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Old 6th April 2009, 06:16 AM   #1
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Default Current drive for speakers

Sorry to bring up this old topic, but I've been thinking (again, my apologies). Does current drive really help out with power compression? It doesn't make speakers behave like they do with voltage drive, but that doesn't necessarily mean that current drive is perfect. We know that joule heating causes the voice coil to heat up, which means that unless the conductor is made of constantan or a similar material, the resistance will go up. This means that current drive does the opposite that voltage drive does - it'll put more power into a hot voice coil.

This should also apply to inductance effects. It should be safe to say that varying inductance (impedance) causes distortion by modulating the voltage. As before, instead of fixing the power to the speaker, current drive will do the opposite thing that voltage drive does.

So why did Hawksford measure a decrease in distortion by using current drive?

http://www.essex.ac.uk/dces/research...nt%20drive.pdf

EDIT: You can use constantan for voice coils. You just lose about 10dB of efficiency compared to copper.
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Old 6th April 2009, 02:53 PM   #2
DougL is offline DougL  United States
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I apologize for the lack of a more rigorous answer, but my intuition is to model a driver as a zero resistance coil in series with a resistor. Further, current through the coil is proportional to electromagnetic force.

In the case of constant voltage, as the resistor heats, the current and EMF decrease. In a constant current situation, EMF is constant. The resistor does receive additional heating.

Similar logic works for the inductive component of the voice coil.
The 20 DB reduction makes sense to me. I assume that other mechanical non-linear systems such as surrounds prevent a more dramatic reduction in distortion.

Doug
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Old 6th April 2009, 07:16 PM   #3
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Quote:
Originally posted by DougL
In a constant current situation, EMF is constant.

Doug
Yes, but isn't the acoustic power proportional to the electrical power?
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Old 6th April 2009, 10:15 PM   #4
DougL is offline DougL  United States
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Quote:
Yes, but isn't the acoustic power proportional to the electrical power?
I think that is the point, that the resistive heat loss is dynamic and adds nothing to the acoustic output. So power is not proportional to output. Current is proportional to output.

Doug
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Old 6th April 2009, 11:38 PM   #5
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Quote:
Originally posted by DougL
So power is not proportional to output.

Doug
I'm still stuck at this point. Are you saying that acoustic power is derived from the coil current ONLY and has nothing whatsoever to do with the electrical power through the coil?
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Old 7th April 2009, 12:27 AM   #6
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by 454Casull

I'm still stuck at this point. Are you saying that acoustic power is derived from the coil current ONLY and has nothing whatsoever to do with the electrical power through the coil?
It's possible to work it out from a few simple equations:

i * Bl = f
f / m = a
v = int(a)dt
f * v = P

therefore

P = i * Bl * int(i * Bl / m)dt
(do please point out if I've made a mistake!)

where
i = current through voice coil
Bl = force factor
f = force on cone
m = moving mass
a = acceleration
P = instantaneous power

Note that there is no relationship to the electrical power wasted in the voice coil resistance.

EDIT: Oops, calculus.
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Old 7th April 2009, 02:18 AM   #7
DougL is offline DougL  United States
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Mr Evil

Well said.

Doug
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Old 7th April 2009, 02:38 AM   #8
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Quote:
Originally posted by Mr Evil

Note that there is no relationship to the electrical power wasted in the voice coil resistance.
That clears everything up for me. Thanks!
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Old 7th April 2009, 05:27 AM   #9
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Wait, why does output go up near resonance?
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Old 7th April 2009, 11:11 AM   #10
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by 454Casull
Wait, why does output go up near resonance?
That calculation only shows the total mechanical power applied transferred to the cone - some more of it will be lost before it finally emerges as sound power e.g. losses through damping in the surround. Those losses vary with frequency.
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