Max Fidelity PR65NEO mods

Dustcap, remove or leave in ?

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

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.

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)

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.

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

Therefore, I don't recommend doing it.

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.

20190118_140200

20190118_141334

20190118_140200

1/5

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?

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.

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.

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.

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.

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

new big dustcap

92mm instead of 60mm

felt inside

on top of the 4x 1.5" is 2x 3/4" felt pieces. Glue not dry yet...

frequency response

new big dustcap

92mm instead of 60mm

1/6

Phase 2

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.

Well, the results against the stock unit are debatable.

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.

Stock = Green

Phase 2 = Red

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