What does a speaker prefer to eat?

[rcollege]

Hello all,

Excuse my ignorance, but I have often wonderes ( and haven't found the answer for) what drives a speaker? I know it is watts...but what is the most substantial part of watts drives the speaker? Is it Voltage? or mainly amperage...(current).

[Andy5112405]

Both.

However, it is the current flowing through the voice coil that provides the motor effect that pushes the cone backwards and forwards.

In order to get a current to pass through the resistance of the voice coil, you must apply a voltage across the coil.

Some speakers are more current hungry than others. Good quality speakers generally respond better with amplifiers with substantial power supplies.

This is a very general overview of a subject that is very complex.

Look up Faraday's Law on Wikipedia.

[aardvarkash10]

at a superficial level its not that complex... A current will flow through a resistance if a voltage is applied across the resistance.

No voltage, no current.

A speaker has a winding in it that has a resistance. Apply a voltage to the winding and a current will flow through it.

The greater the voltage, the greater the current that flows.

This implies two things - as long as the voltage is applied from a source that has limitless current capability, an increase in applied voltage will lead to an increase in current flow.

Which gets to the point that Andy makes - a good quality amp starts with a top quality power supply.

Of course not all speakers are the same (some are more equal than others). A low impedance speaker of a particular rating (say, 4 ohm, 91db/w) will require a power supply capable of delivering higher current than an equivalent high impedance speaker (say, 16 ohm, 91db/w). The high impedance speaker will need an amp with greater ability to swing a voltage.

Which is, in part, why tube amps tend to be happier with higher impedance speakers - they are largely voltage amplifiers and not designed to deliver high currents to the speakers, while ss amps are largely current amps and don't apply huge voltages...

I generalise wildly for simplicity - hope it helped.

[gainphile]

What is the difference between using current (transconductance amp) like those of FirstWatt with "normal" amps ?

[Andy5112405]

There are hundreds, if not more, parameters that need to be considered when designing a loudspeaker. However, here is a bit of food for thought.

Some novices think that a BASS woofer flapping is a sign of volume. A good speaker will provide 100dB with no real visual movement of the speaker cone. You will certainly be able to feel it moving.

This is generally achieved by making the suspension of the speaker cone quite stiff.

Immediately we have a conflict of requirement. We want the speaker cone to be able to respond rapidly to small signals from the amplifier AND we want minimum distortion from the cone.

Stiffening up the cone suspension increases the inertia of the cone so that it requires more current to get it moving.

If you prefer "Twing Twang" music then a lighter suspension may sound better to you. If you like "Heavy Rock" or "Orchestra" then tighter suspensions may sound better.

Valve amps are better with more sensitive speakers. However, a good output transformer will be good at transforming the High Voltage/Low Current output of the valve into Low Voltage/High Current for the speaker.

You will notice that a lot of HIGH-END amplifiers are only around the 100W mark. However the output stages contain devices that could easilly cope with 10 times that power. The Power Supplies are also very over-rated.

My Linsley Hood 80W Power Amp can actually deliver nearly 17A peaks, that certainly overcomes any inertia problems in my B&W speakers.

[chris661]

Current drive amplifiers (where they're designed to stick a certain amount of current through, regardless of voltage) don't respond too well to peaky impedances of say, ported cabinets. Where the twin impedance peaks are, the amplifier still tries to deliver the same current as everywhere else. However, because the impedance is very high (can be 10x nominal), the volts are ramped up to try to push the current through. This leads to an "interesting" sound.

Voltage drive amps tend to care little about impedance spikes. It'll apply the volts, supply current when current flows. Of course, a very low impedance will destroy lesser amplifiers as they try to shove too much current through.

That's about as much as I know. If there's errors, let me know and I'll revise things.

Chris

[T101]

Generally it doesn't matter how the watts are derived.
This is the OHM's Law: Ohm's law - Wikipedia, the free encyclopedia

I=V/R

and W=VxA where A (ampere) is actually "I" from the Ohm's Law.

Watt is a unit for power and when we refer to power, we always have in mind amount of work for a given amount of time.

It is the same with horsepower of an internal combustion engine - it is derived by the engine torque and engine speed in revolutions per minute.

1 Watt can be result of endless number of combinations, such as 10V and 0.1A or 10A and 0.1V or even 500A and 0.002V

At 100 mph on level road the air resistance of a vehicle can be overcome with not less than 50 (36.76kw) horse power and the air resistance doesn't care if it is achieved by a petrol engine with 2000 n.m. (1475 lbft) and 500 rpm or with 4000 rpm and 100 n.m. or even by a electric motor driven by 200V and 180A or with 50V and 735A...

If we know the impedance of the speaker and the voltage across it's voice coil, we can find the watts driving it at the moment.
For instance a 8 ohm speaker with 63 volts across the input terminals is being driven by 496 watts.

I=V/R so I=63/8, I=7.875A
W=IxV so W=7.875x63, W=496.125 - and that's how a normal amplifier works.

A current drive amplifier will keep the voltage the same and alter the current in accordance with the signal form. - I don't know how that happens without voltage rise when the load is a constant value? But my guess is that a current drive amplifier would supply greater dynamics than the voltage drive ones?

After all Watts do the work and everything other is just it's component.

So the answer would be: The speaker eats Watts.

And a more specific answer will be, that the speaker can not discriminate from volts or watts in theory. In the real world it would be very unpractical to move huge amounts of current through enormous cables or the opposite - to deal with life threatening voltages.
The issue for overcoming the initial resistance in order to be able to drive the voice coil does mot apply to materials which are conductors by nature, this only applies to dielectric substances such as plastic insulators or the human skin. - the human skin if it is dry does not conduct electricity with less than 30 volts no matter of the current.

Qurious Fact: a 100W into 8 ohms amplifier actually delivers around 28.3 volts and 3.5375 amps

[Andy5112405]

Oh if it were as simple as Ohms Law.

Speakers are far from simple resistors.

Even the most basic 2-Way effort from the local Flea Market will be a complex combination of Inductance (The windings of the voice coils), Resistance (The wire itself) and Capacitance (The crossover to the Tweeter).

More complex speakers will also have L,C and R in varying combinations.

This all plays havoc with the impedance and what the amplifier is trying to drive.

My speakers vary from 2 Ohms to over 500 Ohms over the main frequency spectrum. Just do a search on any popular speaker and look at its impedance / frequency graph.

[T101]

Yes a speaker is definitely a complex load, but it all still works according the OHN's Law.

For instance if fed with 63V your speakers will be driven with no more than 7.938 watts in the 500 ohm impedance region and with not less than 1984 watts in the 2 ohm region of frequencies. (if frequencies with the corresponding amplitude are present in the signal of cource)

As a consequence you will have excessive cone motion in the 2 ohm region of frequencies and a hole in the SPL response in the 500 ohm region. And the amplifier will be unstable.

In practice impedance peaks correspond to something - too big separation in the crossover for instance or a resonance peak.

If it is the first there most probably would be a hole in the frequency response.

if it is the second the frequency response might be either even in that area or it can peak too... Resonance means that the natural retraction to 0 position speed of the driver suspension is equal to the speed it is driven, and due to that the coil presents lesser load to the driving current.
A lesser load in terms of electricity would be always a bigger resistance - less current draw. - The speaker resonates at it's natural frequency and the amp only adds up for the work done other than overcoming speaker's internal resistance.

Impedance dips correspond to voice coil inductance and better membrane to air coupling.

Here is a funny conclusion: a 8 ohm speaker with a resonance peak of 50 ohms at 40 hertz will draw only 8 watts at 40 hz if it is driven with 20V, even though that 20V into 8 ohms is 50 watts.
That means that on every similar, but non resonant frequency 42 watts are wasted only for overcoming of membrane inertia and suspension resistance...
42 out of 50 is 84%
But we know that a speaker rarely has acoustic efficiency of over 3.5%...

We know where 84% of the power goes and where 3.5% of the power goes.

That leaves us to wonder where the other 10.5% of the power goes? - any clues?
note that depending on the impedance peak at resonance frequency and on the actual acoustic efficiency we might have less percents of power to wonder for...

[Andy5112405]

Simple

HEAT

A lot of the energy sent to the speaker is lost as HEAT. 10.5% is very conservative.

Let's no lose the poor guy that started this thread im detailed conversation. He's asking a simple question and we are now baffling him with science.