SpeakerTalk

ColinR · Posted: Tue Aug 29, 2006 10:52 am Post subject:

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 · Posted: Tue Aug 29, 2006 1:10 pm Post subject:

Brilliant!

Lenzs Law to me means dropping big Neodymium magnets between large aluminium blocks or down copper tubes. The students love the demonstrations!

greg · Junior Contributor 10+ Joined: 28 Aug 2006 Posts: 10

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

proffski · Posted: Tue Sep 05, 2006 7:25 pm Post subject:

Ah another visitor to Beoworld then Wink

It looks like it is Griffins own I reckon.

ColinR · Posted: Tue Sep 05, 2006 10:21 pm Post subject:

proffski · Posted: Tue Sep 05, 2006 11:32 pm Post subject: Griffin 85

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

greg · Junior Contributor 10+ Joined: 28 Aug 2006 Posts: 10

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...

ColinR · Posted: Wed Sep 06, 2006 10:58 am Post subject:

greg · Junior Contributor 10+ Joined: 28 Aug 2006 Posts: 10

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.

ColinR · Posted: Tue Sep 12, 2006 11:01 pm Post subject:

proffski · Posted: Wed Sep 13, 2006 8:06 am Post subject:

I'd not be so polite as "ratty" Wink

Horrendous!
All my designs crossed over much lower with steep slopes Smile

ColinR · Posted: Thu Sep 14, 2006 4:57 pm Post subject:

proffski · Posted: Thu Sep 14, 2006 5:22 pm Post subject:

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

greg · Junior Contributor 10+ Joined: 28 Aug 2006 Posts: 10

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

ColinR · Posted: Fri Mar 09, 2007 8:33 pm Post subject:

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.
_________________
This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.