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Old 5th November 2010, 10:04 AM   #11
andyr is offline andyr  Australia
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Quote:
Originally Posted by bentoronto View Post

Manufacturer says that driver is purely resistive. Sounds like a bad concept, at least theoretically, and can't be true, at least theoretically, but certainly makes it easier to design for.
Ben, I'm interested to know why:
1. a purely resistive driver is a bad concept, and
2. why it can't be true.

Magnepan drivers are almost purely resistive, whether they are:
* the original round-wire on mylar drivers
* the newer flat-wire ("quasi-ribbon") on mylar drivers, or
* true-ribbons.

They sound magnificent - so they certainly can't be a "bad concept". I measured the impedance of my 3 drivers and, from memory, it was about 1/30th of the published figures for a Peerless cone driver. Hence, with this low inductance, they remain a constant load - which is good:
a) for a passive XO, and
b) for the amplifier, in an active setup.

Maybe your only experience has been with conventional cone drivers?

Regards,

Andy
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Old 5th November 2010, 02:38 PM   #12
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andr -

The answer is I don't believe in perpetual motion, and I was talking at least theoretically earlier, if you recall.

Terms like "purely" should not be used in reputable descriptions. And if such terms are allowed to be introduced by the folks in the Marketing Department, you ought to be skeptical about the rest of the description too.

It is correct to say that being able to anticipate impedance is to (an unknown degree) helpful in designing passive crossover (where you deserve what you get anyway if you still use them) and to some probably insignificant degree for poorly designed amps (if you still use them... ). Moreover, constant-like resistance is more so, as you suggest.

Footnote: I'm a big believer in motional feedback. So any time I hear that the driver has nothing to say back to the amp, I am puzzled.
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Last edited by bentoronto; 5th November 2010 at 02:41 PM.
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Old 5th November 2010, 02:46 PM   #13
a.wayne is offline a.wayne  United States
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The phase angle is what determines if the amp will consider the driver a pure resistor not the actual impedance value...

A direct ribbon is a pure resistor , it's there biggest advantage over ESL's "capacitor" ...


regards,
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Old 5th November 2010, 02:48 PM   #14
tinitus is offline tinitus  Europe
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Its a ribbon, or more correctly a planar

due to the princible of having the lead as part of the diaphragm instead of an attached voice coil, they are commonly known to be similar or close to a resistive load
the point is, impedance is very linear, hence the called a resitive load

but its still placed in a magnet gap, and moving, and thus may not be that simple
but is known as an easier load
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Old 5th November 2010, 08:55 PM   #15
andyr is offline andyr  Australia
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Quote:
Originally Posted by bentoronto View Post

Footnote: I'm a big believer in motional feedback. So any time I hear that the driver has nothing to say back to the amp, I am puzzled.
The property of being a resistive driver that doesn't have any inductive properties (and so displays a contact resistance, rather than an impedance which increases with frequency) has got nothing to do with feedback from the driver back to the amp. Maggie drivers do exhibit back-EMF.

Regards,

Andy
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Old 5th November 2010, 09:05 PM   #16
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Quote:
Originally Posted by andyr View Post
The property of being a resistive driver that doesn't have any inductive properties (and so displays a contact resistance, rather than an impedance which increases with frequency) has got nothing to do with feedback from the driver back to the amp. Maggie drivers do exhibit back-EMF.

Regards,

Andy
... in which case, unless I don't have my grasp right, means they are not purely/theoretically resistive.
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Old 5th November 2010, 10:18 PM   #17
andyr is offline andyr  Australia
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Quote:
Originally Posted by bentoronto View Post
... in which case, unless I don't have my grasp right, means they are not purely/theoretically resistive.
I believe your grasp most certainly isn't right!

AIUI, in the case of Maggies, back EMF is generated through Newton's Law (due to the coupling between the large panel area and the air).

And as to being "purely resistive", if the inductance is at so low a level as to make no real-world difference in terms of the operation of an XO, the driver can be called non-inductive. After all, we never say "<i>Speaker X goes down to 10Hz</i>" - we always say "<i>Speaker X goes from 25Hz to xxKHz</i>". The "25Hz" generally refers to its -3dB point (some mfrs probably cheat and refer to the -6dB point) and the speaker ,i><b>does </b></i>produce sound below this frequency ... it's just that you won't be able to hear its output at 10Hz.

Regards,

Andy
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Old 5th November 2010, 10:43 PM   #18
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Quote:
Originally Posted by andyr View Post

AIUI, in the case of Maggies, back EMF is generated through Newton's Law (due to the coupling between the large panel area and the air).

And as to being "purely resistive"...
Back-EMF is a result of magnetic coupling - the same coupling that produces a force on the ribbon when you run a current through it. Force constant (N/A) and back-EMF constant (V*sec/m) are directly related.

The in-circuit result of back-EMF is an increase in the apparent resistive impedance seen by the amp.
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Old 6th November 2010, 12:35 AM   #19
andyr is offline andyr  Australia
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Quote:
Originally Posted by Jacques Merde View Post

Back-EMF is a result of magnetic coupling - the same coupling that produces a force on the ribbon when you run a current through it.

Thanks, Jack, for your succinct explanation.

Regards,

Andy
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Old 6th November 2010, 01:08 AM   #20
sreten is offline sreten  United Kingdom
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Hi,

This thread has wandered into total irrelevance.
The consequences of resistive attenution are well known.

rgds, sreten.
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