High sensitivity dipole tweeter - in my dreams - diyAudio
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Old 29th August 2009, 02:25 AM   #1
cuibono is offline cuibono  United States
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Default High sensitivity dipole tweeter - in my dreams

I've been looking, but not finding, so I thought I'd ask. I'm cooking up a new design, with the goal of it being a high sensitivity fully active dipole - with the dipole peak around 3k (read no baffle).

So here is my 'dream' tweeter: 96dB/2.83V/1m, dipole or sealed back (probably have to be neo motor), with a small enough separation to keep a smooth dipole pattern off axis into the treble, and inexpensive. Usable from 1.5k to 15k.

I have two partial solutions so far - BG Neo3 and Vifa BC25SC06-04. I've been using the Neo3 and am pleased with it - not expensive, dipole, and a small dipole separation. My only contention is that its about 90dB sensitive when used OB. The Vifa looks promising (96dB, smooth, cheap), except for two things - one, it has an Re of 2.9ohms - paralleling them (to create dipole) would be a bad idea. Two, the separation is a little larger than I would like.

I bought a pioneer fp66 cone tweeter from PE, which is supposed to be sensitive, cheap and small, but it rolls off before 10k. Too bad.

So right now, I'm thinking stay with the Neo3 - I like it, and I'm not sure high sensitivity matters as much with such a small diaphragm mass. Another possibility is run the Vifas off of the amps for the woofers, crossing passively, which is an interesting possibility.

If I were doing a big budget design, I'd look at TPL-150 air motion transformers. Another possibility is a DIY ribbon, I don't know if I can get the sensitivity I need though.

Any ideas?
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Old 29th August 2009, 02:37 AM   #2
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Two ribbons in opposite polarity? Or these? Getting the dipole part with separation isn't that hard to do with two tweeters, but the 96 dB sensitivity is a bit tougher...Two SB29's at 93db...
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Old 29th August 2009, 01:10 PM   #3
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by cuibono View Post
So here is my 'dream' tweeter: 96dB/2.83V/1m, dipole or sealed back (probably have to be neo motor), with a small enough separation to keep a smooth dipole pattern off axis into the treble, and inexpensive. Usable from 1.5k to 15k.

I have two partial solutions so far - BG Neo3 and Vifa BC25SC06-04.

Any ideas?
The BG Neo3 certainly is a good choice. My - much cheaper - choice was the Monacor DT-25N with this factory frequency response. This FR graph is a bit optimistic, but you will get true 94 dB/2.83V/m.

I have no idea how you could get this tweeter in the USA - and at what price.
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Old 29th August 2009, 01:26 PM   #4
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Hi Cuibono;

Why do you feel that you have to wire the Vifas in parallel? Couldn't you wire them in series?
Matt
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Old 29th August 2009, 03:16 PM   #5
el`Ol is offline el`Ol  Germany
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This 4 Ohm dome mid/tweet has an open back and got a very good test result in a diy magazine.
http://www.intertechnik.de/Lautsprec...1174,4417.html
As far as I know the Neo 3 has high sensitivity in the top octave when used open back and could be combined with it.
I don´t know whether there is something like this in the US as well.
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Old 29th August 2009, 04:32 PM   #6
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How low can you cross the BG Neo? Is 1.4kHz possible? Thanks.
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Old 29th August 2009, 08:42 PM   #7
cuibono is offline cuibono  United States
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Thanks everyone for the suggestions - just what I was looking for. The DT-25N looks very promising. I wonder if I could get them reasonably.

But Matt may have the winning suggestion - maybe there is no reason to wire the Vifa's in parallel. Although the driver would need more voltage to drive it to the same SPL, it would still retain its inherent sensitivity of 95dB... Something to consider!

Gainphile, I'm going to guess you could cross them at 1.4k, but I'll let you decide - here is a graph of an open back Neo3, at 0, 15, 30, 45, and 60 degrees, with NO baffle at all. I've been using it this way for a couple of weeks, and think it is a great improvement - particularly, vocals are noticeably more articulate. I had previously been using them on a 28cm baffle, which in retrospect, made them sound like they had baffles step problems. Although I hadn't noticed it before, vocals now sound like they are noticeably less nasal.

The only deviation from an ideal dipole pattern is between 7 and 9k (although above 2k the pattern narrows a little due to driver dispersion, ie, above 2k, at 60deg, the response is down 7-8db versus the ideal 6db - but still the off axis response is very smooth compared to baffle mounted drivers. ). The tweeter is supposed to be 90.5dB/2.83V/1m, although I haven't measured.

Enjoy


PS - it is hard to read the graph at 8k, because the 0deg and 45deg lines are both brown - at 8k, the dip is 0deg (unfortunately) - the upper trace is 45deg. I guess the good news is that the on axis null is filled in off axis. I listen to them at a 30deg angle. What I think is going on is the grill of the driver is causing an on-axis cancellation at 8k, which disappears off-axis due to the changing path geometry, combined with off-axis 'baffle' peaking (the baffle being the tweeters frame), usually seen at much lower frequencies with drivers on baffles.
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Last edited by cuibono; 29th August 2009 at 08:55 PM.
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Old 29th August 2009, 11:16 PM   #8
cuibono is offline cuibono  United States
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Whoa, check this out: http://www.diyaudio.com/forums/showt...=1#post1909992

Right on time! Now just to make it dipole..
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Old 30th August 2009, 12:29 AM   #9
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http://www.usspeaker.com/beyma%20tpl150-1.htm

Watch out for a fudged response chart, though.
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Old 30th August 2009, 08:58 AM   #10
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by cuibono View Post
So here is my 'dream' tweeter: 96dB/2.83V/1m, dipole or sealed back (probably have to be neo motor), with a small enough separation to keep a smooth dipole pattern off axis into the treble ...
I don´t see how one can achieve that 'small enough separation' with an AMT construction.
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