Compact Class D Amps - Misleading Specs?

[Tin_Ears]

Why is it that all the compact class D amps I see for sale have misleading specs? For instance, I just had a look at a 100x2 (8 ohm) and the distortion curve indicates it's really only safe at 20x2 (allowing for plenty of headroom) or 50x2 (if "really" pushing it into distortion on peaks). IMO, the 200x2 I'm referring to would often sound like crap if pushed beyond 20x2 RMS.

Why do our "trusted" resources continue these misleading advertising tactics? Don't their marketing departments understand this behavior literally destroys our trust in those retailers?

Okay... had another look. I guess the rated output power is max. Even so, the rated power output is about 3-5x what I'd call appropriate given the distortion figures.

Oh... and I'll bet those ratings are with using the best PS at the highest voltage.

Anyone who knows no better, like me, can really get burned by these advertising tactics... not advertising "lies" but taking advantage of those who "trust" their retailer.

[Sidi]

Re: Compact Class D Amps - Misleading Specs?

All of those amps will make rated power at the published distortion. they all say what chipset they use. all the datasheets show the typical distortion curve. I really don't see the problem here

[greywarden]

Re: Compact Class D Amps - Misleading Specs?

To make money.

Honestly, I don't know why someone would rate an amplifier's max power at 10% THD when everyone strives for 1% or less THD in speakers.

[generic]

Re: Compact Class D Amps - Misleading Specs?

Because their is no legal over site, rules or regulations to follow. They can measure peak output at a high frequency and slap it on the specs.

Topping seems to be honest about their specs. They still rate them at their highest output, but have the ratings at low distortion levels as well (in the smaller print).

In some cases, I think many manufactures are a little more honest, but they put that info in the small print. If you read the specs to a middle of the road Onkyo, or Denon or whatever. They might only be rated at low distortion at half the rated watts. Just gotta read the fine print. They might be up front about it, just not putting a huge spot light on the limitations of their products.

[donradick]

Re: Compact Class D Amps - Misleading Specs?

I, for one, feel a little peaved and bewildered over this change.
In the 60s - you saw specs like "music power" or "peak power" or even
"dynamic inverse power" - then in the early 70s the FTC or the RIAA or someone said:
"Distortion specified at rated power, all channels driven" - and you
could compare specs and "apples to apples". For the last 20 or 30 years at least.
Now it's changed....

[AJ]

Re: Compact Class D Amps - Misleading Specs?

It's up to the consumer to understand what the specs mean. The fact that the THD graph and details about the designs is available makes them more open than your typical AVR.

[jon96789]

Re: Compact Class D Amps - Misleading Specs?

I, for one, feel a little peaved and bewildered over this change.
In the 60s - you saw specs like "music power" or "peak power" or even
"dynamic inverse power" - then in the early 70s the FTC or the RIAA or someone said:
"Distortion specified at rated power, all channels driven" - and you
could compare specs and "apples to apples". For the last 20 or 30 years at least.
Now it's changed....

Ha! You haven't shopped for an AVR recently, have you? 99% of the products do not rate their products with all channels driven. They will just say 100-watts @ x% distortion... That's it. Some don't tell you the ohms it's rated at, or how many channels. Most will say two channels, some one channel. Some say 6-ohms, some say 8-ohms.

That's why the sub-$500 junk may be rated higher than a $1000+ AVR. But a lot of junk only weighs 20-lbs, the best will weigh about 50-lbs, and it all goes to the power supply. I have seen some amps test for 30-wpc with all channels driven and others at 120+-wpc, even though the specs are for 120-wpc on the web site.

[tyger23]

Re: Compact Class D Amps - Misleading Specs?

To make money.

Honestly, I don't know why someone would rate an amplifier's max power at 10% THD when everyone strives for 1% or less THD in speakers.

OK - let me dispel some myths. There has not been some "great shift" in marketing perspective. What you're seeing here is the chip amp distortion specifications. In the silicon-amp industry, we've always used the 10% distortion mark to rate the power of the chips themselves.

The actual amplifiers that use the chips can then be rated via distortion numbers of their choosing.

As Don mentions, some amplifier vendors choose to use "music power" or "peak power" ir "dynamic power", and some choose to call out the amount of power with a given amount of dynamic headroom.

Basically, though, even the amp manufacturers have bowed down to the shiny ***** of marketing and the idiocy of consumers. "More is Better", right? Therefore, more watts will sell more amps. So, some amp maufacturers have started using the 10% rating to compete in the watts number. This is because of the cheap D amps and other solutions coming from China.

What you should really be concerned about is less that the amp can do 100W at 10%, and more than these cheap class D amps can't actually do the rated power with the power supply given. Most of these chip amps are capable of the powers that are listed, but the power supply required to get those chip amps up to that power is much higher than provided.

Let me pick on the venerable DTA-100 (yesterday's Deal Of The Day). It is rated as 50W per channel, comes with the TK2050, and comes with a 24V, 5A power supply. The rear panel of the amp even states 24V required.

However, let's check out the TK2050 chip amp itself. Here's the datasheet:

http://pdf.datasheetcatalog.com/datasheet/Tripath/mXyzwqtx.pdf

Note that right on the top, it claims to be a 50W amplifier. So does the DTA-100. We're copacetic here. Now, scroll down to page 3 and look at the Pout characteristics. Note that the 8-ohm characteristics are calling out a power supply of 30V!! Sadly, Tripath did not provide a chart for output power vs supply voltage, but I can guarantee you that they will match the same charts provided by virutally any class-D manufacturer. Assuming that efficiency is the same (which for most all Class-D implementations, it is), then the output power vs supply voltage will be identical into the same loads across all manufacturers. Basically, we're talking about ohm's law and, try as I might, I simply can't disprove ohm's law.

So, let's look at the output power vs supply voltage for virtually any class-D amplifier. I'll pick on the Apogee DDX amplifiers, which were the main competitors to Tripath in their hey-day. Here's a datasheet for the DDX-2100, which was the comparable part to the TP2050, and I believe was in the same exact package. Both were made by STMicro.

http://www.apogeebio.com/ddx/PDFs/DDX-2100.pdf

Scroll down to page 8, and you'll see a graph of output power vs power supply voltage at 1% THD+N. The note states that 10% THD+N numbers should be 30% higher. So, using this chart, we can see that hitting 50W into 8-ohms actually requires about 33V for the 1% distortion number and it requires about 28V for 10% THD+N.

So, the DTA-100, with it's 24V power supply, should actually be rated at about 28W into 8-ohms at 1% distortion, or about 37W at 10% distortion. This is far from the 50W at less than 5% distortion the amp actually claims. They might be able to claim those numbers if you use the 6ohm minimum load, but that's not how the amp is marketed, and I doubt 6-ohm loads will even be able to produce those numbers.

Don't believe me that all class-D output power vs power supply voltage is similar? Check out the TI TAS5142, which is remarkably similar to the above two devices:

http://www.ti.com/lit/ds/symlink/tas5142.pdf

Scroll down to page 11 and look at Figure 2. Note that 50W into 8-ohms at 10% distortion requires 28V. Note what I said was required for the Apogee (28V).

So - marketing on these class-D amps is the least of your concern.

Maybe later I'll go into the second biggest concern of these cheapy class-D amps - frequency response due to the output filters. However, I need to go get food and that rant will take quite a while. The nut of it is - all these class D amps have a L/C filter to mitigate FCC EMI concerns. This L/C filter should be specifically tuned to the speaker loads, or else the filter will be operating at the wrong frequencies, and the frequency response of the amplifier will be completely off at high-frequencies.

For example, a L/C filter tuned for 8-ohm loads will be affecting the in-band frequency response when connecting a 4-ohm load.

At least the DTA-100 calls out "8-ohm loads only". This frequency response issue could be one reason for that. Heat is probably the actual reason, though....

[SirNickity]

Re: Compact Class D Amps - Misleading Specs?

Maybe later I'll go into the second biggest concern of these cheapy class-D amps - frequency response due to the output filters. The nut of it is - all these class D amps have a L/C filter to mitigate FCC EMI concerns.

Huh? I thought it was to shape the PWM output into a continuous waveform. Interesting post, if you feel like continuing the rant, I'll be happy to read it.

While it bugs me that manufacturers aren't more up-front with ratings, I have to say... most consumers wouldn't know what "50W" sounded like anyway, much less how to compare the difference between 1% and 10% THD, sensitivity rating of speakers, or how to choose a power supply (if that was left to them). Even in electronics hobby forums, you get people measuring wall warts and posting things like "ZOMG! This 5V adapter reads 7.4V!" until someone informs them about how transformer output sags with load...

Put things into perspective. What's the difference between 28W and 50W? 3dB... so not much. Now consider what a manufacturer has to expect when they put a 24V rating on the DC input, and weigh that against the cost of bullet-proofing the input enough to accept the range of real-world power supplies that are also rated for "24V". If you're the kind of guy that's going to find the Tripath data sheet and look up the absolute max ratings, you probably also know enough to crack open a case and see what the PSU caps can handle, and find a power supply that will ride that line of tolerance.

[Jonathan Anspach]

Re: Compact Class D Amps - Misleading Specs?

I believe the old FTC regulations are still in effect, so those published amp ratings are essentially illegal. You could report these manufacturers to the FTC, but don't hold your breath that anything will come of it. The reason some people think the FTC rules no longer apply is because they are for stereo amps only, not multichannel amps. Since the majority of amps and receivers produced these days are multichannel the manufacturers have not published specs in the form required by the FTC for stereo amps.

[generic]

Re: Compact Class D Amps - Misleading Specs?

I believe the old FTC regulations are still in effect, so those published amp ratings are essentially illegal. You could report these manufacturers to the FTC, but don't hold your breath that anything will come of it. The reason some people think the FTC rules no longer apply is because they are for stereo amps only, not multichannel amps. Since the majority of amps and receivers produced these days are multichannel the manufacturers have not published specs in the form required by the FTC for stereo amps.

What about this? http://www.parts-express.com/pe/show...umber=310-2014

Read measured numbers in the reviews.

[Tin_Ears]

Re: Compact Class D Amps - Misleading Specs?

I remember when most medium grade manufacturers of amps listed power output at rated RMS power at (usually) 1KHz but were often rated at 20-20KHz and we could "assume" at least 3dB dynamic headroom. Some even rated with both channels driven. Of course, back then voltage input wasn't part of the rating process... all commonly available amps were 120v/60Hz input (USA).

I haven't shopped for amps in a long time except out of curiosity regarding the budget-priced class D offerings so I don't know how far downhill the average consumer-grade amp manufacturer ratings have gone. I just find it disheartening that our "trusted retailers" are running these shell games with consumers who don't know any better. This has always been done but only with cheap products marketed through retailers most people "knew" weren't trying to tell the entire truth... they knew the products were cheap and didn't care about quality nor full transparency regarding product details.

Where have all the retailers gone who are there to, not "just" make profits, but be completely honest about the products they sell?

[tyger23]

Re: Compact Class D Amps - Misleading Specs?

Huh? I thought it was to shape the PWM output into a continuous waveform. Interesting post, if you feel like continuing the rant, I'll be happy to read it.

Well - the filter does both. It removes the high-frequency switching portion of the waveform, which makes the output look more like a standard class-AB. Note, though - the filter is not always required. In wholly embedded systems (where the amp and speakers are contained in a small, compact unit - think laptops, GPS, clock radios, etc.) the Class-D amps are often run "filterless", which means the L/C filter is omitted for cost concerns. EMI emissions are abated by the enclosed case, so the L/C is not necessary. The limitation here is distance between the amp and the speakers. If the distance is too large, then the traces or speaker cables will emit the high-frequency switching noise and cause interference with other components. In the case of a stand-alone class-D amp, you have an infinite length cable to assume, so an L/C filter is necessary.

So, back to my original statement, the filters are REQUIRED for FCC and radiated emissions testing, but they are not required for anything fidelity or speaker related. The speakers don't care about this switching frequency because their voice coils can't respond that fast (>250KHz).

Put things into perspective. What's the difference between 28W and 50W? 3dB... so not much. Now consider what a manufacturer has to expect when they put a 24V rating on the DC input, and weigh that against the cost of bullet-proofing the input enough to accept the range of real-world power supplies that are also rated for "24V".

Agree that cost and reliability are definitely at play here. Also the fact that 12V and 24V power supplies are much more common than 32V or 36V power supplies. However there's more at play than just 3dB of difference. When you factor in headroom and distortion, the delta between this amp at 24V and 33V is more like 6dB, which is a tremendous difference.

If you're the kind of guy that's going to find the Tripath data sheet and look up the absolute max ratings, you probably also know enough to crack open a case and see what the PSU caps can handle, and find a power supply that will ride that line of tolerance.

Tru dat. Check out my AMP-100 mods :D

[fastbike1]

Re: Compact Class D Amps - Misleading Specs?

I'll call BS here. The manufacturers aren't violating any requirements. The given specs match the requirements in that they give the power at a given frequency into a given load with a given distortion, for a given number of driven channels. Further, those products will meet the given specs.
The consumer, as always, needs to be able to underderstand the meaning of what they are reading. The consumer also needs to realize that you don't get something for nothing.

There have never been any legal requirements to protect the ignorant, lazy, or foolish. Ever heard the phrase "caveat emptor"?

[tyger23]

Re: Compact Class D Amps - Misleading Specs?

I'll call BS here. The manufacturers aren't violating any requirements.

In 1974 the FTC passed Rule 46 CFR 432 (1974), which basically states that amp manufacturers must use RMS methods to rate their amplifiers. In Car Audio land, there's the CEA-2006 logo, which specifies real-world power measurements and results.

The CEA-2006 logo is voluntary, but the FCC rules are supposedly required for any HOME AUDIO amp sold in the US. Note, though - the DTA-100 is following those rules by PUBLISHING a spec that conforms to the spirit of the FTC regulations.

The given specs match the requirements in that they give the power at a given frequency into a given load with a given distortion, for a given number of driven channels. Further, those products will meet the given specs.

Not true. Given the 24V power supply, the DTA-100 CANNOT meet the published specifications. It is phyiscally impossible. The TK2050 amp solution is capable of hitting those specs @ around 32V, but the DTA-100 is NOT.

The consumer, as always, needs to be able to underderstand the meaning of what they are reading. The consumer also needs to realize that you don't get something for nothing.
There have never been any legal requirements to protect the ignorant, lazy, or foolish. Ever heard the phrase "caveat emptor"?

1000% agree.