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Old 20th December 2005, 02:42 PM   #1
Nuuk is offline Nuuk  United Kingdom
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Default Pseudo co-axial?

I'm considering the next stage(s) in the developement of my open baffle speakers.

I wondered about moving the tweeter so that it sits directly in front of the Goodmans 201 driver.

I wondered what problems I may encounter doing this and also which tweeter is best suited to the job.

The dustcap of the 201 has an area about 9 cm (3 1/2 inches) diameter.

All input is welcome!
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Old 20th December 2005, 03:28 PM   #2
dnsey is offline dnsey  United Kingdom
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Interesting...I'm thinking of doing just the same!
I have a pair of (I think) Jensen 12" woofers, which give pretty even response subjectively well into the midband. Whilst looking for suitable partners for them from my 'stock', I tried some little Nokia tweeters taken from a scrap TV. They worked beautifully together, apart from some phase cancellation which depended on elative positioning.
Whilst contemplating this, I suddenly realised that the tweets would fit within the voicecoil dameter of the Jensens. A swift lash-up worked very well, so that's the way I'm going. The sound was very like my Tannoys, but a rather less explicit in the midrange. A rough cardboard horn on the tweeter overcame that, so I'll have to make some proper ones.
Of course, all this is reinventing classic drivers like the Goodmans dual- and tri- Axioms!
Go for it! I think fine-tuning the crossovers may be the most taxing bit, but isn't it always! The interesting thing is that with a simple protective cap on the tweets, there was a subtle variation in sound when reversing the HF phase, but I still can't decide which polarity is best.
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Old 20th December 2005, 03:35 PM   #3
Rudolf is online now Rudolf  Germany
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Sorry that this description is in german only, but the pics and diags will give you a first impression:
http://www.visaton.de/vb/showthread....&threadid=8317
What was learned from this development:
  • Donīt try to put the tweeter into the center of the woofer cone. Its like placing a loudspeaker in the middle of a round baffle. Asymmetry rules.
  • Donīt try to be "time aligned". The reflection issues of keeping the tweeter close to the woofer cone are more serious than the time delay caused by a distance between cone and tweeter.
  • A tiny tweeter is a better tweeter. So donīt look for maximum quality if it is only avaliable in a large packet.
FYI: There had been experiments at the VISATON forum to integrate tweeters in the woofer voice coil. Since the woofer cone and the gap between tweeter and voice coil form a complex and moving waveguide, results had been disappointing. There is a reason why true time aligned coax speakers are rare!

Rudolf
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Old 20th December 2005, 03:58 PM   #4
Nuuk is offline Nuuk  United Kingdom
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Thanks guys - that's a good start!

Rudolf- if I understand things from those pictures, I have to use the smallest tweeter that I can find and position it off-centre!

So I need suggestions for a smaller tweeter (or start looking at broken televisons ) .

At present the 201's are used full-range and I bring my existing tweeter in at around 10 Khz. Of course, I could use a crossover to cut the 201's off lower down if that would be an improvement.
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Old 20th December 2005, 04:08 PM   #5
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Old 20th December 2005, 04:13 PM   #6
Nuuk is offline Nuuk  United Kingdom
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Well I guess that Visaton SC 4 ND 4 is one candidate. Does anybody have experience of using one?
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Old 20th December 2005, 04:18 PM   #7
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Quote:
http://www.me-geithain.de
Interesting design again although I like the Visaton arrangement better (aesthetically at least)!
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Old 20th December 2005, 04:23 PM   #8
Rudolf is online now Rudolf  Germany
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Quote:
Originally posted by Nuuk
Rudolf- if I understand things from those pictures, I have to use the smallest tweeter that I can find and position it off-centre!
By positioning the tweeter off-center you can "smear" the unavoidable diffraction/reflection effects.

The smallest tweeter will not automatically be the best, but your compromise between best and smallest should tend to the latter.
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Old 20th December 2005, 05:27 PM   #9
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Quote:
The smallest tweeter will not automatically be the best, but your compromise between best and smallest should tend to the latter.
Thanks again Rudolf. That should narrow it down. But then I get the question come into my head - is it better to use a lower quality small tweeter in front of the woofer or go for a higher quality tweeter in the 'normal' position!
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Old 20th December 2005, 06:36 PM   #10
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Quote:
Originally posted by Nuuk

- is it better to use a lower quality small tweeter in front of the woofer or go for a higher quality tweeter in the 'normal' position!

Good question I'll be very interested in the answer.

I'm using a "good" tweeter in the normal position to fill in above 6KHz on my 201's and can find little to criticise with the imaging, it's rock stable and nicely defined.

Kev
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