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Dustcap, remove or leave in ?

I've used the Max Fidelity PR65NEO on few occasions.

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They are a super efficient 6" mid-range, have a huge 2" voice coil, exhibit no power compression at all and sound very dynamic.

They also require an aggressive EQ to be flat and have a huge break-up at 4 kHz, which renders passive crossover use almost impossible.

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On active crossovers (with a steep crossover slope) and digital sound processing, (need a big EQ notch at 4 kHz to tame the breakup), they can be spectacular. (covering 220 Hz-1800 Hz decade with ease)

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Since they’re in storage and its unlikely I’ll use them, I've decided to investigate the strong break-up at 4 kHz.

 

I started at the obvious point, by removing the dust cap.

Sometimes this leads to good improvement and sometimes, it worsens the performance.

Unfortunately, it's irreversible.

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In the PR65NEO case, removing the dust cap led to detrimental performance of the driver.

Therefore, I don't recommend doing it.  

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Rather, and instead of spending hours manufacturing a bullet shaped phase plug, I just turned a solid piece of metal cylinder to fill the voice coil cavity and tested again.

Once again, results were worse than the stock unit, but slightly better than without a dust cap.

Using strong mental deductions, this means that the break-up is from the cone itself.

  • Is the entire cone area entering resonance?

  • Does the inner part, or the outer part vibrate more?

  • Could a cone treatment get rid of the issue? (use of dammar, puzzle glue, varnish, or dope would be good idea?)

  • Is it the glue between the voice coil and the cone that plays a factor here?

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Lots of questions. Unlike the dust cap that can be re-installed (they sell those on Ebay), messing with DIY cone treatment is irreversible and you only have one shot to hopefully improve it.

I know that a cone treatment works… but in moderation. Add too much stuff and the speaker is pretty much ruined.

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I decided to gamble and order a 92mm replacement dust cap (the original was around 60mm)

Thanks to eBay, the dust caps were $2.40 a pair.

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The PR65NEO cone is 125mm in diameter (before reaching suspension)

Having a big dust cap will add lots of support close to the outer edge of the cone. This will hopefully strengthen the cone and get rid of that nasty 4 kHz peak, and hopefully restore the lost the efficiency between 100 Hz - 700 Hz intact.

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On the plus side…I can add sound absorbent between the magnet and the front of the dust cap.

See picture. This will help attenuate cavity resonances at higher frequency.

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If it doesn't work… well… this is the joy of DIY.

You can't hit homerun every time, but the process is still fun

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Phase 2

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  • Three coats of PVA glue on cone

  • 92mm dust cap installed (heavier compared to the stock 60mm dust cap)

  • Four pieces of 4mm felt on the pole piece (slightly off-set to try and avoid possible cavity resonance.

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Well, the results against the stock unit are debatable.

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Not sure that I've improved it, as it now has a big dip at 3 kHz.

Frequency response is a bit flatter. The distortion is a bit higher from the increased mass of the heavier dust cap, but the impulse response is somehow better.

The step response is about same.

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Stock = Green

Phase 2 = Red

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This driver was good ‘till ~1800 Hz, with the mods extending that to ~2200 Hz.

 

A bit of a bummer for the 4 kHz peak and associated 2nd and 3rd distortion peaks are still present.

I might re-use the PR65Neo one day

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