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Topic: Amp power and current (Read 52279 times) previous topic - next topic
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Amp power and current

Reply #225 –
Watts are volts x current in to a load. Current gives control (like torque) and voltage is loudness (speed).
You can achieve a 100 watt rating with current in reserve or not.

Amp power and current

Reply #226 –
Quote from: Yahzi on
Watts are volts x current in to a load. Current gives control (like torque) and voltage is loudness (speed).
You can achieve a 100 watt rating with current in reserve or not.


I thought control was a byproduct of amplifier damping???

Amp power and current

Reply #227 –
Quote from: Rich B on
Quote from: Yahzi on
Watts are volts x current in to a load. Current gives control (like torque) and voltage is loudness (speed).
You can achieve a 100 watt rating with current in reserve or not.


I thought control was a byproduct of amplifier damping???


I don't think thats a great analogy, and I wouldn't worry too much about trying to make sense of it.

Amp power and current

Reply #228 –
Quote from: Rich B on
I thought control was a byproduct of amplifier damping???


Amplifiers don't "damp" anything.  Any damping is done by the speakers themselves.

Take a woofer and tap it's cone (gently) while it is unconnected to anything, and you will excite it's natural resonance and hear a nice resonant "thump".  Short circuit the terminals and do the same thing and the "thump" largely disappears. No amplifier is needed. This is a result of the back emf generated by the voice coil in a magnetic field when it's circuit is closed.

A coil moving in a magnetic field is a generator.  In this case the voltage generated by moving the speaker voice coil opposes the motion of the voice coil and so resists it.  Since the loudspeaker moves most at resonance the back e.m.f. resists most effectively at resonance, thus "damping" the main resonance.

The "damping factor" of an amp is merely a measurement of it's internal resistance.  The closer this is to zero the more the back emf can act and so the speaker's resonance is damped more.  The amp doesn't damp anything, it merely gets out of the way and allows the damping to occur.

This is how I understand things, at least.  Anyone with more knowledge about this than I is invited to contradict me.
Ed Seedhouse
VA7SDH

Amp power and current

Reply #229 –
Quote from: Rich B on
Quote from: Yahzi on
Watts are volts x current in to a load. Current gives control (like torque) and voltage is loudness (speed).
You can achieve a 100 watt rating with current in reserve or not.


I thought control was a byproduct of amplifier damping???


That is correct. The best technical term is source impedance. An ideal amplifier has a source impedance of zero. Very good amplifiers have source impedance that are close to zero.

Control of a speaker by an amplifier is observable as an absence of change to the voltage across the speaker's terminals as the frequency changes all other things being equal. If this voltage remains the same or close to it as the impedance of the speaker changes with frequency, then the speaker is well-controlled. If this change is less than 1% then further improvements in this area of performance are ensured to be inaudible. If this change is 10% or more then the control of the speaker by the amplifier is questionable.

Questionable control of speakers is typically demonstrated first and foremost by tubed amplifiers with SETs being in general the worst.

The situation where the current and voltage capabilities of the amplifier enter into this discussion is when the amplifier is clipping.  Amplifiers can clip due to current limiting or voltage limiting. All amplifiers have limits in both areas.

Limits due excess voltage are acceptable if they are in accordance with the specifications of the amplifier.  Our model of an ideal amplifier is a device whose operation is accurately characterized by its input and output voltages.

Amp power and current

Reply #230 –
Does impedance vary with signal amplitude as well as frequency? Someone told me that, for example, a 4 Ohm speaker can easily reach down to 2 Ohms or less when driven hard?

Amp power and current

Reply #231 –
Quote from: Rich B on
Does impedance vary with signal amplitude as well as frequency? Someone told me that, for example, a 4 Ohm speaker can easily reach down to 2 Ohms or less when driven hard?

If anything this will raise the temperature of the voice coil, increasing its impedance.

On the other hand, as the temperature of the permanent magnet increases this could decrease its magnetic strength, making the speaker less efficient.

Amp power and current

Reply #232 –
Quote from: Rich B on
Does impedance vary with signal amplitude as well as frequency? Someone told me that, for example, a 4 Ohm speaker can easily reach down to 2 Ohms or less when driven hard?


In extreme cases, there is some variation in driver impedance with drive level.

The most common situation is where the driver voice coils heat up and their impedance increases to as much as double or more. This takes pretty serious abuse.

If a speaker voice coil is driven out of the magnet's gap, its impedance can decrease, but it won't drop below the voice coil's resistance.  Therefore the claim that a 4 Ohm speaker can easily reach down to 2 Ohms or less when driven hard is an exaggeration.  Most 4 ohm speaker voice coils have DC resistance of no lower than 3 ohms, and that puts a floor under how low the driver's impedance can go. In order to have this sort of thing happen, the audio system has already stopped being anything like hi fi.

These are the sorts of thing that are experienced by bass guitar players who are too drunk to care what their instrument sounds like.

Amp power and current

Reply #233 –
Rich B, you gotta distinguish between nominal impedance, impedance vs. frequency, and changes in impedance due to various factors (heat, mechanical stress, damage ..).


Here's the impedance vs. frequency of a single driver (woofer in this case). The nominal or rated impedance is taken at impedance "valley", usually roughly 20% above DC resistance:


Here's the impedance vs. frequency of a full loudspeaker. Ignore the phase (dashed line):


The manufacturer specifies a nominal impedance of 8 ohms for this loudspeaker, but at ~180 Hz we dip down to 4 ohm.

So if you played a pure sine tone through this loudspeaker at 180 Hz, the amplifier would be loaded quite a bit more than at 80 Hz, for example. Even if you amp would blow up in such a test, it could be completely fine when playing real music that doesn't have all energy concentrated at one frequency, at the highest possible level.


Here's a loudspeaker where the manufacturer specifies a nominal impedance of 4 ohms, even though the impedance stays above 5 ohms at all frequencies:


So as you can see, manufacturers take some freedoms in their specifications... hence the need for proper measurements if you really wanna know what's going on.
"I hear it when I see it."