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How do amps drive speakers?

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Posted on September 22, 2024 at 21:39:39
George S. Roland
Audiophile

Posts: 1507
Location: N W Pennsylvania
Joined: March 20, 2004
 I posted this question a few years ago. Answers were not especially informative, likely because I'm not an electrical engineer or physicist and do not know how properly to frame a question to get the answers I'm looking for.
I do know basically how a dynamic loudspeaker works--cone, voice coil, magnet, spider, surround and so on. My interest here is exactly what the amp does in driving the speaker.
Let us assume for this question that we are talking about one dynamic driver connected to an amp.
There is a circuit here, two wires from the amp to the speaker. So when there is a need to push the cone forward, a certain force is applied to the voice coil and the cone moves forward. What is happening at that moment to the rest of the circuit? Does the amp output a signal to draw the cone back from its forward movement or does the cone just try to restore itself to its normal position?
Is the amp sending alternating power to both push the driver out and pull the driver in?

 

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RE: How do amps drive speakers?, posted on September 23, 2024 at 03:52:36
cawson@onetel.com
Audiophile

Posts: 2488
Joined: September 27, 2004
 Your last sentence has the secret!

Looking at its simplest where the amp is delivering a single frequency, say 100 Hz, the amp delivers an alternating current that fluctuates 100 times per second. The magnet within the speaker reflects this by changing its polarity 100 times per second and this fluctuating signal induces movement of the voice coil and therefore the cone - also 100 times per second. Thus the amp's alternating signal will result in movement of the cone at the same frequency. The cone will stop moving in and out the instant the signal from the amp ceases.

The clever bit is that the amp can deliver hundreds of these AC frequencies simultaneously and at different intensities (strengths) and this results in "music". Because large cones cannot be expected to move 15,000 times per second. a crossover circuit is used to split high and low frequencies that are sent to 2 or 3 drivers that are each designed to handle a smaller range of frequencies.

The other way to answer your basic question of how the amp's signal can be made into music is simply "by magic". It is pretty remarkable that music from an orchestra or band can be reasonably well reproduced by an amp and a couple of paper cones. I quite like the "magic" theory myself and leave it at that!

 

RE: How do amps drive speakers?, posted on September 23, 2024 at 05:40:37
George S. Roland
Audiophile

Posts: 1507
Location: N W Pennsylvania
Joined: March 20, 2004
 Thank you for this clear, comprehensible explanation. I agree with you that it's magical. Transducers are amazing. How the stylus in an LP groove, wiggling around in that spinning ditch can reveal choruses singing behind a symphony orchestra with the degree of detail it does is astonishing. The speaker/amp interplay is equally astonishing. It's easy to grasp the principle when thinking about sine waves, but music? Magical indeed!
Cheers!

 

RE: How do amps drive speakers?, posted on September 23, 2024 at 07:06:36
Inmate51
Industry Professional

Posts: 2805
Location: Dallas, TX
Joined: August 12, 2022
 "The magnet within the speaker reflects this by changing its polarity 100 times per second and this fluctuating signal induces movement of the voice coil and therefore the cone "

You've got it backwards, maybe accidentally. I'll spot you that.

The magnet in a dynamic loudspeaker is a permanent magnet. It's polarity doesn't change. Whan curent is passed through the voice coil (a coil of wire), the coil generates its own magnetic field. This field interacts with the magnetic field of the permanent magnet, causing the coil to move. Since the curent running through the coil alternates from positive to negative repeatedly, and this movement is controlled by the shape of the waveform of the signal, the movement of the coil is not strictly just back-and-forth, but rather it has many undulations along its travel, which are (ideally) a mirror of the current's waveform.

But, since a speaker is a transducer - converting an electrical signal into physical movement, there are "issues" with regard to how well the moving assembly follows the waveform. So, it's the alternating current running through the voice coil which makes the voice coil-spider-diaphragm-surround assembly make sound, which often is a reasonable facsimile of the electrical signal.

Many years ago, I wrote a "white paper" on the fundamental failure modes of dynamic loudspeakers. Basically, they come in two flavors: Heat-related failures, and mechanical failures. I'll try to find the diagram I drew regarding basic speaker design components.

*********

We are inclusive and diverse, but dissent will not be tolerated.

 

RE: How do amps drive speakers?, posted on September 23, 2024 at 08:30:41
cawson@onetel.com
Audiophile

Posts: 2488
Joined: September 27, 2004
 > You've got it backwards, maybe accidentally.


Thanks for correcting my schoolboy error!

 

The basic physics for magnetic speakers, whether cones or Magneplanars is , posted on September 23, 2024 at 16:44:30
kff
Audiophile

Posts: 1063
Location: SE PA
Joined: October 19, 2006
 that electricity (from the amp) moving in a magnetic field (the voice coil gap, the gap between the bar magnets on Maggies, produces perpendicular movement.

The voice coil gap is the empty ring between the pole piece and the magnet or the gap between two adjacent bar magnets in Maggies and there is a strong magnetic field here.

The amp power goes through these magnetic fields generally at 90


This post fell apart when I used alt+248 for the degree symbol, lost 2/3 of the content. Oh well, guess I won't use those ascii codes on this forum.

 

Right hand rule. nt, posted on September 24, 2024 at 17:17:22
G Squared
Audiophile

Posts: 8793
Location: Washington, DC Metro Area
Joined: November 16, 2004
Contributor
  Since: May 23, 2023
 .
Gsquared

 

This tells you the direction of the movement in or out based on , posted on September 25, 2024 at 11:58:26
kff
Audiophile

Posts: 1063
Location: SE PA
Joined: October 19, 2006
 polarity of the voltage in the fixed magnetic field

 

RE: How do amps drive speakers?, posted on September 26, 2024 at 15:34:25
tomservo
Manufacturer

Posts: 8714
Joined: July 4, 2002
 The amplifier is a Voltage source with a low output impedance.
The Electrodynamic loudspeaker is a DC motor being driven with an AC signal that makes it change directions.

The wire in a magnetic field is two things, it produces a force proportional to the current flowing thorough it AND it is a generator that produces a voltage proportional to how fast it is moving. That generated voltage is called back EMF and is the counterpart to the force producing part of the motor. That force is often specified as BL which in force terms is "newtons of force per amp of current). The larger the BL is, the larger the back emf is, they are connected.

For a woofer, it's moving mass and the suspension "spring" appear to be a parallel capacitor and parallel inductor in the woofer equivalent, those parts driven through a resistor (the driver's Rdc) and series inductor (driver le).
There is more but does this make sense?
Tom



 

RE: How do amps drive speakers?, posted on September 26, 2024 at 15:50:29
Craiger56
Audiophile

Posts: 5648
Location: San Jose CA
Joined: April 3, 2002
Contributor
  Since: December 29, 2003
 Hi Tom

Could you comment further on the use of autoformers and how they do what they do, and why they aren't universally popular?

 

RE: How do amps drive speakers?, posted on September 26, 2024 at 17:29:02
tomservo
Manufacturer

Posts: 8714
Joined: July 4, 2002
 Umm, if there is a disadvantage to them, it is that compared to a transformer, an auto-transformer does not have DC isolation between the input and output, in other words, with a regular transformer, one winding can "float" at a DC voltage like in a tube output transformer where the primary is floating at say 500Vdc while the secondary is at ground potential.

The auto transformer has an advantage in that it can be physically a little smaller for a given power because there is less copper wire. The greatest advantage is when what you need is a 2:1 voltage ratio because then you have two equal windings.

The low frequency response corner is set in both cases by the inductance of the primary winding, looks like a parallel inductor and at say .1Hz, what you see is only the DC resistance of the winding.
There is a saturation limit in both cases but it is not what most think. This "limit" is a 6dB per octave slope and a level in which the core become nonlinear.

Take a power transformer at 60Hz and a 120V winding, the core is nearing this level but not there yet. There is a connection to the Voltage per turn and the core flux and area.

BUT at 30Hz that same saturation limit is now 60V but at 120Hz, that limit is 240V, so this is only a low frequency issue, a 6dB/oct slope (that is because as you go up in frequency, the core flux needed to produce a given voltage falls).

Both types have high frequency limits as well, the main one called leakage flux is when the primary winding produces a magnetic field that does not couple to the secondary and so, looks like a simple series inductor (low pass filter). The fix is putting the primary and secondary closer together or even layering them or intermixing them "multi-filar" winding.

I wound a large output transformer for a 250W tube amp once, wound on a form and the coupling was so good, it would have worked fine above about 5Hhz, without any of the core lamination in place (no iron).
The higher you go, the less the core is involved and the more inter winding coupling matters


 

RE: How do amps drive speakers?, posted on September 26, 2024 at 17:49:57
George S. Roland
Audiophile

Posts: 1507
Location: N W Pennsylvania
Joined: March 20, 2004
 Hi Tom,
Thanks for this explanation, but I'm afraid it is way over my head. I was sort of with you until the third paragraph where I became completely lost.
Thanks again for your time in trying.

 

RE: How do amps drive speakers?, posted on September 27, 2024 at 05:46:27
tomservo
Manufacturer

Posts: 8714
Joined: July 4, 2002
 I jumped too far ahead haha, well so far the power and internet is still on here, so...

The moving wire in a magnetic field (voice coil etc) is a conversion mechanism where the electrical impedance shows what's going on mechanically on the other side of the input terminals.

After conversion, the cone/ coil moving mass appears electrically to be a capacitor and the suspension "spring" appears to be an inductor. These two are electrically in parallel and make the large impedance peak at the drivers free air resonance or box resonance if in a box.

Those parallel parts are electrically in series with the wire's DC resistance and the coil inductance.

ok better stop, power in going on and off here in north Ga
Tom

 

Thank you sir (nt), posted on September 27, 2024 at 10:22:54
Craiger56
Audiophile

Posts: 5648
Location: San Jose CA
Joined: April 3, 2002
Contributor
  Since: December 29, 2003

 

RE: How do amps drive speakers?, posted on September 30, 2024 at 07:41:04
John Elison
Audiophile

Posts: 23976
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 Any time an electrical current is sent through a speaker's voice coil, it produces a magnetic field in the voice coil. This magnetic field interacts with the magnetic field of the speaker's permanent magnet thereby causing the voice coil to move. Since the power amplifier delivers an alternating electrical signal to the voice coil, it makes the voice coil and speaker diaphragm move in both directions creating sound.

The reason you need a power amplifier is because the electrical signal coming from the preamplifier does not produce enough power to make the speaker play loud enough. Therefore, the power amplifier is designed to increase the power of the electrical signal coming from the preamplifier before it's delivered to the speaker in order to make the music loud.

Happy listening!

 

The Physics of how a speaker works is just this statement, posted on October 1, 2024 at 17:27:58
kff
Audiophile

Posts: 1063
Location: SE PA
Joined: October 19, 2006
 Electricity (from the amp) moving in a magnetic field (the voice coil gap where the electricity is moving in the voice coil, produces perpendicular motion.

The magnetic field goes from the pole piece in the center of the speaker to the magnetic material surrounding the voice coil. This is across the speaker, generally in the plane of the soundboard that the driver is mounted to.

The electricity from the amp is in the voice coil. This is generally perpendicular to the magnetic field as the voice coil curves around in the gap.

The perpendicular motion from the rules of physics is the in and out motion of the voice coil connected to the cone. If the signal is just DC the cone will move a certain amount in or out depending on if the DC is + or - and how the amp is hooked up to the driver. Constant DC will position the cone away from its normal no signal position. This is why we check amps for DC offset, we don't want it.

If it is an alternating current like music the cone moves in and out making hopefully audible sounds we enjoy.

 

RE: How do amps drive speakers?, posted on October 2, 2024 at 15:34:51
seancuster71@gmail.com
Industry Professional

Posts: 28
Location: Florida
Joined: August 22, 2024
 Finally, a good question to answer. Unfortunately, it is complex and if you are not familiar with a speaker's specs aka Thiele/Small parameters it may be a little hard to understand. I am a speaker designer, not a teacher but I will do my best to answer.

An amplifier does not output a signal to pull the cone back to its original position an amplifier does contribute to cone control via dampening factor. To answer your question properly we need to first learn more about speakers and how they stop/dampen a cones movement.

First, we need to know the basics of how a speaker works. Whenever you have wire moving in a magnetic gap it will create electrical current. Do you have an old speaker and a voltmeter? Hook the voltmeter up to the speakers' terminals and set it to read DC voltage the voltmeter will read 0 volts DC now start pushing the cone down rapidly and you will see it generates DC voltage.

Now let's look at a speakers Qms this is the mechanical resistance of the suspension aka spider & surround the lower the Qms the stronger the suspension so the faster it is going to return to its original position this is called dampening a speaker with low Qms has more/higher dampening.

Qes is a speaker's electrical resistance/attenuation as the voice coil moves in the gap it creates opposing current this counteracts the flow of current from the amplifier and helps stop/dampen speaker cone movement. So, it is the combination of a speakers Qms & Qes working together to dampen/stop cone movement.

Qts is the total Q of a speaker so a speaker with low Qes,Qms will have a low Qts the lower the Q the higher the dampening the higher the dampening the faster and better the cone movement is controlled. Also, your amplifiers dampening factor also contributes to dampening/cone control.

This is why I always tell Audiophiles you must learn as much as possible if you want to put together a nice sounding system. things are not just plug and play everything is working together in an audio system things need to be matched to work together.

For example, say you have the opposite of a low Q speaker say you have speakers that have a high Q less dampened less cone control you do not want to hook them to a tube amp that has a very low dampening factor you will end up with slow muddy sounding bass. But if you have a Low Q highly dampened speaker and you hook them to a good amp with a high dampening factor you will have punchy tight articulate bass.






 

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