What would an "FTC watts" statement look like for these Class-D econoboxes?

[richardm]

Specifically referring to TPA3255-based amps with the typical 32 or 36V/5A brick supply. I see a lot of data in the various review threads -- are the power quotes peak (and I'd multiply by .707 for RMS)?

Looking at the Fosi V3 here:

Converting db to % it seems we're crossing the 1% threshold at 50W (with 32v supply). So perhaps an FTC statement for this amp would be something like: Minimum 35 watts per channel RMS at 8 ohms from 20-20,000Hz, both channels driven, with no more than 1% THD. Far cry from the "300w" number or whatever on the box. What do you guys think?

PS. My fingers are crossed for a 1974-style government smackdown. Get out your poly leisure suit and your flare-bottom corduroys; we're making honest watts great again!

 

[staticV3]

Amir's measurements always show W RMS.

 

[richardm]

Amir's measurements always show W RMS.

Cool. So I reckon we could call 'em an "honest" 50 watts per channel with their stock power bricks? Interesting how it balloons to ~120w at 48 volts supply. Not what I'd expect...

 

[TonyJZX]

i always take those 8 and 4 ohm measures as the most realistic real world power with the lowest thd and so everything else quoted from every other source will be worse off that what you see here

a good comparison is this:

AUDIOPHONICS MPA-S250NC Amp Review

This is a review and detailed measurements of the AUDIOPHONICS MPA-S250NC stereo Ncore class D amplifier. It is on kind loan from a member and costs 415,83 € tax excl. (US $444). The enclosure is absolutely adorable! While it is fair bit deep, the other two dimensions are quite small...

www.audiosciencereview.com

a LOT of amps use the same board and here you will see a real hard tested 100w @ 8 ohms and 190w @ 4 ohms

you see other sources quoting up to 125-150w @ 8 and up to 250w @ 4 ohms and so this is sort of like the old "box of truth"

 

[staticV3]

Cool. So I reckon we could call 'em an "honest" 50 watts per channel with their stock power bricks?

Yeah. I wonder how they arrived at their claimed numbers:

Source

 

[richardm]

Yeah. I wonder how they arrived at their claimed numbers:
View attachment 323401
Source

32v, 8 ohms, 50w -- at least that one looks right.
48v/5A, 8 ohms, 115w -- looks OK too, no?
48v/5A, 4 ohms, 230w -- uh, it's a 240w power supply so the unit would have to be 95.8% efficient here. Not buying it.

 

[staticV3]

48v/5A, 4 ohms, 230w -- uh, it's a 240w power supply so the unit would have to be 95.8% efficient here. Not buying it.

Nah, not necessarily. Sufficient capacitance allows PSUs to discharge more power than what they can supply continuously.

For audio applications, this is perfectly viable.

 

[richardm]

Nah, not necessarily. Sufficient capacitance allows PSUs to discharge more power than what they can supply continuously.

For audio applications, this is perfectly viable.

You're referring to supply caps getting juiced-up while the amp's output signal is crossing zero?

 

[fpitas]

You're referring to supply caps getting juiced-up while the amp's output signal is crossing zero?

Music average power is frequently 15dB below the peaks.

 

[DSJR]

Possibly silly question, but those power graphs - are they done on level sweeps and could said Fosi amp do say 35W continuous into 8 ohms for prolonged periods without melting? If Amir has already answered it then I apologise for not reading further. I only ask because my fifty year old prosumer amps were designed to run flat out all day at still quite low distortion levels without overheating (or heatsinks barely getting warm in hard use, let alone hot).

 

[fpitas]

Possibly silly question, but those power graphs - are they done on level sweeps and could said Fosi amp do say 35W continuous into 8 ohms for prolonged periods without melting? If Amir has already answered it then I apologise for not reading further. I only ask because my fifty year old prosumer amps were designed to run flat out all day at still quite low distortion levels without overheating (or heatsinks barely getting warm in hard use, let alone hot).

It's a good question in a technical sense. But domestic music isn't like that, it's more like noise, with huge peaks on a small average. In PA applications you do obviously need gobs of continuous power at times, and no one cares if the peaks get sawed off.

 

[pma]

Possibly silly question, but those power graphs - are they done on level sweeps and could said Fosi amp do say 35W continuous into 8 ohms for prolonged periods without melting? If Amir has already answered it then I apologise for not reading further. I only ask because my fifty year old prosumer amps were designed to run flat out all day at still quite low distortion levels without overheating (or heatsinks barely getting warm in hard use, let alone hot).

You never know until you test it seriously. I have posted numerous measurements of AIYIMA A07 with power supplies covering range 24V - 50Vdc, not restricted by their output current. At 50V supply voltage, the achievable output power is only short term rather than continuous and the amp had never reached the rated power. These are rather toys than serious amplifiers.

 

[fpitas]

These are rather toys than serious amplifiers.

I agree for pro audio they are a poor choice. I wonder how many home users will ever challenge them with continuous output.

 

[DSJR]

I agree for pro audio they are a poor choice. I wonder how many home users will ever challenge them with continuous output.

I suspect very few, but we're a fussy bunch of bods here though aren't we?

I remember one of my inherited Crown D-60's having intermittent swishing type noises and located the source in the ic input op-amp socket. After cleaning, I foolishly bent a pin re-inserting said chip and it seems, put the supply rail(s) into the input, or something nearly as bad Switched back on, plugged my headphones into the headphone jack which in this model is tapped directly off the output sockets with NO attenuation and left it for a goodly while. of course, the headphone modules fried almost immediately and after the time (twenty minutes?) the case got rather warm but not burning hot. I discovered my mistake, sorted the chip out and the thing is running just fine in my second system to this day ten years or so later. I put it down to the 'modular supply/limiting' design where each gain stage has V/I limiting in case of failure, preventing damage acting akin to knocking down a row of dominoes. Glad the chip wasn't fried as they're like hens teeth today although I believe they can be found here and there - nice old amp that and with kind-of similar 8 ohm power to the Fosi (44WPC at 8 ohms and 64WPC from memory into 4 ohm loads - sounds like it's farting if pushed to clipping I remember) and makes for a totally effortless headphone amp of course - cough...

 

[rdenney]

Amir's measurements always show W RMS.

Yes. The word “peak” means “the RMS of the waveforms that sustain the highest amplitudes for some defined period of time.” The length of that period is a whole other discussion.

Music average power is frequently 15dB below the peaks.

This is the difference between “continuous” and “peak” in a practical sense, but this may point more closely to “headroom”. Amps with regulated power supplies don’t have much headroom.

Probably the difference between rated and peak power more like the point where distortion passes the rated amount and where it crosses 1%. The actual power could double across that difference. Lots of stuff these days reports peak power at 10% distortion.

And then there’s the whole bandwidth issue.

Rick “not really sure the FTC really cleared all this up though it addressed the most egregious claims” Denney

 

[restorer-john]

An equivalent "FTC watts statement" cannot be inferred from Amir's testing.

 

[Blumlein 88]

An equivalent "FTC watts statement" cannot be inferred from Amir's testing.

True, but we can infer that an FTC rating won't be better.

 

[restorer-john]

True, but we can infer that an FTC rating won't be better.

Define "better".

An FTC power rating is a very detailed statement across specific parameters for the purposes of compliant advertising. One that has legal standing and can be compared by a (savvy) consumer to other similar ratings.

Amir's testing offers much greater insight into many areas and insufficient in other areas, some of which cover the FTC power output claims which the OP was specifically asking about.

 

[richardm]

We're getting pedantic here. I've found the answer I was looking for.