Seeking clarification stated amplifier rating.  - Page 1
Forum section: electronics / general

author: Jbates58
author_rank: Regular Contributor
timestamp: July 10, 2019, 01:29:51 pm
content: Hi all, I have found a set of party/dj speakers that I would like to get to expand my setup. However with my knowledge of electronics, reading the specs for them, i am somewhat dubious that they actually perform they are rated. I rang and spoke to the manufacturer today, and he assured me that they do, but it still dosent make sense, so perhaps my understanding is incorrect.They are listed as 1100wrms , so a massive step up from my 300w rms mackies.But, to my knowledge that would mean they need more than 1100w in, however they are listed with 250w next to power draw.The manufacturer tells.me.its something to do with the class d amplifier. But I dont know if it's TRUE or not.The link to the spec sheet ishttp://www.scpaudio.com/product/456/T12a.bossif someone could look at that and tell me if I am right or wrong. That would be great, anod perhaps an explanation as to why if its true anaccurate.ThanksJason

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 10, 2019, 01:37:36 pm
content: Not completely sure what they mean by that, but they state: "Power Draw: 250W - 1/8 Rated Output".That's the power draw at 1/8 rated output, not at full power!Assuming they are talking about output power, I would understand this as 1100/8 = 137.5WRMS output power.

author: Jbates58
author_rank: Regular Contributor
timestamp: July 10, 2019, 01:40:50 pm
content: Is there an effective way in can verify the output power? Power in is easy, but I have no idea about power out

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 10, 2019, 02:08:21 pm
content: Unless you have direct access to the output of the integrated amplifier, nope. But you can measure the sound pressure with an appropriate device, in dB SPL, typically with a 1kHz input signal and at 1m facing the speaker.Assuming the rated 130dB SPL is obtained at 1100W RMS, at 1/8 of the power, that would be -9dB, thus theoretically 121dB SPL. So if what they state is correct (or at least if I have understood them correctly), you should get ~121dB SPL when the speaker is itself drawing 250W.If you're going to do this kind of measurement, do not stay close to the speaker. 121dB SPL is VERY loud.

author: nctnico
author_rank: Super Contributor
timestamp: July 10, 2019, 04:03:10 pm
content: Sounds like the good old PMPO rating to me. Having a class-D amplifier isn't the reason to draw much less than 1100W. If a speaker is rated for 1100W then it will need an amplifier capable of delivering 1100W and that amplifier will need at least 1100W from the mains.

author: james_s
author_rank: Super Contributor
timestamp: July 10, 2019, 04:20:40 pm
content: These sort of bullshit inflated specs have plagued audio gear for decades. They find all sorts of ways to inflate the number for marketing reasons.The wattage is largely irrelevant either, the efficiency of a speaker can vary dramatically, one may require an order of magnitude more power to produce the same sound pressure. As has been mentioned already an amplifier cannot possibly deliver more RMS power to the speaker than it draws from the power source, that would be over unity and violate the laws of energy.

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2019, 04:48:23 pm
content: It could theoretically drive the speaker with a much higher peak power, than the RMS mains power draw, but in reality it's bullshit. The actual power output is probably under a tenth of what's stated on the label.

author: nctnico
author_rank: Super Contributor
timestamp: July 10, 2019, 05:41:10 pm
content: Quote from: Zero999 on July 10, 2019, 04:48:23 pmThat is PMPO!

author: dmills
author_rank: Super Contributor
timestamp: July 10, 2019, 09:11:11 pm
content: Um, not necessarily... A power amplifier actually has a couple of limiting numbers, peak voltage, maximum current and how much heat it can get rid of.Now real program audio has a rather large crest factor so designing an amp where the peak handling is maybe as much as 10dB above the continuous rating is actually entirely sane and in the PA game is done routinely (Nobody likes humping continuously rated heatsinks around to no point, them things heavy).Industry consensus is that average (thermal) power of the order of 1/8th the voltage^2/impedance limited power is quite sufficient, and much the same sort of factor works for typical supply input sizing (circuit breakers being rather slow on the thermal part of their curves).The PMPO thing was a 1970s hifi (And modern car audio!) perversion, but cont. power not usually being particularly relevant is not news to anyone running LabGruppen FP, Camco or similar in their rigs.An amp good for 500W (Cont) and 4kW peak at say ten percent duty cycle is a LOT more useful then one rated for say 1kW (Cont) but that bangs into the rails at 1100W.Regards, Dan.

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2019, 01:18:26 pm
content: One of my pet peeves for at least 50 years is the term "rms power", when people mean "average power". If you measure rms voltage into a resistive load, the usual formulaP=V2/Rgives the "mean" power. One could calculate rms power, but it is not a useful quantity. The mean power is the power that a DC voltage of the same value would deliver to that resistor.

author: dmills
author_rank: Super Contributor
timestamp: July 11, 2019, 01:54:56 pm
content: Trouble is the American consumer products folks specified RMS Power as being the required label (Or at least that is how the industry interpreted the regs)!What you really want is a curve of power Vs duty cycle into some specified load impedance, sort of a limiting curve on a CCDF plot with a specified average value, but a lot of the consumer market would not know how to read such a thing.You will note that nowhere in my post did I mention that RMS Power nonsense, it does grate somewhat.Regards, Dan.