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Old 12th September 2013, 10:17 PM   #21
tinitus is offline tinitus  Europe
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... my DIY ribbon

does this mean I can just mount it with a 1ohm resistor and have 4ohm ?


Quote:
Originally Posted by hitsware View Post
Qts went from ~1 to ~0.3
not sure I understand that
I thought Qts would rise
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Old 13th September 2013, 12:09 AM   #22
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> does this mean I can just mount it with a 1ohm resistor and have 4ohm ?

You lost me on that one .

> not sure I understand that ......... I thought Qts would rise

Me neither .... I thought it would stay the same
Perhaps because the tester is reading the resistance
of the xfmr winding rather than Re .......
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Old 13th September 2013, 07:13 AM   #23
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by hitsware View Post
> does this mean I can just mount it with a 1ohm resistor and have 4ohm ?

You lost me on that one .
because ribbons have zero impedance without transformer
and with a modest 2:1 autoformer trafo it needs 'something'
hence the 1ohm

btw, is this correct connection ?
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Old 13th September 2013, 04:24 PM   #24
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Other way around. The impedance from the center tap to ground
is multiplied by 4 accross the full winding ........
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Old 15th September 2013, 08:39 PM   #25
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by hitsware View Post
Other way around.
ofcourse ... thanks
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Old 15th September 2013, 09:35 PM   #26
gootee is offline gootee  United States
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Quote:
Originally Posted by tinitus View Post
because ribbons have zero impedance without transformer
and with a modest 2:1 autoformer trafo it needs 'something'
hence the 1ohm

btw, is this correct connection ?
Your ribbon has significant resistance, which is impedance without any imaginary component.

To calculate how much resistance it has, see the post at:

Magnepan Ribbon Tweeter Question

Or here's the short version:

R = pL/A,

where R is in Ohms, and the conductivity of aluminum, p, is 2.6 x 10^(-8) Ohm-meters (where 2.6 x 10^(-8) = 0.000000026), and L is length in meters, and A is cross-sectional area in square meters.

From the equation above:

Ohms per Meter = .026 / (cross-sectional area in sq mm)

for aluminum.

OR, you could just measure the resistance with an ohmmeter.

As mentioned in the linked thread, if your ribbon is pleated, you can measure its resistance with an ohmmeter and then calculate the original un-pleated length (assuming you know the thickness and width).
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Last edited by gootee; 15th September 2013 at 09:43 PM.
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Old 15th September 2013, 09:47 PM   #27
tinitus is offline tinitus  Europe
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ahh, you think of Apogee clones etc, with long foil lines glued to a film
yeah, I build a few long time ago
and I might do it again
they can sound very good

but this one is just a small and simple experiment with a piece of kitchen foil, 12mm wide and 150mm long
resistor alone was not good
attenuated too much
never got around winding a trafo
and actually dumped the whole thing

but this one I have to try
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Old 15th September 2013, 10:16 PM   #28
gootee is offline gootee  United States
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Oh!

Well, standard kitchen foil is about 0.016256 mm thick. So maybe you could just add some length (and/or lower the width) to get a higher resistance.

At 12 mm wide, the cross-section is presumably 0.1951 sq. mm.

That gives 0.026 / 0.1951 = 0.1333 Ohms per meter!

So you really do have only 0.15 * 0.1333 = 0.02 Ohms; basically zero.

I'm guessing you don't want to make 15 2-meter ones in series, to get 4 Ohms.

Let's see, if you cut it down to 3 mm wide, you could get 0.5332 Ohms per meter. Then you'd "only" need 7.7 m to get 4 Ohms.

Nope. So let's see, to get 4 Ohms while keeping the same length, it would need to be 4/0.15 = 26.67 Ohms per meter. So the cross section would need to be 0.026 / 26.67 = 0.000975 sq mm. Wow. If the foil is 0.016256 mm thick, then the width would need to be 0.06 mm. Or make it 1 Ohm with a width of 0.24 mm. Ummm... No.

Never mind!
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Old 15th September 2013, 10:28 PM   #29
tinitus is offline tinitus  Europe
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nice try, thanks

well, I may have at least one small trafo, just to try the experiment
hey, since its only a tweeter, I have some 1A 2x 22V C-core
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Old 15th September 2013, 10:42 PM   #30
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Needs to be toroidal .....
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