Ok Justin I have one for you. Why do some amplifiers double their power from 4 to 2 ohms and some do not? What is the difference in the power supply? For example I’ve noticed that many Kicker amplifiers literal double their power from 4 ohms to 2 ohms. How come Rockford Fosgate amplifiers do not double their power like Kicker?
I admit amplifier design is not my specialty. Everyone please be careful quoting what I write below since I still have a lot to learn.
JCsAudio might enjoy this since I ducked a question earlier. I've been thinking about it ever since though!
Amplifiers have two parts that can affect the power output: power supply stage and output stage. The power supply can be regulated or not, and the output stage can be "smart" or "dumb".
Power Supply (the voltage side of Ohms law)
Non-regulated power supplies will allow the rail voltages to increase or decrease as the battery voltage is increased or decreased. If the battery voltage goes up or down, then the "power" output of the amplifier can go up and down. This is how many amplifiers on the market work and I believe the "P.O.W.E.R. Supply" on the Rockford Prime amplifiers work this way. Non-regulated power supplies, as I understand it, can be an advantage in some SPL competitions because your amplifier will increase power output if you increase the battery voltage which leads to more more deebeez. An analogy would be holding your foot steady on the gas pedal of your car and your car's speed will change depending on if you have a tailwind or if you drive into a headwind. The speed of the car (similar to the power output of the amplifier) will depend on the road conditions (similar to the battery supply voltage) if you hold the gas pedal constant (like a non-regulated power supply).
Regulated power supplies will hold the rail voltages constant when the battery voltage changes. If your battery voltage sags while you're cranking your engine or your battery is weak and your headlights are dimming with the music, then the amplifier will attempt to keep the rail voltage constant by dynamically adjusting how much energy it is drawing from the electrical system. Using the car analogy, this would be the equivalent of cruise control where the vehicle speed is held constant by adjusting the gas pedal automatically based on the changing road conditions. This is also hilarious on the freeway since people don't do a good job maintaining their speed and they will speed up and slow down to pass you over and over again. It's hilarious when you have nothing else to do on a long drive.
Output Stage (the current side of Ohms law)
I was going to write a big thing here but Manville Smith of JL Audio wrote a sweet post over at DIYMA about ten years ago.
How does RIPS (JL Audio) work? post #21
Please read Manville's post, it is really well written and it goes into a lot of really good detail.
In short, the output stage is usually made up of transistors that can supply the speaker with voltage up to the rail voltage limits that the power supply has. The speaker will present an impedance (or a load) and then current will flow. The output stage is always designed with a current limit in mind. If the speaker's impedance is too low, then too much current is passed through the output stage to the speaker and the output transistors will overheat and -if you're lucky- trigger an over-temperature protection circuit. Or the power supply will be unable to keep up and it will overheat and (hopefully) trigger an over-temperature protection circuit.
Continuing with the car analogy, a given car can drive 65mph on the freeway if the road is flat. The same car will go much slower up a steep hill, even with the gas pedal on the floor. The same car can go much faster than 65mph if you're driving down a steep hill, so fast that it can easily loose control and crash into a fireball and cause the road to be closed and inconvenience literally everyone else in a supremely selfish act of dumbassery.
Relating the car to the amplifier: an amplifier can supply 100watts of power if the impedance of the speaker is a good match (like driving the speed limit on a flat highway). The same amplifier, using the same rail voltage, can supply less power if the speaker impedance is too high (like driving up a steep hill). The same amplifier can supply much more than rated power if the speaker impedance is too low (like driving down a steep hill) and if the impedance is too low then the amplifier can overheat and blow up (like the car going too fast and crashing).
A "dumb" output stage might just let the amplifier overheat and blow up. A less dumber amplifier might sense the temperature is getting too high and shut down similar to truck driver taking the "runaway truck" offramp and ditching the truck in the gravel. A "smart" amplifier might sense the current being delivered to the speaker is too high and then do *something* to reduce the current of the output stage so that nothing Bad
™ happens.
Constant Power
To get constant power out of an amplifier at 1Ω and 2Ω and 4Ω, you need two main things: 1) enough voltage amplifier power supply so that the power can be realized on the highest impedance load and 2) a current sensing/limiting technique so that the amplifier doesn't overheat or fail when the lowest impedance load is used.
An example amplifier might claim:
100w @ 4Ω
100w @ 2Ω
100w @ 1Ω
To get 100w at 4Ω the rail voltage would have to be 20 volts (see ohms law). In this setup, the output stage would deliver 5amps of current to the speaker.
When you attach a 2Ω speaker then your 20v rail voltage will attempt to deliver 10amps of current to the speaker which is 200w of power. One of two Bad™ things happen: the output stage transistors will overheat from too much current or the power supply stage which was designed to handle 100w will overheat when it tries to deliver twice that much power.
The "smart" amplifier will sense the 10amps of current in the output stage and then make an adjustment so that the amplifier is capable of delivering 100w instead of 200w. The amplifier can do this many ways and I think the JL RIPS technology will step down the amplifier rail voltage. In this case the rail voltage might go from 20v down to 14.1v so that a 2Ω load would only require 7amps of current from the amplifier output stage which is in the safe operating range of the transistors, and the amplifier will deliver 100w of power which is in the safe operating range of the power supply stage.
Another way the amplifier could reduce the current in the output stage is to lower the volume of the audio signal by some amount. In this case a reduction of -3dB in audio volume would result in half power, or going from a potential 10a down to 5a and a potential 200w down to 100w.
This technique can continue down to some low impedance threshold where the current required from the output transistors becomes too high. For example a 0.5Ω load would use 7volts and 14amps of current. If the transistors are only capable of 10amps of current, then the system would either sense the over-current condition and go into protect or it might limit itself to ~70w of output or something else completely .... but you won't get 100w out of the amplifier anymore.
The trick of all this is to use a "smart" output stage that can sense the over-power condition and correct for it in a way that keeps the amplifier safe and also fools the user into never hearing a difference. These kinds of techniques can also be used for over-temperature protection too since any kind of power roll-back like this will reduce the heat the amplifier generates. This could be triggered not just by a lower-impedance speaker but maybe by installing an amplifier in a super clever spot underneath all the carpet and trim panels in your car where it can't breathe and the heat gets trapped.
Well, so much for "not writing a big thing" and "in short..." haha
