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Why Sony watts aren't as good as _____ watts

One again I’m writing something as an electrical engineer who knows just enough to be dangerous. Please keep this in mind. One more note, I’m not deliberately picking on Sony. This really applies to any low cost receiver.

A friend of mine is considering buying some new equipment. He currently has a Sony receiver rated at 100wpc. I was showing him a Rotel amp rated at 75wpc and I assured him that it should have just as much real world power as his Sony. This got me thinking about why are Rotel, McIntosh, Bryston etc watts better? After consulting with my brother, who unlike me has designed and built an amp, I came up with this idea.

It all comes down to power supplies: This is not entirely true but I’m going to assume that both amps have all the heat sinking they could use and I’m not looking at little things like tonality and such. I think it’s safe to assume that receivers have a lot of sound quality compromises but I’m looking at watts of power. Power needs a good power supply.
Most amps (all?) will clip when one of two things happen:
1. The output voltage can’t go any higher.
2. The output current can’t go any higher.

All power supplies have voltage and current limits. For the sake of comparison I’m not going to look at loss in the amp circuit, I’m really interested in what can be delivered to the speakers.
An amp delivering 100watts into 8 ohms is delivering 3.54A at 28.28V. ie the voltage at the speaker terminal is 28.28V, the current through the wire is 3.54A.

Amp manufactures like to advertise big power supplies which can deliver high current. A voltage limited amp (ie all the current it needs) will double in output power every time you halve the speaker load. The opposite is true in a current limited amp. In a current limited amp the power HALVES each time you halve the load (lets assume you Sony can drive those Magnepans)
Some quick math (remember Power = Current^2 * Resistance) yields the following numbers:

Good beefy amp (lots ‘o current)
Ohms-------8-------7-------6-------4
Power-----100-----114-----133-----200
Current--3.54----4.04----4.71----7.07
Voltage--28.28---28.28---28.28---28.28

Low $ amp (meek current)
Ohms-------8-------7-------6-------4
Power-----100-----88------75------50
Current--3.54----3.54----3.54----3.54
Voltage--28.28---24.75---21.21---14.14

In both cases the limiting factor at 8 ohms is assumed to be voltage (this is usually the case.)

Numbers mean a lot in low cost products. The manufacture will rate the highest power possible, but cost is everything. The manufacture will use a power supply just big enough to get the numbers marketing asks for (100watts into 8 ohms). Since the power supply can provide more current the amp’s actual power drops as speaker impedance goes down. This is just the opposite of those big He-Man amps.
But my speakers say 8 ohms… A quick look in a Stereophile speaker review shows all sorts of pretty charts. One of them is speaker impedance vs frequency. This numbers are all over the place. A quick look at the B&W 302 shows this 8 ohm baby swings from 3 to 20 ohms. At impedances over 8 ohms both amps are limited by voltage. When things go below 8 ohms beefy becomes stronger while meek gets meeker.

Now the question of how many Rotel watts = 100 Sony watts. That really depends on what impedance you are interested in but… a 75 watt good amp will deliver 86 watts into 7 ohms. This is the same at the 100 watt meek amp. Drop the impedance any lower and the Rotel looks better and better.

Any questions?
nikkidanjo
jwrobinson's avatar
jwrobinson
61 posts
 
I'm not sure what your take is on power, but I think we probably agree. Here's my take:

In a perfect world each time you halve the load impedance,
the power will double. Unfortunately, all amps are not created equal in this regard. Take a well known amp like a Krell. It has such a huge PS that it WILL double it's power output as the load is halved. The reason for this is it's voltage doesn't droop. Another amp not quite so beefy can't maintain its output voltage and so power will only increase by little, if at all.

Another thing I like to look at is the way an amp is rated.
Some amps rated at 100 watts actually only deliver that power into a narrow band of frequencies. As an example, the FTC mandated spec is supposed to be 100 watts from 20hz-20khz at .1 thd. But many manufacturers fudge on this, especially tube amps. But hey, I own tube amps and they deliver all the power I need driving magnaplanars.

The bottom line is that if you have speakers with a tough load (low impedance, high phase angle, etc.) you need a Krell or something like it. Otherwise, you're spending a lot of money on a big PS. Huge PSs are what mostly drives the cost of amps into the stratosphere. Remember, an amp puts out voltage, the speaker(load) draws current. Of course, if the amp can't deliver the current required then its power will not increase.

Jim Robinson
timo's avatar
timo
610 posts
 
Power supply is much larger and current delivery is higher on quality amps.
sugarbrie
3,854 posts
 
Well, all printed specs are only a guideline. No one takes the time to design and amp that is exactly XX watts. The watt specs are always on the low side, partially for legal reasons (false advertising on power), and many times geared to get a certain low distortion reading. It is more likely that a mass market receiver will be less conservative to impress Joe Consumer with the power.

I can remember checking out HT receivers for a friend. A certain brand had a receiver that was rated at 50 watts at 8 ohms and 110 watts at 4 ohms, which does not make sense. The 100 watt receiver at 8 ohms was 150 watts at 4 ohms. It is more likely the 50 watt receiver was a 75 watt receiver, but they probably wanted to justify asking $100 more for the 100 watt model, so they understated the other model. They probably costs the same to manufacture in China, so if they can get you to spend an extra $100 to get double the power, they'll profit.
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