thanx: RE: [sdiy] ot: please recommend a low power pentode power amp circuit

[Czech Martin]

Speakers are really funny,
it is hard to belive that they really work.

A speaker has a so called "moving mass", i.e. cone, coil, glue, everthing that moves,
then it has a spring that helps to keep the cone into place (one at the magnet,
and one at the open end of the chassis, really).
Then it has a magnet wich interacts with the coil, this can be described
as the force-factor, how much Neweton force per ampere voice coil
current. Usually the speaker sits into an enclosure, so the air
which is in there will act like a spring, too.
I do not want to talk about vented enclosures or resonators.

All the springs are nonlinear, I hope that the motor will be almost linear.
The nonlinearity is required to hold the cone, otherwise it will pop out.
So it is a nonlinear mass-spring-motor problem.
It is further complicated in that the magnet will induce a voltage
proportional to magnet field B and cone speed v in the coil.
This will counteract the driving force and will damp the system.

{
At higher frequencies the cone will "brake up", i.e. it will not
move as a whole, but separate regions will move in different modes.
These regions should be avoided.
}

So, it is not so easy to tell when the speaker is "easy" to drive.
Low mass, high B field and a lot of coil turn give strong force and
low inertia, but then the cutoff frequency may be too high...

After all, you want sound output, and this depends also on cone size,
because the radiation resistance depends on cone size and frequency...


I did never wonder why speakers work, until I found out that pressure
is proportional to cone speed. OTOH the pressure should be proportional
to the amp VOLTAGE. How can this work at all??
The solution is the said B x v coil voltage. If the amp gives a large
voltage, a very large current will flow. This accellarates the cone,
until the velocity will give enough counter-voltage to keep the current
low. A kind of feedback process. Now you tell me that you want to avoid
feddback in your audio chain, because it is a bad thing... ;->
As a result cone speed will be quite proportional to amp voltage.

Of course the are phase shifts and the damped mass-spring system has
it's own resonance...


m.c.




-----Original Message-----
From: James Patchell [mailto:patchell at cox.net]
Sent: Samstag, 16. August 2003 00:28
To: Synth-Diy
Subject: RE: thanx: RE: [sdiy] ot: please recommend a low power pentode
power amp circuit


I try to avoid getting involved in these threads...

But.....

A speaker is nothing more than a Motor....yep, a linear motor, with a 
spring (to keep it centered).  There will be many factors that determine 
how easy the speaker is to drive.  But you can't forget about the magnet 
and the voice coil (which, is the motor).  My experience has shown quite 
the opposite, however, from what you stated.  I noticed that generally the 
bigger the speaker, the easier (less power) required to drive it....however 
bigger speakers generally also have bigger magnets and coils....

-Jim

At 03:09 PM 8/15/2003 -0600, Brousseau, Paul E (Paul) wrote:
>Also, recall speaker efficiency.  I think (no PhD here, only a BS...) that 
>a smaller speaker (i.e., headphone) will be easier to drive than a 10", 
>12", or 15" speaker.
>
>--PBr
>
>-----Original Message-----
>From: Nils Pipenbrinck [mailto:np at inverse-entertainment.de]
>Sent: Friday, August 15, 2003 6:04 AM
>To: Synth-Diy
>Subject: AW: thanx: RE: [sdiy] ot: please recommend a low power pentode
>power amp circuit
>
>
>I refused 'cause I don't liked the idea to build an amplifier that can
>permanently ruin the ears. Now he has 300mW, and I doubt he ever pushed it
>harder than 100mW.