Max Output Limit on Amplifier? - Page 1
Forum section: electronics / beginners

author: Shredhead
author_rank: Regular Contributor
timestamp: May 28, 2014, 06:06:54 pm
content: I see many pro audio amps on the market that claim 4000 to 14,000 watts bridged on a 120V line. How in the hell is this possible? Even if the amp was 100% efficient wouldn't ohms law limit it's output to 120V x 20Amp(circuit) = 2400 watts? I don't get it.

author: Monkeh
author_rank: Super Contributor
timestamp: May 28, 2014, 06:12:31 pm
content: Peak power != continuous power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2014, 04:08:34 am
content: Also, "Watts PMPO" ~= 10000 * SHP (Sears Horsepower). In other words, BS on top of BS...Proper hi-fi amps are usually rated in RMS like they should be, and I don't think I've heard of anyone claiming over 1000W from a domestically powered unit. Typically, you'll be limited to, say, 120V * 15A = 1800 VA, which after a capacitor-input rectifier gives a power factor of maybe 0.6, or 1kW maximum. After efficiency of the linear amplifier, this will come down further to more like 600W real undistorted sinewave output at nominal load impedance. The peaks of course are twice this (because the voltage/current peaks are sqrt(2) times the average and power goes as V or I squared), but that's not you measure when you heat up a resistor.Taking liberties with peak-average levels, large capacitors and slow-blow fuses, one can easily draw several kW for long enough to power most music. To be completely honest, it would have to be specified as quasi-peak RMS, or some nonsense like that -- RMS over a 100ms period, say. Which would be a reasonably useful way to rate such equipment. But complicated to measure, test and protect.Marketing types would probably be more inclined to simply jump right to peak watts, regardless of the duration (who cares if it's for 1us or 1 min?), so you'll see that quite often, especially in mid to upper level equipment I think?Tim

author: Shredhead
author_rank: Regular Contributor
timestamp: May 29, 2014, 04:22:10 am
content: My brain just melted...PMPO. Wow, I never heard of that until now. Just looked it up and how disgusting. What you said made total sense, I appreciate it T3.

author: David_AVD
author_rank: Super Contributor
timestamp: May 29, 2014, 04:23:21 am
content: Around here, we refer to PMPO as Peak Marketing Power Output.

author: GK
author_rank: Super Contributor
timestamp: May 29, 2014, 04:49:47 am
content: Quote from: Shredhead on May 28, 2014, 06:06:54 pmThere are plenty of legitimate high power PA amplifiers (particularly class D) out there that can deliver in excess of 2400W on a short term basis. A real world music signal isn't a continuous sinewave."RMS power" is also a BS/abused term. The figure given (the product of the RMS voltage and the RMS current) is actually average power.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 29, 2014, 05:34:42 am
content: Quote from: GK on May 29, 2014, 04:49:47 amThe problem with using other than average power ratings is that they all use different test setups,so there is no easy way of comparing them.At one time one such test involved using a massive external power supply so the output stage could produce peak power which was impossible with the built in supply.There was going to be a "crackdown" on such practices,but it may not have eventuated."RMS" power is one of my "pet hates"---people use it all the time without thinking of,or even knowing about the original reason for the use of RMS for ac voltage or current.Perhaps this is due to the teaching of RMS as a purely Mathematical process,with no reference to the practical reasons for its adoption in Electrical Theory.

author: GK
author_rank: Super Contributor
timestamp: May 29, 2014, 05:53:51 am
content: Quote from: vk6zgo on May 29, 2014, 05:34:42 amDid someone say otherwise?Quote from: vk6zgo on May 29, 2014, 05:34:42 amI have never come across an elementary text delving into and explaining the mathematical derivation of “RMS” that did not mention that the RMS figure for AC voltage is equal to the DC figure for the same “heating effect” into a load.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2014, 06:19:30 am
content: Quote from: GK on May 29, 2014, 04:49:47 amTo be perfectly accurate, Vrms * Irms = S, the apparent power in VA. P / S = PF, the power factor. For a resistor, PF = 1 and S = P is the average power.I suppose it's worth noting that a speaker is quite far from a resistive load, but now that I'm thinking of it, I've never once heard even the merest suggestion regarding the reactive power of a speaker. It's always assumed resistive, because that's what's easy to test, and whatever effective power factor you actually get in practice is just chocked up to speaker inefficiency.Now I'm kind of curious what the real world power factor of a speaker system, under given musical conditions, is! A shame I don't have a wideband PQA on hand!Tim

author: GK
author_rank: Super Contributor
timestamp: May 29, 2014, 06:30:27 am
content: Quote from: T3sl4co1l on May 29, 2014, 06:19:30 amWell, as you state yourself, the “RMS” power figures given always assume a resistive load, so it is hardly inaccurate to state that the product of Vac and Iac is in fact average power.

author: kaindub
author_rank: Regular Contributor
timestamp: June 03, 2014, 03:49:00 am
content: It does not matter what units we use to rate equipment. RMS watts, peak watts, wombats per week.The real issue amongst audio suppliers is that each uses their own 'standard' to usurp the others.The EIA and FTC have standardised tests for amplifiers and the AES has standardized test for speakers.Do theses rogue audio manufacturer ever quote which test standard they have tested to? NO!If every manufacturer tested to the same standard then my first statement would be 100% true.You know they don't test to any standard because none of their technical literature. let alone the sales literature refers to the standard or method of test.Us poor buyers just want to know the comparative goodness of one product to the next. The manufacturers just cloud our heads.BTW RMS is a mathematical derivation. (Take the square of the signal; take the average of that value; take the square root of the average)For Power which is volts times amps, one uses the vector product of the volts and amps, which takes into account the phase angle.Robert

author: GK
author_rank: Super Contributor
timestamp: June 03, 2014, 04:27:44 am
content: Quote from: kaindub on June 03, 2014, 03:49:00 amThe point that you appear to have missed is that "RMS watts" is a technically bunk term. An amplifier that can deliver a maximum of 20 Vrms into a 4 ohm (resistive) load, just prior to clipping, on a continuous or a semi-continuous basis, is typically called a “100W RMS” amplifier.That “100W” is derived from the product of the rms voltage and the rms current, but that does not make it sensible to label the result “RMS”. The result is in fact average power.This is a silly convention that exists (as far as I’m aware) only in modern* audio. For example a 100V/10A transformer is not called a “1000W RMS” transformer; it just has a “VA” rating. A Broadcast AM radio transmitter has an unmodulated carrier power capacity rated, as it should be, in just plain watts, not “RMS watts”. Etcetera, etcetera......*None of my proper audio engineering texts <~1980 use this silly convention. A 20W amplifier is specified as just that – a “20W” amplifier; or "20WPC" if stereo (PC meaning “per channel”).

author: Shredhead
author_rank: Regular Contributor
timestamp: June 03, 2014, 03:53:18 pm
content: I think it would be much more simple to just stick to Volts. Like 30V to clipping. So much easier.Also, what circuit for power the amp is hooked up to would come into play right? A mains plug from a 30 amp circuit would therefore test different from a 15 amp circuit in a high current amp? So why wouldn't that be on the spec sheet?The thing is with the high powered stuff is they have such a large capacitor reservior for transients, there WOULD have to be some kind rating for a short blast peak but it would have to be graphed -output vs. time.It's a shame that marketing departments get to cheat standards instead of engineers deciding what they should be.