Question on box rise and constant power amps.

[Brian617]

I'm having a tough time understanding the interaction between amp and sub at different frequencies. My understanding of a resonant frequency of a speaker is that particular frequency at which the speaker is easiest to move, and which requires the least power to maintain that movement. As a consequence of more movement per amount of current, the voice coil generates more back emf resulting in impedance rise. I'm assuming "box rise" is due to the box facilitating movement and increasing back emf as a result as well.

My question is what is the point of a constant power amp? It seems the back emf at different frequencies compensates for how easily the speaker is driven at that frequency allowing for a flatter curve. Wouldn't a constant power amp overdrive the speaker at resonant frequency introducing the need for more equalization? Wouldn't damping factor be more important for speaker control at resonance?

[Krakin]

I'm interested in this as well, as I have only related this to nominal DC current.

[MrSkippyJ]

I'm not sure about this, but I don't think resonant frequency has anything to do with power.

I'm interested in this as well, as I have only related this to nominal DC current.

related what to DC current?

[Brian617]

I'm not sure about this, but I don't think resonant frequency has anything to do with power.

I'm interested in this as well, as I have only related this to nominal DC current.

related what to DC current?

The impedance rises as the signal approaches the resonant frequency. The impedance seen by the amp is highest at that point, so the power output is reduced. But the power needed is reduced too. Go figure.

http://www.caraudiohelp.com/newsletter/speaker_impedance.htm

[MrSkippyJ]

I meant measuring resonant frequency doesn't involve power. I could be very wrong though.

[finkster]

I asked the exact same question a while back to Boon via PM. Here's what he had to say.

I would say that they use current limiting on the outputs, with quite high rail voltage.

So the amp is capable of, say, 70vac(rms) at the outputs, giving it 2450wrms into 2 ohms.

But let's say the output section can only handle 60A of continuous throughput. This is heaps for running 2450wrms @ 2 ohms (Only takes 35A) but 70vac at 1 ohm is going to result in 70A of current and blow the amp. So my guess is that they limit the output current to, say, 50A. This way if you run it at 1 ohm you still get 2500wrms (50A at 50V) but as your impedance rises the voltage can rise too so you get the same output.

All this probably makes them quite expensive.

Regards,

Logan

[Dwn4BassAlan]

I'm not sure about this, but I don't think resonant frequency has anything to do with power.

I'm interested in this as well, as I have only related this to nominal DC current.

related what to DC current?

The impedance rises as the signal approaches the resonant frequency. The impedance seen by the amp is highest at that point, so the power output is reduced. But the power needed is reduced too. Go figure.

http://www.caraudiohelp.com/newsletter/speaker_impedance.htm

I just read through very quickly so I might have missed something but if I understand one of your inquiries is about the resonance and impedance rise? Basically, this is the point where the coil will be the easiest to move as you stated, so it also is passing thorough the most amount of magnetic field, therefore the highest amount of back EMF starts to come into play, along with other things like eddy currents, basically bob's your uncle, your highest impedance rise. That's my dumbed down version of it.

I notice the article I used to teach myself over summer has answers to almost all, if not all of your questions. You're Welcome :

Resonance:

The moving system of the loudspeaker (including the cone, cone suspension, spider and the voice coil) has a certain mass and compliance. This is most commonly likened to a simple mass suspended by a spring that has a certain resonant frequency at which the system will vibrate most freely.

This frequency is known as the "free-space resonance" of the speaker and is designated by F_s. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak amplitudeand velocity, the back-emf generated by coil motion in a magnetic field is also at its maximum. This causes the effective electrical impedance of the speaker to be at its maximum at F_s, shown as Z_max in the graph. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches F_s and is inductive in nature.

At resonance, the impedance is purely resistive and beyond it—as the impedance drops—it behaves capacitively. The impedance reaches a minimum value (Z_min) at some frequency where the behaviour is fairly (but not perfectly) resistive over some range. A speaker's rated or nominal impedance (Z_nom) is derived from this Z_min value (see below).

Beyond the Z_min point the impedance is again largely inductive and continues to rise gradually. The frequency F_s and the frequencies above and below it where the impedance is Z_max/√2 are important in determining the loudspeaker's T/S parameters which can be used to design a suitable enclosure for the driver, especially for low frequency drivers. Note that F_s is itself one of the T/S parameters of the loudspeaker.

Load impedance and amplifier

The variation in loudspeaker impedance is a consideration in audio amplifier design. Among other things, amplifiers designed to cope with such variations are more reliable. There are two main factors to consider when matching a speaker to an amplifier.

Minimum impedance

This is the minimum value in the impedance vs. frequency relationship, which can sometimes be slightly higher than the DC resistance of the voice coil, i.e., as measured by an ohmmeter. Minimum impedance is significant because the lower the impedance, the higher the current must be at the same drive voltage. The output devices of an amplifier are rated for a certain maximum current level, and when this is exceeded the device(s) sometimes, more or less promptly, fail.

Nominal impedance

Due to the reactive nature of a speaker's impedance over the audio band frequencies, giving a speaker a single value for 'impedance' rating is in principle impossible, as one may surmise from the impedance vs. frequency curve above. The nominal impedance of a loudspeaker is a convenient, single number reference that loosely describes the impedance value of the loudspeaker over a majority of the audio band. A speaker's nominal impedance is defined as:

The graph above shows the impedance curve of a single loudspeaker driver in free-air (unmounted in any type of enclosure). A home hi-fi loudspeaker system typically consists of two or more drivers, an electrical crossover network to divide the signal by frequency band and route them appropriately to the drivers, and an enclosure that all these components are mounted in. The impedance curve of such a system can be very complex, and the simple formula above does not as easily apply.

The nominal impedance rating of consumer loudspeakers systems can aid in choosing the correct loudspeaker for a given amplifier (or vice versa). If a home hi-fi amplifier specifies 8 ohm or greater loads, care should be taken that loudspeakers with a lower impedance are not used, lest the amplifier be required to produce more current than it was designed to handle. Using a 4 ohm loudspeaker system on an amplifier specifying 8 ohms or greater could lead to amplifier failure.

Impedance phase angle

Impedance variations of the load with frequency translate into variation in the phase relationship between the amplifier's voltage and current outputs. For a resistive load, usually (but not always) the voltage across the amplifier's output devices is maximum when the load current is minimum (and the voltage is minimum across the load) and vice-versa, and as a result the power dissipation in those devices is least. But due to the complex and variable nature of the driver/crossover load and its effect on the phase relationship between the voltage and current, the current will not necessarily be at its minimum when the voltage across the output devices is maximum - this results in increased power dissipation in the amplifier output stage which manifests as heating in the output devices. The phase angle varies most near resonance in moving coil loudspeakers. If this point is not taken into consideration during the amplifier design, the amplifier may overheat causing it to shut down, or cause failure of the output devices. See Power factor for more detail.

Damping issues

A loudspeaker acts as a generator when a coil is moving in a magnetic field. When the loudspeaker coil moves in response to a signal from the amplifier, the coil generates a response that resists the amplifier signal and acts as a "brake" to stop the coil movement. This is the so-called back EMF. The braking effect is critical to speaker design, in that designers leverage it to ensure the speaker stops making sound quickly and that the coil is in position to reproduce the next sound. The electrical signal generated by the coil travels back along the speaker cable to the amplifier. Well-designed amplifiers have low output impedance so that this generated signal has little effect on the amplifier.

Characteristically, solid state amplifiers have had much lower output impedances than tube amplifiers. So much so that differences in practice between a 16 ohm nominal impedance driver and a 4 ohm nominal impedance driver have not been important enough to adjust for. Damping factor (ratio of output impedance (amplifier) to input impedance (driver voice coil)) are adequate in either case for well-designed amplifiers.

Tube amplifiers have sufficiently higher output impedances that they normally included multi-tap output transformers to better match to the driver impedance. Sixteen ohm drivers (or loudspeakers systems) would be connected to the 16-ohm tap, 8 ohm to the 8 ohm tap, etc.

This is significant since the ratio between the loudspeaker impedance and the amplifier's impedance at a particular frequency provides damping (i.e., energy absorption) for the back EMF generated by a driver. In practice, this is important to prevent ringing or overhang which is, essentially, a free vibration of the moving structures in a driver when it is excited (i.e., driven with a signal) at that frequency. This can be clearly seen in waterfall measurement plots. A properly adjusted damping factor can control this free vibration of the moving structures and improve the sound of the driver.

[Wicks]

Well, we could look at what Fosgate says about their Constant Power technology:

Constant Power

U.S. Patent 7,994,857

Constant Power is a patented circuit topology that provides instantaneous delivery of output power over varying impedance loads.

Music and speakers have a dynamic relationship. As the frequency of music varies, so does the impedance of the speaker. Rockford Fosgate’s patented Constant Power circuit design actively monitors and increases voltage and current as the speaker’s impedance rises with music, resulting in up to a 25% increase in total amplifier power.

Here's an impedance sweep I did on a Hertz ES200 8" subwoofer of mine:

Here you can see how the impedance changes with frequency.

And also greatly depends on how the driver is used.

A raw driver contains a single large impedance peak (almost 30 ohms) at resonance.

In a sealed box it still has a single peak which has moved in frequency, and surprisingly has a reduced impedance at resonance compared to the raw driver.

So therefore it appears to have a negative "box rise".

In a ported box, the driver actually has an impedance dip at the box tuning frequency (38Hz) with peaks before and after it.

[Dwn4BassAlan]

oh yeah, almost forgot, you seem knowledgeable enough to understand these links too for any more detailed questions you might have:

http://sound.westhost.com/patd.htm

http://www.churchsoundcheck.com/imp1.html

http://www.prestonelectronics.com/audio/Impedance.htm

[Brian617]

I asked the exact same question a while back to Boon via PM. Here's what he had to say.

I would say that they use current limiting on the outputs, with quite high rail voltage.

So the amp is capable of, say, 70vac(rms) at the outputs, giving it 2450wrms into 2 ohms.

But let's say the output section can only handle 60A of continuous throughput. This is heaps for running 2450wrms @ 2 ohms (Only takes 35A) but 70vac at 1 ohm is going to result in 70A of current and blow the amp. So my guess is that they limit the output current to, say, 50A. This way if you run it at 1 ohm you still get 2500wrms (50A at 50V) but as your impedance rises the voltage can rise too so you get the same output.

All this probably makes them quite expensive.

Regards,

Logan

I asked the exact same question a while back to Boon via PM. Here's what he had to say.

I would say that they use current limiting on the outputs, with quite high rail voltage.

So the amp is capable of, say, 70vac(rms) at the outputs, giving it 2450wrms into 2 ohms.

But let's say the output section can only handle 60A of continuous throughput. This is heaps for running 2450wrms @ 2 ohms (Only takes 35A) but 70vac at 1 ohm is going to result in 70A of current and blow the amp. So my guess is that they limit the output current to, say, 50A. This way if you run it at 1 ohm you still get 2500wrms (50A at 50V) but as your impedance rises the voltage can rise too so you get the same output.

All this probably makes them quite expensive.

Regards,

Logan

Ok, That makes sense. I believe you are saying that the rail voltage is high enough that any load up to "x" ohms can be supplied with the current to meet the amp's max output. I for some reason was thinking output power was controlled by the relative value of the input signal. IE a 1000 watt constant power amp presented with an input signal of 50% the amplitude required to drive a set impedance load would deliver 500 watts regardless of variations in impedance. I don't know what kind of processing, feedback, and sorcery would be required to do that, but that's where my head jumped. So a constant power amp behaves like any other amp once it's gains are set; it's just capable of delivering max power at more than it's lowest impedance rating, making it much more flexible in system design.