Question about voice-coil gaps and dust-cap necessity

Race Bannon

New Member
A strange question, I know, and I'm probably going to get mixed opinions, but...

I have a pair of Altec 604-Cs with shredded cones (imagine that!), so I am going to recone them, of course. But once I've done that I plan on experimenting with differing horns, since they're simply mounted by two screws. 3D printing ought to make for easily made horns for experimentation...

But the 604 series use the funny felt "diaphragm" to seal out dust, as a dust cap would do on a regular woofer.

Is dust infiltration really such a big deal? I see so many "full range" drivers designed for the SET Tube-amp Crowd for use in rear-horn or TL enclosures that have a "phase plug" on the center pole, preventing the use of a dust cap. They seem to run open without dust protection.

I can guess it might depend on how dusty the environment is. Maybe how much iron dust is in the air. Don't laugh; as cars' brake discs wear, they throw off iron dust!

Any thoughts from the Altec gang?
 
All it takes is one magnetically attracted bit larger than the minimum clearance to get pulled in the gap, all your questions will be answered, and you'll know exactly why magnetic gaps, especially ones with tight tolerances are very well protected from the outside environment.

An ounce of prevention is worth a pound of cure.
 
If you do end up trying out some different horns, pls let us know what you figure out. I am also curious about other designs than the Urei, Mantaray, and normal eggcrate.

Maybe you can find some way to attach the felt in a non permanent way. Velcro? Lol
 
Or a milder adhesive sparingly, like maybe the alene's tacky glue.

That would probably work. As long as it would seal between the horn and the felt it would be fine. He could attach the felt to the cone and not mess with that part

Its interesting to me with as many 604's as there are around, I have heard very little about changing out the horns. Maybe because they sound so damn good in their stock form.
 
I'm interested in making a horn that is more like the original 604 / 604B, which is closer to a true multicell, as well as something that doesn't unload the driver at so high a frequency as the 604C horn does. Maybe that's more a function of size than design, but it would be fun to try some different strategies / compromises.
Perhaps cure some of that midrange honk, too.
 
If you load the HF with a horn that provides adequate acoustic load down to XO, you have a horn of physical proportions that create a phase plug/lens that only serves to muck up the upper response of the LF section.

Depending on what you intend to drive them with and what your max SPL requirements are, i would probably experiment with small conical waveguides as well as increasing/decreasing the HF driver's back chamber volume.

The long exponential throat of the alnico duplexes is an obstacle with no practical detours around. This exercise would be better visited on a more modern ferrite motor duplex with it's greatly reduced HF throat length.

I would also recommend you give them a listen with no horn at all. It will give you a good idea how little a 604's horn actually does.
 
Yes the 616 duplex I have sound pretty good. They have no horn. They have a much bigger sweet spot than my Urei horn 604G's.

Neither driver has any honkiness with the right crossover (Markwart)
 
Thank you, gentlemen.

Bowtie, I like the idea of the "small conical waveguide" and was thinking to try that, but had not considered the back chamber volume. Looking at a cross-sectional illustration, it does become apparent that the throat is indeed very long, but I do want to retain the Alnico motor, so no-go on the shorter ferrite units.

A no horn listen, you say... The little horn is really that ineffective? Not too surprising. Makes me wonder about re-profiling the end of the throat to blend into the LF cone, a la Tannoy. But it is said elsewhere the Tannoys have issues with IM distortion caused by the moving cone. I can imagine a HF horn made to oscillate at lower frequencies, and how that would color the sound. It would seem that there is a set of compromises inherent to the duplex design that no amount of juggling will completely solve.

Elitopus, I have read good things about that Markwart crossover. I will definitely go that route!
 
I'm pretty sure I'm "riding bareback" with my highly modded VMPS FF1's, but there's nothing I can do about it except avoid using steel wool around them.

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