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28th December 2017, 03:36 AM  #11 
diyAudio Member
Join Date: Jun 2011

That particular gap is intended for a wider diaphragm. A narrower gap will increase sensitivity, but not enough to negate the need for a massive amount of current to keep up with any normal cone speaker.
Opposing flat magnets cause a sharp suckout at a frequency inversly proportional to the distance between the magnets. A more narrow gap pushes the suckout frequency higher. The Magnepan IIIC tweeter has a deep suckout at 9 kHz. The ribbon is narow and the magnet gap is narrow compared to the tweeter pictured above. Using stronger magnets makes assembly more difficult and actually dangerous. The magnets want to collapse on to each other in the gap pinching a finger quite easily. 
28th December 2017, 05:58 PM  #12 
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28th December 2017, 11:08 PM  #13 
diyAudio Member
Join Date: Jun 2011

The sensitivity of the midrange was calculated from the T/S parameters. It just happens that the SPL meter measured very near the dB sensitivity at 3mA with the SPL meter very near the midrange dust cap.
The tweeter is so quiet that the SPL meter does not deflect on the lowest range. I have to get an amp involved so that I can drive the tweeter enough to get a measurement. Then, ... I will have to calculate the power into each driver and correct for the disparate powers. I will post better info when I have it. 
29th December 2017, 12:33 AM  #14 
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Join Date: Jun 2011

Better info is below
3kHz signal. Near field measurements on on both drivers. Current through driver is set for 85 db near field for each driver The ribbon diaphragm is about 0.5 ohm The midrange is about 12 ohms at 3kHz Ribbon 85 dB at 1.3A, 850 mW Midrange 85 dB at 32mA, 12 mW The tweeter requires 850 / 12 = 69X more power, 18 dB less sensitive Let me know if I made a miscalculation somewhere. I used the 2 most significant digits of the current measurements and rounded subsequent calculations to two significant digits. 
29th December 2017, 12:43 AM  #15 
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Join Date: Sep 2013

Additionally can you put 2.818Vrms through each driver and record the dB value on your meter.
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"If you leave this point unattached to some circuitry, an ideal constant current source will emit a small lightning bolt which will travel until it connects to something." Nelson Pass 
29th December 2017, 10:58 AM  #16 
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Join Date: Jun 2011

The drivers would not be at equal input power at 2.82 vrms and you would have different SPL readings. You would have to calculate the power ratio , convert to dB and then add dBs to one of the drivers and then subtract the two.
The way I did it seems simpler to me. 
29th December 2017, 03:22 PM  #17 
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OK, so let's say you have 86 dB at 1 watt at .5 ohms, which is 1.4A:
( 1 W = I^2 * .5 ohms ) 106 dB at 100 watts, 14A 120 dB at 2,500 watts, 70 amps (100a peak) Correct me if my math is wrong.... 
29th December 2017, 05:54 PM  #18  
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Join Date: Jun 2011

Quote:
1. My measurements were very near field, about 1 inch from the driver. At a listening position, the dBs will be lower. The 86 dB sensitive midrange only required 12 mW to achieve 85 dB right at the cone. 10*Log (1/0.012) = 19 dB. So, as a wet thumb in the air, subtract 19 dB from your numbers. 2. The ribbon measured is 1/2 inch wide. The skinny ribbon is more than 10X lower impedance. Last edited by woofertester; 29th December 2017 at 05:58 PM. 

29th December 2017, 11:41 PM  #19 
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Join Date: Sep 2013

I would be taking more measurements depending on how good that dB meter is.
At least 3 different points.
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"If you leave this point unattached to some circuitry, an ideal constant current source will emit a small lightning bolt which will travel until it connects to something." Nelson Pass 
30th December 2017, 12:14 AM  #20  
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Join Date: Jun 2011

Quote:
3kHz is in the passband of both the tweeter and the midrange. At this point, I am satisfied that the ribbon will require much more than 100A peaks. It makes sense to have peak current capability of 300 to 400 amps. 

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