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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Speaker phase and current vs voltage drive
References: <3DBC76E9.F9DA95C2@managesoft.com> <3DBDC5A7.1F5A6706@managesoft.com>
Date: Tue, 29 Oct 2002 00:51:10 GMT
NNTP-Posting-Date: Mon, 28 Oct 2002 16:51:10 PST
Clifford Heath wrote:
> > ...No matter how it works...
> >> ...The current technology appears to work...
> Some people have problems reading. It obviously works. I asked why!
> Thanks to those who gave partial technical explanations (3 Johns
> and Chuck). Although I understand what you've said, I don't feel
> entirely enlightened. It seems to me that:
> * it's possible to reproduce the SPL fairly accurately,
> * it's unlikely that our technology does,
> * our ears are remarkably competent at hiding deficiencies,
> * a number of justifications are used to avoid thinking
> about it, such as "our ears are not sensitive to phase-shift".
> Though the justifications are likely true, that doesn't seem enough
> of a reason not to attempt a combined mechanical/electrical end-to-end
> analysis that might yield an improvement.
> > Voice coils are usually spring suspended
> How linear are those springs? The construction techniques I've seen
> incline me to think they get stiffer with greater displacements, which
> would create 2nd harmonic distortion. My experience here is with a pair
> of Altec-Lansing 8's, both the original drivers and modern (Kevlar and
> carbon) mids and tweeters I had fitted earlier this year - very sweet
> speakers! The replacement got started when the original woofer cone
> suspension died and needed fixing, a job competently carried out by
> Total Recoil in Melbourne - thanks guys, very happy with the result!
Liearity is not bad over a short range of motion however, acoustic
suspensions are not very linear except when the speaker is in a sealed
enclosure. I've heard of but never seen systems that include a cone
position or velocity sensor and close a loop around the sensor, power
amp and speaker. Damping can be increased as desired and linearity is
pretty much an issue of usable open loop gain. This is pretty expensive
to do and the power amplifier has to have pretty high bandwidth.
Crossover is more than interesting in this kind thing.
What we normally do is assume that the speaker above the principle
resonance is nearly ideally a current to force converter, that is to say
a motor. A voltage driver working into a resistor gives a current
proportional to voltage. Thus the resistance of the speaker essentially
changes the speaker to a voltage to force converter. This is actually
pretty good. The damping is mostly determined by the motor constant and
the speaker resistance.
The voice coils you have from disk drives may be rather strange animals
when compared to speakers. I think it was Oswald at IBM who came up with
the idea of improving mechanical response by the use of a shorted turn.
Shorted turn voice coil motors need to be driven by a transconductance
amplifier because their impedance is not nicely behaved. These motors
have excellent linearity over their useful travel. This is required so
that the servo will have fairly uniform phase and gain margins. The
motors are designed specifically to optimize position and velocity
control loops. Tracking is really a regulator loop and seeking is a
velocity loop usually with some anticipation (feed forward). IBM also
developed some novel applications of anticipation in drives with
multiple actuators situated with a single spindle.
In short, disk drive voice coil motors are really a lot different from
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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