracy of my turn counting.
The ribbon leads are still intact and I might be able to solder to them.
What kind of glue would one use to fasten and seal the winding.

Well, I have, but only as a desperate stopgap measure until somebody could get in contact with an Air Hostess who was willing to go to downtown NY from the airport and buy one ... easily U$150 or more plus up to 3 months wait
It works, sort of, unless you hear it side by side with an original one in which case difference makes your ears cry
Wire is hair thin, proper glue is Industrial Epoxy, the kind which takes 8 hours to overnight to cure, and preferrably inside an oven, I used a heat gun while winding and then let the still sticky coil in the Sun if by day, or near a 60W table lamp if by night or on bad weather.

Not a job for the fainthearted .

When installing new diaphragms I see that one should attempt to align it.
To do this I read somewhere that you would run signal to the diaphragm and move it around in the slot such that there is no extra buzzing. Is this feasable for a normal human being. There are no locating pins on my driver but the mountin screws are a very close fit and don't allow for much lateral movement.

If you have an original Altec driver and an original or well cloned replacement diaphragm (+ coil), either it has a well machined mounting ring which tightly fits a guide circular slot turned on the main assembly or it has 2 or 3 (don't remember now) mounting pins which tightly fit in prioper alignment holes (or viceversa, pins in frame, holes in diaphragm, it's the same idea) so you can not, and do not need to center by ear or otherwise.

The centering by ear, which I also use, applies to much cheaper turned sheet metal PA horns , think sports fields or racetracks or large outdoors in general, where unmounted diaphragms are centered by driving them straight from a cheap AM pocket radio until sounding best.
Yes, full range, we need the lower frequencies to make poor centering more apparent.
No, the 200mW output power won't damage them at any frequency.
I saw the suggested $33 replacement diaphragm: go for it.
You should even at $91, don't tell me you'd junk an original Altec driver to save what a couple Pizzas cost

But please protect bthem as suggested above.

[Shadrock2]

$91.00 is pricey if I can buy a whole PRV driver from Parts express for about $40.00 ea. Probably not as good but might be hard to tell the difference. At $33.00 (TY Dude) all that rebuilding stuff becomes moot.

They had big power amps in the '80's. I used that horn with a rig that consisted of a single scoop JBL 2225 loaded bass bin with a sealed JBL E-140 for mid's and that horn with a 3.3 uF cap in series. One stack each side. All was powered by 1 BGW 750c power amp which I believe was rated at about 250W/ch @ 8 ohm's. Simple rig... worked well for more than 200 gigs. I did blow the diaphragms once when the lead singer stuck his mic in the exhaust pipe of a running Harley during "Born to be Wild". When they blew that time I had a 4.7 uF motor cap in series and reduced the value after that incident. I didn't know about diaphragm alignment then but I was careful to clean the slot out.

JMF... you did anticipate my next question though. I was going to ask about putting an automotive light bulb in series with the horn driver. Someone had suggested that a long time ago.
I decided that a 211-2 type bulb should work. Used as courtesy light in Toyota's. These will just barely fit into a 6mm fuse clip. If someone can source a better socket I'm all ears.

I had shied away from using a more complicated crossover. Mostly because because I want to use the horns as an add on and am concerned about phase issues. And also because the math gets more complex in a hurry.

[Leo_Gnardo]

I had shied away from using a more complicated crossover. Mostly because because I want to use the horns as an add on and am concerned about phase issues. And also because the math gets more complex in a hurry.

Parts Express has some very nice passive XO components & kits PLUS tables/charts so you don't have to get lost in the math. I fully agree with Juan, raising the frequency on the horns a very good idea.

FWIW I run a 3 way system, active XO, bring the horns in at 3 KHz. Nice & crisp top end, never blew a diaphragm. For protection against an amp that might fail DC, a 10 uF 250V film cap in series with the horn driver.

One thing you must understand in a passive XO system, when you clip the power amp, that creates a lot of high frequency energy which the passive XO happily transmits to the horn driver. You'll hear a lot of "clack" then the voice coil opens up. Who needs that? Steeper cutoff curves (12 or 18 dB) reduce this effect. With just a cap to "stop" low frequencies you have 6 dB/octave, OK for a cheap home stereo speaker, not so much for a pro PA where you have to have reliability besides good sound quality.

If you elect to use light bulb limiter on your horn, keep in mind the lamp can get very hot when it's being driven hard. Community uses lamps as overdrive protection in their PA speakers - in some cases I've seen the circuit board get charred - and one fine day in Augusta GA (15 March 1992 in fact) set the PA cabs on fire. Best to think ahead, use ceramic lamp connectors & plenty of space around any lamps used as current limiters.

[g1]

I decided that a 211-2 type bulb should work. Used as courtesy light in Toyota's. These will just barely fit into a 6mm fuse clip. If someone can source a better socket I'm all ears.

That's called a festoon base, the 211-2 uses the 42mm length. A socket/holder should be fairly easy to get from an automotive supplier.
https://www.superbrightleds.com/more...b-sockets/242/

[Shadrock2]

Thanx G1 I have been looking for those. Did not know they were called "Festooned" sockets. I looked at that exact site but could find the sockets.

I'll draw up a schematic for my horns that includes a 12dB/octave XO and the light bulb and post it for your approval.
Might not be until Sunday though.

[Shadrock2]

I have ordered a pair of new diaphragms for the 908 drivers along with some festoon sockets and 10 more 211 lights.

So next I am trying to design a high pass crossover for my drivers and I have some conflicting parameters to talk about. When I bought the horns new, I was under the impression that these drivers were good for 40 watts and that the crossover point should not be lower than 1200 Hz. But I found a spec sheet online that has some different specs. I don't really think I want the XO point lower than 1200 and used and online program to set up 12dB high pass filter @ 1500 Hz, 1800Hz, and 2400Hz, with some extra's. The basic schematic is attached. Also, the jpg of the data sheet I found. I should have the 30yr old spec sheet that I got with the horns somewhere but I can't put my hands on it. Anyway, the key specs are listed below:

150 watts continuous program 1 kHz to 20 kHz
80 watts continuous program 500 Hz to 20 kHz

75 watts (25V) pink noise band limited 1000 Hz to 20 kHz
40 watts (17.9V) pink noise band limited 500 Hz to 20 kHz

So if I read JMF's post right I should use a 1500 Hz XO?
And the... specs for C & L vary quite as bit from depending on which filter type.
I would like to hear your opinions on the merits of the different filter types.

1500 Hz
Butterworth L = 1.2 mH C = 9.375 uF

Bessel L = 1.47 mH C = 7.6 uF

Linkwitz-R L = 1.7 mH C = 6.63 uF

1800 Hz
Butterworth L = 1.0 mH C = 7.8 uF

Bessel L = 1.23 mH C = 6.33 uF

Linkwitz-R L = 1.42 mH C = 5.53

2400 Hz
Butterworth L = 0.75 mH C = 5.86 uF

Bessel L = 0.919 mH C = 4.75 uF

Linkwitz-R L = 1.06 mH C = 4.15

[Shadrock2]

Follow Up

So I order a couple diaphragms from a place called "Simply Speakers". They look just like the picture on the web site and just like the ones pictured on the E-bay link provided by The Dude. $40 ea. but free shipping for me through Amazon. They claim to fit a wide variety of Altec Horn Drivers. And they fit my 908 drivers... but only with some modifications. First of all, the original diaphragm was 81mm Dia. and set in a machine well that centered it on the magnet. The new diaphragm is 75mm in diameter and needed to be centered. Secondly it is fastened to the magnet with three screws but the bolt circle diameter is about .063" smaller than the original and the holes needed to be re-drilled/enlarged. I used the rear plug as a drill guide and a cheap drill press that uses a hand held drill to redrill the holes. This worked because the rear plug fit snugly in the plastic ring of the new diaphragm and centered it.
I wish I had pictures but I don't have that capability at this time.
Finally, the electrical terminals on top of the diaphragm where incorrect for my driver. The rear plug needs to set flush with the plastic ring and the top mounted screw terminals prevent that. I could have notched out the original rear plug but I was loathe to do that. I instead removed the screw terminals and filed down the raised lands that they were mounted on so that the plug mounted flush on the plastic ring. I soldered spade lugs to the screw terminals, re-soldered the ribbon wire from the voice coil so that it was shorter and then epoxied that to the side of the new diaphragm. That way the spade lugs set approximately where the lugs of the original diaphragm were located.

I used my signal generator to drive the new assembly and wiggled the new diaphragm around to get the cleanest tone possible. About 800mv sine wave at 400Hz and 1kHz. It was almost located in the right place just from the screws.

I see my original post didn't display my XO schematic drawing so here is a new one.
Crossover _Highpass2_Basic.pdf
The calculated XO point is 2100 Hz. Linkwitz-Riley.

I could not come up with ceramic Festoon sockets so I mounted the plastic ones with a small strip of brass to act as a heat shield.

Connected the horns to my PA in the basement with new XO and light bulb limiter/fuse and sang/yelled/screamed to test and they worked ok. With my highest pitch scream I could just get the 211 bulb to light up. But my power amp is only 120 watts into 8 ohms. We will see Saturday how they work with the Crown XLS power amp at the gig.

[Leo_Gnardo]

So I order a couple diaphragms from a place called "Simply Speakers". They look just like the picture on the web site and just like the ones pictured on the E-bay link provided by The Dude. $40 ea. but free shipping for me through Amazon. They claim to fit a wide variety of Altec Horn Drivers. And they fit my 908 drivers... but only with some modifications. .....

What a lot of work to mount the bargain diaphragms, whew! I wonder if you'd have to do all that with Radian replacement diaphragms.

Good on you getting better crossovers & best of luck at the upcoming gig. I expect festoons won't turn out a lot of heat. The lamps in those Community boxes that caught fire were high intensity, needed about 4 amps to start to glow & about 8 amps full brightness, with 4 lamps in parallel. That adds up to a lot of heat.

[Shadrock2]

The picture of the Radian driver shows the top mounted screw terminals. But it also shows a larger plate at the bottom of the plastic ring.

I did a little study on the light bulb with my adjustable power supply. Light bulb in series with 1 0hm 1% resistor.

Resistor Volts Bulb Volts Bulb ohms

0.668 (First glow) 2.7 3.98
0.850 5.3 6.24
0.900 5.8 6.44
0.950 6.93 7.29
1.000 8.04 8.04
1.05 9.00 8.57
1.10 10.08 9.17
1.15 11.14 9.68
1.20 12.3 10.25
1.23 13.3 10.81

At 1.23 amps the bulb filament didn't blow but the end cap came loose and the bulb was indeed very hot.
That was max volts for my DC supply.

[The Dude]

......Connected the horns to my PA in the basement with new XO and light bulb limiter/fuse and sang/yelled/screamed to test and they worked ok. With my highest pitch scream I could just get the 211 bulb to light up. But my power amp is only 120 watts into 8 ohms. We will see Saturday how they work with the Crown XLS power amp at the gig.

2x120W feeding a pair of horns crossed over at 2.1k should be able to tear your head off. Those things are rated at 150W continuous if you crossover at 1k (not recommended). I would be careful about putting much more power than that to them.