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Where can I get decent foam surrounds for Pentas

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GTS
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GTS posted on Sun, Jul 1 2012 12:55 AM

Hi there

Had a great experience with Goodhifi for my CX100 foams, the units supplied for my Pentas (I've got two pairs that need refoaming) is not so good. The foams are too small, with the roll not clearing the cone and the outer diameter being a good 1.5-2mm too small (critical, as the basket frame has a notch and these won't seal if it's not done right). 

I could 'make' the parts fit though being too small, this would at best preload the driver and at worst not provide a full seal at the frame perimeter. Neither ideal.

Not a slight on the seller - the package was lost in post the first time and they resent at their cost without delay, and have been very proactive in attempting to placate me, I'm just getting the impression that getting a truly well-fitted foam is going to be a bit of a lottery.

Who's had good experience elsewhere?

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GTS
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GTS replied on Thu, Aug 30 2012 12:59 AM

Søren Mexico:

GTS:
Did the Penta 2 set similarly. It's so easy and effective, I'm not likely to not shim ever again.

We love pics, good job-

Sure. Here is a brief problem definition - four of the six culprits. L-R we have a Penta 1, Penta 1, Penta 3, Penta 2 (with the yellow lens swapped out). The Penta 3 have been my daily speakers for a few years. They are fed by some decent interconnects, an upsampling DAC and a generally lossless audio sources. It's said that blind tests between lossless audio and high-bitrate MP3 prove things to be indistinguishable, however most people that come over can pick FLAC/ALAC from 320kb/s MP3 with this setup. This from a speaker regarded as 'cloudy in the midrange' - go B&O! Settings are minimum bass, least sensitivity on the Pentas.

The photos below are of the cosmetically worst driver from each driver pair.Taken with a cameraphone, they look much better to the naked eye.

This is a Penta 1 driver. Of those refoamed, these came in at the lowest impedance by a half ohm consistently and 0.15 ohm variance among them. They're badged ITT or SEL (4 each). The dust caps were purple when I got them (through age? Environmental conditions? Who knows. They'd come from a smoker's environment - lots and lots of cleaning here - and are the last time I trust an eBay seller with 'Perfect condition! Professionally refoamed!'. The refoam was awful. The foams weren't run in, which was good as three drivers had light contact and another three were borderline. They needed a light sanding to get glue off in pieces. Really not good. Since salvaged.

This is a Penta 2 driver. These speakers were otherwise in pretty good condition. The drivers are all badged Nokia, and the dust caps were yellow/brown. Under the dust cap they're identical to the ITT/SEL drivers - six vent ports, some pretty loose tolerances for the piston from what I could make out.

This is a Penta 3 driver. I used to think these were refoamed, as when I bought the speakers there was a receipt for this. Two local professionals assured me they could provide results that were cosmetically comparable with reshimming too. Despite the age, these could be the original foams. Who knows, opinions welcome.

Results? They all sound great. In pairs, though, they sound different. And they only sound cohesive when run in their original pairs. The 1's are definitely the brightest (impedance? Crossover specs?) The 2's are closer to the 3's, possibly a little tighter in the midrange (the foams will wear in I'm sure) - though possibly a little lacking up top?

What would I change if doing things again?

  • I'd hunt for some suitable dustcaps mercilessly. It keeps the cone mass closer to original, despite the notion that T/S parameters vary significantly between drivers of the same type and that the same modification to a set of identical drivers should affect each identically. Despite it being under a gripe, I kind of like things looking pretty too. Above all a new one would be easier to work with, though given the glue width on the original item, finding something suitable might be tricky.
  • I'd use the SimplySpeakers glue again - it's light years better than PVA - but with a much smaller nozzle, something in the 1-1.5mm range. What's supplied is great for refoaming woofers, not 8cm drivers.
  • I don't regret reshimming my Penta 1's, they were in very poor shape. But the 2's? Not sure there was much need, though no regrets. Don't get me wrong, they sound great, though they were pretty much on-tolerance. Shimming didn't reveal any great lateral misalignment issues in these speakers, which had been well looked after. Possibly just weighing down the cone on the centre of the dust cap would have done it just fine. The 1's had taken a beating by comparison, shimming fixed these properly - I was surprised how much alignment force is taken in the foam - it really is important to get it right, particularly if you're to run your speakers hard. The foam will weaken over time and what was centered when pressing up and down a few times using clothes pegs to hold it in during installation, probably won't work when it's oscillating hundreds to thousands of times a second at high amplitude a few years later.
  • I don't regret not using the GoodHifi foams, whoever needs to redo these in 10-15 years - be it myself or someone else - won't be tearing the cone to get remnants of the last job off. (They have great customer service though and could probably very simply redesign their foams, after which I'd be a happy customer).
  • We'll never recreate the original sonic properties of the drivers with new foams unless we're getting original foams from ITT - the two foam brands I have feel different in the hand, and would offer different damping properties. There's likely a bigger effect on T/S parameters here than in removing a bit of cone material, as the moving mass - for an 8cm midrange - is high.
  • I still don't know that I'd let a professional refoam them. The best quote I had here was USD$30/ea - or $240/set. Quite steep. If money wasn't an object and time was, I'd probably have focused on the signal path. Given there are 18 moving assemblies with a pair of Pentas and a bunch of other things that age depending on environment and usage, we shouldn't be surprised that there are sonic differences in these three pairs. Replacing tired components in amplifiers and crossovers probably has a more tangible effect on rejuvenation than anything else - the midrange drivers are honestly a pretty robust design. They're clearly not the 'Rolls Royce' of drivers but they're judged here as part of a complete assembly, and considered on their own from a maintenance perspective, I've learned that they'll take a good amount of abuse regardless.

Now if I can only work out how to make a new Penta top cap… hmm...

GTS
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GTS replied on Tue, Sep 4 2012 2:42 AM

Of interest, two more photos showing shimming.

The Penta midrange with the dustcap cut out (this one was in pretty good condition to start with - no contact/misalignment). The small imperfections on the former (the metal bits around the pole and bonded to the voice coil) are not the knife I used to cut things out, and were present on a fair few of these drivers. At this stage the new foam is glued on (appears distorted in this photo for some reason but was measured concentric within 0.5mm).

Shimmed with 5 thou acetal sheet. Sit the shims in, move the cone up and down to settle the shims appropriately, lift up the outer of the foam and apply a bead of glue to the basket, let it tack (the wrinking seen here) then tap down to a smooth finish.

Remove the shims, glue the dust cap down. Remove any excess glue, let it dry, check to see you've a seal and it's good to run.

Fred Buist
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Dear Sir,

At most pictures I see a kind of surround that almost must be a standard surround that incidentely also 'fits' on a Beolab or Beovox Penta. What do you think of under beneith repair?

Best regards, Fred

Fred Buist
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Beautiful isn't it? The Penta repaired in it ’s full greatness….

 

Best regards, Fred

 

GTS
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GTS replied on Wed, Mar 19 2014 9:06 PM

Fred, there's nothing special about the speaker size - they're certainly standard-sized foams.

Whether they're mounted above of below the driver really doesn't matter. I think above is aesthetically better as we're mounting to a paper, not a poly driver cone, so mounting under will likely leave residue and may not tack as well given the surface quality.

Foam is better than rubber but we really don't know a great deal unless B&O releases the T/S parameters for the drivers and we do a pre/post test with each one for conformance. I didn't find anyone through B&O that'd actually go that far... so I did it myself.

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