Question on box rise and constant power amps.

[finkster]

Don't over think it. They made an amp powerful enough (or smart enough) to make the same power at different loads.

Think of a powerful amp that can make 1000 watts at 10 ohms. Naturally it will make more power at 1 ohm, but not if it was limited by design to an output of 1000 watts. Get it?

[Brian617]

Wait, wait... I'm lost again. If a Constant Power amp delivers max power into various resistances using current limiting... lost man, lost.

OK. Let's say a CP amp has rail voltage 75 VAC (rms), and a 1 VAC input signal results in 75 VAC out across a resistive load of 4 ohms giving you 18.75A for 1406.25W total. It follows that .5 VAC input signal will result in half the power (don't know how to figure it). But if the load is halved to 2 ohms, the 1 V signal would cause current limiting and rail voltage drop for the duration of the signal. But if the amp is then given a .5 V signal, the rail voltage not having caused max current draw would not be reduced so overall power would be greater than 50%, leading to an effective compression of dynamic range.

What I'm trying to say, is that it seems the max rail voltage would need to be constant for each given load. I could see a trailing set point that would reduce the voltage permanently if current limitations were exceeded, but how long would that limit exist? What if a transient peak at the Zmin frequency limits output down the road? Is the rail voltage set for an impedance range automatically at turn on or after playing for a bit?

[finkster]

As far as I know it is constantly changing. JL had something similar to this. When the amp would first turn on, it checked the impedance of the subwoofer and locked into that. The CP amps are constantly scanning and re adjusting. How it is done I can't tell you. It is patented for a reason and a very cool feature.

[ChevyBoy95]

[Brian617]

Actually I seem to be back to the original question after reading this on RF's site:

"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."

This is not simple current limiting to allow max output into lower impedance loads, it more resembles variable equalization of the output signal which would need to be counteracted with input signal equalization. Even that wouldn't work completely since the output could vary based on current load... ie a 40Hz drum beat may be output differently if combined with a 100 Hz synthesizer note. Sounds like a mess.

[finkster]

Notice that box rise was not included in the statement.

I haven't studied it enough to know how much enclosure environment affects power delivery to a speaker. I know how it affects power handling, just not power delivery. Will more power be delivered to a free air sub playing the same frequency as one inside of a box? I remember seeing a bunch of posts about box rise but I don't think they have been tested correctly. It is VERY hard to keep variables constant in car audio.

[poorfish88]

As far as I know it is constantly changing. JL had something similar to this. When the amp would first turn on, it checked the impedance of the subwoofer and locked into that. The CP amps are constantly scanning and re adjusting. How it is done I can't tell you. It is patented for a reason and a very cool feature.

The JL amps were regulated power supplies. Which meant they made the same power no matter how much extra voltage they received. RFs CP is nothing like that, it's far

superior, it makes more power at lower HZ than standard amps.

[Wicks]

Actually I seem to be back to the original question after reading this on RF's site:

"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."

This is not simple current limiting to allow max output into lower impedance loads, it more resembles variable equalization of the output signal which would need to be counteracted with input signal equalization. Even that wouldn't work completely since the output could vary based on current load... ie a 40Hz drum beat may be output differently if combined with a 100 Hz synthesizer note. Sounds like a mess.

Sounds like you're WAY over-thinking this.

From the website statements:

RF CP amps monitor the load impedance. On higher impedance's, it raises the power supply to compensate.

On normal impedance's, the power supply stays the same.

Notice that box rise was not included in the statement.

I haven't studied it enough to know how much enclosure environment affects power delivery to a speaker. I know how it affects power handling, just not power delivery. Will more power be delivered to a free air sub playing the same frequency as one inside of a box? I remember seeing a bunch of posts about box rise but I don't think they have been tested correctly. It is VERY hard to keep variables constant in car audio.

Nobody saw my post above..?

RF doesn't mention "box rise" because they don't need to. They are able to monitor the "loads" impedance and compensate.

There's no other secret thing to factor in.

The load impedance is the load impedance.

Check out my plot from above. The ES200's peak impedance actually went down when I put it in a sealed box.

[finkster]

Actually I seem to be back to the original question after reading this on RF's site:

"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."

This is not simple current limiting to allow max output into lower impedance loads, it more resembles variable equalization of the output signal which would need to be counteracted with input signal equalization. Even that wouldn't work completely since the output could vary based on current load... ie a 40Hz drum beat may be output differently if combined with a 100 Hz synthesizer note. Sounds like a mess.

Sounds like you're WAY over-thinking this.

From the website statements:

RF CP amps monitor the load impedance. On higher impedance's, it raises the power supply to compensate.

On normal impedance's, the power supply stays the same.

Notice that box rise was not included in the statement.

I haven't studied it enough to know how much enclosure environment affects power delivery to a speaker. I know how it affects power handling, just not power delivery. Will more power be delivered to a free air sub playing the same frequency as one inside of a box? I remember seeing a bunch of posts about box rise but I don't think they have been tested correctly. It is VERY hard to keep variables constant in car audio.

Nobody saw my post above..?

RF doesn't mention "box rise" because they don't need to. They are able to monitor the "loads" impedance and compensate.

There's no other secret thing to factor in.

The load impedance is the load impedance.

Check out my plot from above. The ES200's peak impedance actually went down when I put it in a sealed box.

I saw it but didn't read it. Go figure right. I'm more along the lines of impressed how the amp can react so fast. It kind of reminds me of a permanent fuel trim setting for an ecu.

I get it, I just want to to know how it is monitored. Have any of these been amp dyno'd on music?

[Wicks]

Actually I seem to be back to the original question after reading this on RF's site:

"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."

This is not simple current limiting to allow max output into lower impedance loads, it more resembles variable equalization of the output signal which would need to be counteracted with input signal equalization. Even that wouldn't work completely since the output could vary based on current load... ie a 40Hz drum beat may be output differently if combined with a 100 Hz synthesizer note. Sounds like a mess.

Sounds like you're WAY over-thinking this.

From the website statements:

RF CP amps monitor the load impedance. On higher impedance's, it raises the power supply to compensate.

On normal impedance's, the power supply stays the same.

Notice that box rise was not included in the statement.

I haven't studied it enough to know how much enclosure environment affects power delivery to a speaker. I know how it affects power handling, just not power delivery. Will more power be delivered to a free air sub playing the same frequency as one inside of a box? I remember seeing a bunch of posts about box rise but I don't think they have been tested correctly. It is VERY hard to keep variables constant in car audio.

Nobody saw my post above..?

RF doesn't mention "box rise" because they don't need to. They are able to monitor the "loads" impedance and compensate.

There's no other secret thing to factor in.

The load impedance is the load impedance.

Check out my plot from above. The ES200's peak impedance actually went down when I put it in a sealed box.

I saw it but didn't read it. Go figure right. I'm more along the lines of impressed how the amp can react so fast. It kind of reminds me of a permanent fuel trim setting for an ecu.

I get it, I just want to to know how it is monitored.

I agree! It is impressive that an amp can adapt so quickly.

Considering microprocessors are becoming smaller and cheaper and can easily operate well into the megahertz range, it makes sense that it can monitor and adapt to relatively "slow" audio signals (20-20kHz).

I'd love to know the details as well but then I suppose there's a reason why its patented....

Ultimately they need to know the voltage and current at the load.

Monitoring the internal voltage is easy.

Most output FET's have a series resistor since there are MANY in parallel. That ~0.1ohm resistor can also be used to calculate the current from the FET.

Voltage dropped across it divided by its 0.1ohms.

Therefore the amp should be able to collect that data and use it to approximate the impedance and therefore control the PSU.