Power Manipulation in Dolby Atmos AV Receivers

Are Power Claims being fudged with Atmos Receivers?

We've noticed a recent trend with the latest Dolby Atmos/DTS:X AV Receiver releases.  Instead of publishing unclipped full bandwidth power with at least two-channels driven into 8 ohm loads per FTC mandate, many of the major AV receiver manufacturers are now touting power with only one-channel driven, at 1kHz, into a 6 ohm load at up to 10% distortion.  This type of testing scenario inflates the power rating up to 2X the former FTC way of rating power into 8 ohm loads.  Only when you search for the fine print on the manufacturers' websites or spec sheets do you actually find the two-channel continuous ratings.  Will most consumers realize this?  Probably not, hence the purpose of this article and related YouTube video discussion.

Today we take a look at three receivers that are rating power in this manner.  Incidentally they all share the same Class D D3 power section developed by Pioneer.  Since Onkyo purchased Pioneer,  they trickled down Pioneer's amp sections into the comparable Onkyo and Integra models listed below.  All three of these receivers are virtually identical amplifier and power wise.  Only their  cosmetics, features, DSP and room correction options vary between the models.

11.2CH Atmos AV Receivers:

• Pioneer SC-LX901
• Onkyo TX-RZ3100 
• Integra DRX-R1

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Check out our YouTube video discussion on this topic before diving deeper into the editorial below.

Understanding Power Ratings in AV Receivers YouTube Video Discussion

Reading the Power vs Distortion Curve

It's important to understand what you're looking at when reading a typical power vs distortion curve that you may have seen on our website or in other audio magazines.  The X-axis represents power (in watts) while the Y-axis represents distortion (THD +N).  When the curve is relatively flat and horizontal, the amplifier is providing power at very low distortion levels. So low in fact, that good amplifiers operating in this region are doing so at distortion levels at or near their noise floor.  The point at which the curve goes from horizontal to vertical is called the "knee of the curve".  This is where the amplifier starts running out of voltage and the distortion dominates the measurement as you move vertically up the curve or to the right of it.   

Your "conservative rating" manufacturers like NAD and Classe, for instance, will typically quote power ratings for their receivers and amplifiers about 10-20% under the area in which the curve is fairly horizontal while some of the bigger brands like Onkyo, Pioneer, Denon, Yamaha, may do so at the knee of the curve, or sometimes even higher into the distortion region (up to 10%) as we've seen in the latest models listed above. 

Classe CT-2300 Power vs Distortion Sweep

Denon AVR-X5200W Power vs Distortion Sweep

Notice the power vs distortion sweep of the Classe CT-2300 and how flat and horizontal the amplifier is at its rated power of 300 watts.  Classe could have easily rated this a 340 watt amplifier and called it a day. At 340 watts, the distortion is at 0.1% THD+N which is well below hard clipping (10%).  However, they chose to rate their amplifier more conservatively (old school if you will) by giving a power spec that was well below the knee of the graph.  This region also ensures power with very low residual harmonic distortion, but more on that later.

Now let's look at the Denon AVR-X5200W power vs distortion sweep.  This receiver is rated 140 watts/ch x 9 channels, but as you can see in the graph, this power figure is taken after the knee where the amplifier heads into clipping.  We can cut them some slack however since its at 0.1% THD+N which is still fairly low distortion.  However, if Denon wanted to be more conservative with their ratings like the Classe example, they would have rated this receiver to be 100 watts/ch. 100 watts used to be the magic power # in receivers a few decades ago, but now 140 watts seems to be the new holly grail. 

To Denon's defense, it's important to realize that these are two very different products. The Classe CT-2300 is a very expensive ($6,500), top of the line two-channel amplifier while the Denon AVR-X5200W is a high value 9CH Atmos AV receiver meant to do much more than just deliver power to a pair of speakers.  The fact that the Denon can do everything it does at under $2k price tag is commendable.  However, I used these two examples to illustrate specsmanship differences of how manufacturers rate power and also because I have actual apples to apples test data that I took for comparative purposes.

The reason why it's important to rate power well below clipping has to do with the types of distortion that occur well below the 10% threshold and more closely to the knee of the graph itself.

 Amplifier FFT Distortion at near clipping

Shown above is the FFT distortion analysis of a 150 watt amplifier measured at near clipping ( <10% THD+N).  As you can see, there are some nasty harmonics above the 1kHz fundamental.  If the amp was measured at or below the knee, the amplitude of the residual byproducts would have been much less.

What About Full Bandwidth Testing?

Don't forget this is a 1kHz test, not full power bandwidth.  Depending on how the amplifier and power supply is designed, you could expect to see anywhere from 10% or more lower power output when testing full bandwidth as opposed to just a 1kHz sweep. I typically like to do full bandwidth sweeps at the 0.1% THD+N 1kHz point to see how the amp behaves at both ends of the frequency spectrum. Remember bass waves are long and sustained and suck a lot of power up and that's where a really beefy power supply and capacitor bank is needed not only to reduce ripple voltage, but voltage sag to maintain power during those sustained musical peaks. High frequencies are tough on many Class D amps due to restrictions of the output filter and how the amp is tuned around it.  Also you can get slew induced distortion at high power that you don't see at 1kHz or at a low power measurement.  A 1kHz power sweep is a good test to give you a rough estimate of true amplifier and power supply capability but we should demand full frequency power specifications into 8 and 4 ohm loads to better understand the true nature of the amplifier behavior or more appropriately stated, misbehavior.

Classe CT-2300 Full Bandwidth Power Sweep

Previous Pioneer AV receivers such as the SC-07 employed ICE Class D amplifier modules that benched extremely well with a 1kHz power sweep test but fell short for full bandwidth testing into 4 ohm loads for reasons previously mentioned. Pioneer has since then moved away from the older ICE module and now employ their own proprietary D3 power block.  We plan on testing them in the near future to see how they handle 4 ohm loads so stay tuned.  

Fudging the Power Ratings?

Here is how Pioneer and Onkyo are rating the power of the above receivers as captured from their website.

Onkyo and Pioneer both spec their flagship AV receivers 1CH driven, 1kHz, at 0.9% THD

Power Calculations

Let's use a little Ohm's Law (V= I*R)  to figure this out. 

If P = V*I and I = V/R; then V = sqrt(PR)

Where P = Power in watts, V = Voltage in volts, and I = current in amps

=>  Therefore 200 watts at 6 ohms translates to 34.6Vrms and driving an 8 ohm load (which is how most amplifiers are rated), this gives you P = V^2/R = 150 watts. 

Now let's not forget this is at clipping (0.9%).  So in reality, this amp is good for a clean 140 wpc and that's for 1CH driven.  We will give Onkyo/Pioneer the benefit of a doubt that their power supply is large enough to drive at least two channels at this power level as they do appear to be pretty beefy products. 

But surely you can now see this 200 wpc receiver is really 140 wpc more honestly rated.  Let's keep it honest guys and stop the BS power ratings please!

So What Have We Learned?

Making an apples to apples comparison between amplifiers and AV receivers is not such an easy thing.  Unless we know exactly how the amplifier was tested and rated, we simply cannot ascertain which amplifier is truly more powerful.  It's important to realize that nothing revolutionary has changed the amount of power these receivers can deliver.  Yes, Class D allows up to 90% efficiency so you can potentially get more power from your wall outlet to your speakers than a conventional linear Class AB design, but they too have limitations especially when cost is involved and your stuffing all of this technology into an already crammed receiver chassis.  There are no free lunches. Just keep that in mind when one of your favorite receiver companies comes out with a new model claiming XX watts/ch more than its predecessor it replaced and for the same price!  Have they really increased the size of the power supply and/or output devices?  Or have they just gotten a little more cleaver with specsmanship?    

Join our Movement by signing our TiP (Truth in Power) Petition NOW!

If you are concerned about power, take our quiz on When Should You Upgrade the Power of Your AV Receiver and make sure when shopping for a new AV receiver that you choose a model with multi-channel preamp outputs in case you decide to add external amplification at a later date.  Spending an extra $200 on the next model up to get this feature hurts a lot less than ditching your underpowered receiver and starting over again from scratch in a few years because you can't expand your current system.