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Griffin Loudspeakers
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ColinR
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PostPosted: Tue Aug 29, 2006 10:52 am    Post subject: Reply with quote

This is a thread where the Physicists on the “bored” can have a fun time with Lenz’s Law.

An ABR can be treated as a reflex port when the port size becomes unmanageable.

A bass/mid loudspeaker with a shorted voice coil behaves like a resistive reflex port ala Celef/Proac or the Scan-Dyna varovent.

At low excursions the resistance to movement is relatively constant, at higher excursions the resistance increases exponentially.

They are usually called infra-bass radiators, so there must be some patents and literature somewhere.

Examples include, some expensive American jobs whose name I’ve forgotten, some Heathkits (who pinched the idea from the previous high-end design), with multiple B139’s, the Goodmans Dimension 8 and the Griffin.

I’ve never seen this technique mentioned on any of the DIY web sites, anybody want to have a go with “modern” drivers?
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proffski
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PostPosted: Tue Aug 29, 2006 1:10 pm    Post subject: Reply with quote

Brilliant!

Lenzs Law to me means dropping big Neodymium magnets between large aluminium blocks or down copper tubes. The students love the demonstrations!
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greg
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PostPosted: Tue Sep 05, 2006 2:08 am    Post subject: Reply with quote

Graham Hartle confirms some of the info on this thread and suggests that John at Five Ways Hi-Fidelity in Birmingham may be able to offer more info.

Can anyone confirm that the midrange driver in the 85 is not a Celestion MF500? All I have to go on is this picture of the B&O Beovox 5700B, which I believe uses the MF500!



This appears slightly different to the midrange driver on the Griffin speaker. Can anyone confirm? If this is the case, it would suggest the midrange in this case is Griffin's own.

Thanks

Greg
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proffski
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PostPosted: Tue Sep 05, 2006 7:25 pm    Post subject: Reply with quote

Ah another visitor to Beoworld then Wink

It looks like it is Griffins own I reckon.
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ColinR
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PostPosted: Tue Sep 05, 2006 10:21 pm    Post subject: Reply with quote

Quote:
It looks like it is Griffins own


Yes, but it's still missing the Celestion wire grill.

The original design was probably courtesy of Peerless, who used to supply rather good engineering drawings of their drivers on specification sheets.

Quote:
John at Five Ways Hi-Fidelity in Birmingham may be able to offer more info


John Townrow's site

http://www.fwhifi.co.uk/
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proffski
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PostPosted: Tue Sep 05, 2006 11:32 pm    Post subject: Griffin 85 Reply with quote

Are we talking about the Celestion HF 500 here? Surely the unit to my knowledge was always round. The MF on the Griffin is square.

Here again are Griffins own words from the Hi Fi yearbook 1975.

Type 85


Tandem line system (Patented).
Units: Low resonance KEF B200 infra-bass unit.
KEF B139 bass.
Celestion HF 500 OR Griffin titanium dome 2in unit.
STC 40019. (No mention of two) (Yes they have printed a 9, a typo?)

I guess this is a passive example?

Further on an advert claims:

Griffin Studio 85 Electronic.

Tandem Line Bass System. 22Hz - 30 kHz -4dB + 2dB

Amplifier Dividing Network:

Treble 30 WRMS 6kHz - 50kHz
Middle 50 WRMS 750Hz - 6kHZ
Bass 65 WRMS 10Hz - 750Hz.

£218 each in 1975.

Seems there were at least two versions then as well as the electronic active unit. Is that a foam filled port on the photograph supplied?

Shocked
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greg
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PostPosted: Wed Sep 06, 2006 12:28 am    Post subject: Reply with quote

Yes, the Type 85 must be a passive version which may have a few other differences to the Studio Electronic 85. Also, our speakers are a few years younger than those described in the 1975 book.

To the best of my knowledge there is no B200 on this version. It's been a while since I last had a close look but I recall there were two B139s mounted isobarically.

As far as I understand the STC 4001G is the 15 ohm version. The two tweeters in each of our speakers are marked "COLES 4001 - 8 OHM" so are the 4001K version.

Yes, that's a foam-filled port you see...
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ColinR
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PostPosted: Wed Sep 06, 2006 10:58 am    Post subject: Reply with quote

Quote:
Are we talking about the Celestion HF 500 here



I've seen three pairs of large B139 endowed Griffins, with either Celestion or Griffin dome midrange units, all had six point fixing mesh grills, why haven't greg's?

The Peeerles midrange dome was/is a drop-in replacement, but did/does not have a mesh grill.
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greg
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PostPosted: Wed Sep 06, 2006 4:57 pm    Post subject: Reply with quote

Unfortunately, the speakers are in Canada and I'm in Yorkshire so I can't take one of the midrange drivers out to inspect further right now but I will try and get a bit more information as soon as I can.
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ColinR
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PostPosted: Tue Sep 12, 2006 11:01 pm    Post subject: Reply with quote

Quote:
High 30 WRMS 5KHz - 30KHz
Middle 50 WRMS 750Hz - 6kHZ
Bass 65 WRMS 10Hz - 750Hz


I've just noticed the above, I assume 24dB/octave crossover slopes are used, as the B139 is rather "ratty" above 320Hz, with a horrible resonance at ~1kHz, the Celestion MF 500 is crossed over at 500Hz in the Ditton 66 and 600Hz in the Beovox 5700 and the ITT/STC/Coles is normally "good" from ~7kHz.
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proffski
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PostPosted: Wed Sep 13, 2006 8:06 am    Post subject: Reply with quote

I'd not be so polite as "ratty" Wink Horrendous!
All my designs crossed over much lower with steep slopes Smile
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ColinR
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PostPosted: Thu Sep 14, 2006 4:57 pm    Post subject: Reply with quote

Very Happy
Quote:
I'd not be so polite as "ratty" Horrendous!


Funny that's what I'd call the Celestion Ditton 66 when compared to the Beovox 5700.

Just put the B&O's on their sides on top of Target TAP 20 (Naim SBL) stands ala Gale 401C's and listen Laughing .

The smaller bass units with a 1.5" voice coil and a magnet almost as big as the Celestion MF500 seemed to deliver much more when doing an A-B comparison some ten minutes ago.
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proffski
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PostPosted: Thu Sep 14, 2006 5:22 pm    Post subject: Reply with quote

Whoa there! Surprised

I did not mean that the fabulous B139 is rubbish, only so above 400Hz or therabouts!

I was never a fan of Celestion speakers, owned a few Bo Bos in my life though. Still a few languishing in my Hi Fi museum (loft)...

Wit till you see the latest "Centrte Speaker" pictures. Very Happy
CS7 reborn like you would have never imagined. Shocked
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greg
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PostPosted: Mon Mar 05, 2007 5:48 pm    Post subject: Reply with quote

Sorry I let this thread disappear a bit. Other priorities took over...

However, I now need to start thinking about designing some passive crossovers for the Griffin speakers, but unfortunately I feel my theoretical knowledge is lacking. As far as the practical side goes, I have no problem so if some of you guys would be willing to help out with the design side of things I'd be very grateful. Smile

Firstly, I have to say that I've been unable to confirm any more details of the speakers as I simply don't have access to them from 3000 miles away and there's no one who I can ask to check them out. I will be heading out to Canada for a few weeks this summer and would really like to get the speakers working again with passive crossover networks while I'm there. This is obviously far from ideal but unless you feel it's really a waste of time I'd like to build crossovers in the UK (based on the information that we already have in this thread) then take them to Canada to install and make adjustments as necessary. Please advise if you think this is such a bad idea that I shouldn't bother!

Based on what we've discussed so far, it seems that the original crossover points were 750Hz and 6KHz. (Perhaps it would make sense to drop the lower crossover point to account for the B139's weaknesses?) 3rd or 4th order crossovers seem to be necessary. Given that we have two bass drivers arranged isobarically I'm not sure how this affects the design. Is it possible to suggest a starting point (or even some further reading) based on this information?

As I said earlier in this post, if this approach sounds like a very bad idea then maybe I'll have to think of some other way, but it seems to me that if I could get a rough design going then at least I can get the speakers working again this summer and plan further refinements from there.

I look forward to hearing your views.

Thanks

Greg
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ColinR
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PostPosted: Fri Mar 09, 2007 8:33 pm    Post subject: Reply with quote

If you had access to three stereo power amplifiers, I'd suggest getting a ~£120 Alto electronic crossover and having a play.

I'm not sure about the design of a passive 24dB/octave double Butterworth.

BUT

The Celestion MF500 was never corrected in any commercial design, so the midrange filter section should be very close to the theoretical produced by a design program.
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