Why some watts are more expensive than others

All watts are not created equal

By

Buck Pomerantz

Buck Pomerantz was born and raised in Philadelphia. His parents bought their first television set when he was born. He figured out how to run it by the time he was two. Besides athletics, his formative interests included electronics, amateur radio, music, and stage crew work. He got his BA in writing from Brown University. Then he joined a rock 'n roll band as their soundman and moved to Charlottesville, Virginia. After that venture failed, he spent time in Boston, New Orleans, and Berkeley. He worked in a music store in Austin manufacturing, installing, repairing, and operating sound systems for recording studios, clubs, and bands. He moved back to Charlottesville, ran a little recording studio and finally joined Crutchfield as a copywriter. He has 2 grown children and 3 grandchildren, but after a good nap he can still rock out.

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Why pay different prices for the same wattage? Different amps with the same wattage ratings can have wildly different prices. Why pay more for the same amount of power? Because not all watts are created equal. The reason? Build quality.

Rockford Fosgate Power, Punch, and Prime 500-watt amplifiers

Rockford Fosgate Power, Punch, and Prime 500-watt amplifiers


Rockford Fosgate Prime, Punch, and Power amplifiers

To help us explain how this works, Rockford Fosgate sent us a sample of each of their three lines of amps for us to take apart in the Crutchfield Labs. This way, we can show you what makes each different. This  report will concentrate on the physical differences and the benefits of each design.

We start by examining each amp.

Prime R500-1

The Prime R500-1 is Rockford Fosgate’s entry-level 500-watt subwoofer amplifier. At around $0.44 per watt*, it presents a great value for someone who needs enough bass power to keep up with an amplified aftermarket system. It’s over 17 inches long, with a case made of lightweight steel, and long aluminum heatsinks down each side. It weighs 7 pounds, 1 ounce.

Inside the R500-1

View of what's inside a Prime R500-1 (click to enlarge)

Inside the R500-1

Inside, there are 24 transistor-like devices that appear to be bolted and glued to the inside of the heatsinks. These components (called MOSFETs, or metal-oxide semiconductor field-effect transistors) are responsible for combining the energy of the car’s electrical system with the musical signal to ramp up its power. They get very hot. That white glue-looking stuff is thermal grease that helps transfer the heat away from the transistor to the heatsink, which carries the heat further away by air convection.

R500-1 MOSFETs

R500-1 MOSFETs mounted to a heatsink (click to enlarge)

Why do we need to get rid of the heat?

Heat is the enemy of all amplifiers. Electronic components working hard to produce power also produce heat. That’s the nature of work in the real world: nothing is 100% efficient — all physical activity, including electrical, is accompanied by energy loss due to the generation of heat. Heat in a circuit can interfere with the signal by changing the values of the components, burning through copper traces and leads, and even melting the solder that holds everything together. The heavy piece of metal used to draw heat away is called a heatsink because it acts like a sink, making the heat disappear.

The Prime amp's output

Rockford Fosgate designed this Prime amp to deliver Power Output Without Excessive Regulation (P.O.W.E.R.), which means they made it to respond quickly to changes in voltage to take advantage of the electrical system’s peak power, when available. It also means the amp’s output power varies slightly when the voltage changes. But those changes are slight and probably not even noticeable to the entry-level user.

Punch P500-1bd

The Punch P500-1bd amp is Rockford Fosgate’s standard-bearer for what they can do for your subwoofer. Priced at about $0.80 per watt*, this Punch amp comes loaded with technical innovations designed to rigorously maintain musical fidelity while supplying a large amount of power from such a small package. The first thing we notice is that the P500-1bd is much smaller than the R500-1 but just as heavy. It’s only 10-1/4” long and weighs in at 7 pounds, 8 ounces (7 ounces more than the Prime amp). The chassis is the heatsink, with a dense aluminum bottom and 3 sides with cooling vanes.

The Punch 500-1bd's heatsink

The Punch 500-1bd's heatsink

Inside the P500-1bd

Inside the P500-1bd (click to enlarge)

Inside the P500-1bd

The main circuit board of the P500-1bd is only 5-1/2” by 6-1/2”, which makes the heatsink as much as 2” thick in some places. The shapes of the heatsink’s curves and channels induce hot air to flow away from the amp. This is part of Rockford Fosgate’s Dynamic Thermal Management system that rids this amp of excess heat.

Custom MOSFETs

You’ll notice there’re only 12 transistors glued and bolted to the P500-1bd heatsink inner walls, while the R500-1 had 24. The difference is readily visible when you hold them up side by side. These larger MOSFETs each handle much more power than their off-the-shelf cousins, and because there are fewer of them, there’s less interference and imbalances to be compensated for when their signals are combined at the output.

Custom TO-247 MOSFET (left) versus standard MOSFET

Custom TO-247 MOSFET (left) versus standard MOSFET

The consequence of concentrating power like this, unfortunately, is the generation of even more heat. Rockford Fosgate solved this by adding ceramic and aluminum layers between each transistor and heatsink, along with the thermal grease, increasing the heat transfer away from the amp. That’s also why the heatsink’s as big as it is.

Discrete Surface Mount technology

In their Punch amps, Rockford Fosgate uses high-precision, low-tolerance components that mount on a circuit board with no protruding leads or wires. You can see how the leads in a transformer for a Prime amp are long and exposed, and current passing through them tends to radiate energy around the area, increasing heat and causing distortion.

Punch transformer DSM system

Punch transformer DSM system

Prime transformer

Prime transformer

The photo of the Punch transformer, in contrast, shows how each wire terminates separately, and whose length is determined by each coil’s electrical need alone, and not where it gets connected. That’s the circuit board’s job. Discrete Surface Mount (DSM) connections decrease interference from neighboring components, increase circuit efficiency, and lower the operating temperature. And it also makes the resulting circuit neater, so signal flows easier with less interaction or interference.

Power T500-1bdCP

The top of the line for Rockford Fosgate means they push the envelope of what’s possible in an amplifier. They use another proprietary technology they developed, called "Constant Power," to increase the amp’s ability to deliver steady power under varying conditions. The Power T500-1bdCP amp is the heaviest of these three amps at 8 pounds and 9 ounces. It’s a little larger than the Punch P500-1bd, 11-1/2" long, but that’s entirely due to its massive heatsink. And each T500-1bdCP watt costs about $1.00*.

Power T500-1bdCP

Inside the T500-1bdCP (click to enlarge)

Inside the T500-1bdCP

The circuit board for the T500-1bdCP is the same size as the one in the P500-1bd, making the heatsink in the T500-1bdCP up to 2-1/2” thick in some places. The heatsink has more fins and channels for increased heat dissipation. Like the Punch amp, it also employs Discrete Surface Mounted components and the large, custom-made MOSFETs mounted in what Rockford Fosgate calls MEHSA or Maximum Efficiency Heat Sink Application.

MOSFETs on a heatsink

MOSFETs mounted on a heatsink

Rockford Fosgate's MEHSA system

Maximum Efficiency Heat Sink Application

Constant Power

Speakers and subwoofers change their impedances depending on what frequency note is playing. Normally, an amplifier will change its output power according to what impedance it senses. The Constant Power circuitry Rockford Fosgate developed changes all that, allowing the amp to put out exactly the same power no matter what impedance changes happen during play. This actually results in an increase of power delivery over the whole spectrum of sound reproduced. It's not easy to see, but one of the little, vertical circuit boards is where this feature lives.

Deep inside a T500-1bdCP

Deep inside a T500-1bdCP (click to enlarge)


Design and parts make the difference

These are examples of how different amplifier designs and build quality influence the overall performance of the amp, which in turn also has a direct impact on price. Make no mistake, these are all good amplifiers. But the capabilities of the Power T500-1bdCP far outshine those of the Prime R500-1. And that's why the Power amp will cost you so much more than the Prime amp.

You'll see similar differences from different manufacturers, too. Generally speaking, a better-designed and better-built amplifier is going to cost you more, but it will result in a more reliable and better-performing piece of equipment. And that means better sound for you in the long run.

Custom circuit boards signed by their creators

For an extra added bonus, click on the above images of the T500-1bdCP and P-500-1bd circuit boards and bring up the enlarged images. Click on them again to zoom further in. Now, scroll around and discover the funny little initialed pictures the Rockford Fosgate designers etched into their circuit boards. On the T500-1bdCP there’s a hand grenade, a bullet, a steaming cup of coffee, and a funny face. The P-500-1bd circuit board has a muscle man, a lit bomb, and the same funny face. Enjoy.

*The prices per watt were as of the original publishing date of this article. Actual amplifier prices and cost per watt might change over time.

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