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Those class D amplifier boards, not as advertised!

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  • Those class D amplifier boards, not as advertised!

    They are all over ebay. If you are lucky, the relevant data will be included. 200 x 2 watt amplifier board, low distortion! Only 50 bucks! Sounds awesome? Turns out, the only way you get 200 watts a side is if you have no issues with 10 PERCENT harmonic distortion! The low distortion figure is based on one watt output, to get the THD down to 1% you get 128 watts. Every single one of these boards is like this. You get a little better than HALF of the power if you expect a reasonable distortion level. If you run the board at full power supply recommended, even at less than the rated output it will burn up quickly.

    Most listings on ebay don't even tell you the distortion figures. Your best bet is to check out one with a TI chip. They usually tell you the number, then you can look up the chip and get the real truth. TI is awesome for that. Bunches of graphs so you know what the thing will really do. Some of the boards on ebay lack the coils and capacitors on the output. And, look up the amp boards, you get 1000 plus listings. Exclude china and you are down to less than 100. Don't mean to sound racist, but I've been ripped off too many times buying from Chinese sellers. Face it, they are all made in China. The company Sure looks like an American company, but it's not. You can always spot the Chinese stuff by the description. For some reason, Chinese dealers never bother to hire a native American to write them.

    Why can't they be honest? It's not 200 watts! I'd call most of them liars, but if they tell you about the THD, they have covered their butts. At least, boards listed on Parts Express are honest about the distortion. I saw one which claimed 100 watts, but admitted it was at 10 % THD. At 99 watts, it was 1 %. That was unusual. In most cases you have to derate the boards by almost half the advertised power. Why can't these jerks just be honest?

  • #2
    That's pretty common with these boards.

    I don't know about ebay for this stuff but I would tend to just go straight to the sources: Dayton or Wondom (Sure). Wondom is quite clear in the specifications as to what the THD+N specs are.
    What I find quite interesting is that in a 2x50W board using the EXACT same chipset Wondom discloses 10% THD+N @ 50W whilst Dayton is attempting to claim 1% @ 200Hz @50W.

    I mean...it IS 50watts in terms of maximum possible output power. There is nothing that says the rated output power must only be quoted up to a certain level of distortion. Realistically, look at anything that has a power rating - its always advertised/marketed at the maximum possible output. Even your car wont last very long if you run it at maximum power all day.

    I guess all I'm saying is, its a $20-30 amplifier board, Class D, made for Kiosks and the like - they aren't really HiFi and personally I don't expect much from them - so I'm certainly not as bitterly disappointed at the THD specs.

    If you want decent specification in a DIY Class-D amp: https://www.diyclassd.com/diy-amplifier-kits/

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    • #3
      https://www.youtube.com/watch?v=Gxd23UVID7k

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      • #4
        They can't (or won't) be honest because everyone else is also inflating the figures. So, they would sell no units if they told the truth. Amp wattage ratings are, among other things, for bragging rights.
        Francis

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        • #5
          Sadly, this is seemingly SOP for domestic as well as foreign distributors for inexpensive amps.

          Taking some time to understand amplifier tech in general will lead to a better ability to sift through the ad copy and procure what you need.

          In car audio, for example, regardless of advertised claims, the current rule of thumb is ~$.10/W. Most of the inexpensive chip amps out there seems very similar to inexpensive 12V stuff. There are rare exceptions, of course, but if you see a $90 amp advertised as 3000 watts, you can comfortably assume 700-1000 at best, at 1% THD. Some tested amps hit rated power at clipping, others under the "dynamic" mode.

          Welcome to the expensive world of unicorn hunting.
          Don't listen to me - I have not sold any $150,000 speakers.

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          • #6
            If it's any consolation, most of the Class AB boards on Ebay list imaginary wattage ratings, too.
            Francis

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            • #7
              If you've decided on a chip set just go straight to the chip set manufacturer and get a reference/evaluation board. These boards are designed to show off the chip set and are very well documented. They won't have cut corners on design and component quality.

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              • #8
                Most of these specifications are copied directly from the datasheet of the amplifier chip, and are not measured output values. The rated values on the chip datasheet assume an adequate power supply and adequate heat sink, neither of which are typically part of these Ebay designs.

                But... at $50-$100, some of these amplifiers are still flat out amazing, and represent an outstanding value compared to what a simple Class A/B receiver can do for more money.

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                • #9
                  +1 on the power supply. I see corners cut there all the time in the pro-sound genre. You can only stuff so much power supply into a 2x6x8 inch box. Can you really get an honest 3kW/Ch/8 ohms from a Class D amp? Sure, if the innards look like this:
                  Click image for larger version

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                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    99% of people never listen to music louder than what a one of those class D amps is easily capable of. 99% of the listening of the other 1% also isn't at a volume that those little class D amps struggle with.

                    I'm perfectly happy with my $60 Fosi 100x2. Esp paired with a tube preamp. Even if its power isn't as advertised, who cares, its loud enough for any listening I would want to do.

                    Those super cheap class d amp boards are great for boomboxes. I bet boombox parts is the bulk of what PE sells nowadays for DIYers. Unpowered speakers as a concept are becoming retro.

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                    • #11
                      Originally posted by Waldo View Post
                      99% of people never listen to music louder than what a one of those class D amps is easily capable of. 99% of the listening of the other 1% also isn't at a volume that those little class D amps struggle with.

                      I'm perfectly happy with my $60 Fosi 100x2. Esp paired with a tube preamp. Even if its power isn't as advertised, who cares, its loud enough for any listening I would want to do.

                      Those super cheap class d amp boards are great for boomboxes. I bet boombox parts is the bulk of what PE sells nowadays for DIYers. Unpowered speakers as a concept are becoming retro.
                      Retro, or high end. At a guess, serious audiophiles will always want their choice of amplifier to pair with the speaker.
                      Francis

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                      • #12
                        Originally posted by Waldo View Post
                        99% of people never listen to music louder than what a one of those class D amps is easily capable of. 99% of the listening of the other 1% also isn't at a volume that those little class D amps struggle with.

                        I'm perfectly happy with my $60 Fosi 100x2. Esp paired with a tube preamp. Even if its power isn't as advertised, who cares, its loud enough for any listening I would want to do.

                        Those super cheap class d amp boards are great for boomboxes. I bet boombox parts is the bulk of what PE sells nowadays for DIYers. Unpowered speakers as a concept are becoming retro.
                        Yeah, depending on sensitivity, it is generally the first couple watts that ultimately matters on an amp. For me, personally, I rarely listen with peaks over 85db - and that is generally no more than a couple-three watts for most designs. Usually, I listen even quieter.

                        Related: when I started doing all of my modeling based on 90db instead of amplifier power on-tap, my designs became better. I worried less about Xmax, realized I can cross a touch lower for a given tweeter, have plenty of headroom on even smallish amps for EQ purposes, rarely need to implement a high-pass filter on smaller speakers, etc.

                        YMMV, of course, but most speakers are capable of running with very low distortion even at lower frequencies at listening levels that will not cause hearing damage.
                        Don't listen to me - I have not sold any $150,000 speakers.

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                        • #13
                          ...that being said, from time to time I do optimize a design for extremely high volume playback. The Cabrini Redux is one that holds composure at house party volume, and the ol' Barbie Girl demo speakers are another.
                          Don't listen to me - I have not sold any $150,000 speakers.

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