Difference between RMS and PEAK on amplifiers??

http://www.youtube.com/user/theBTman · Post by **thebtman** » 20 May 2008, 19:04

As title. If something says 80 watts rms, 240 peak what does that mean in terms of actual output?

Post by **theEarwigger** » 20 May 2008, 19:12

Peak is "peak to peak" as measured on a 'scope (p-p)

RMS is peak divided by 2 then root 2. RMS is the same as "heating power".

'thought all BT folk would know that..?

-Rob

http://www.youtube.com/user/theBTman · Post by **thebtman** » 20 May 2008, 19:21

theEarwigger wrote:Peak is "peak to peak" as measured on a 'scope (p-p)

RMS is peak divided by 2 then root 2. RMS is the same as "heating power".

'thought all BT folk would know that..?

-Rob

I didnt install Linear amplifiers in my time at BT, we had advanced by then I guess

Edit: and what about pep??!

Post by **crusty** » 20 May 2008, 20:46

http://en.wikipedia.org/wiki/Peak_envelope_power

wikipedia and google are amazing things

http://www.youtube.com/user/theBTman · Post by **thebtman** » 20 May 2008, 23:18

Oh why cant someone just tell me what output it means I get

Post by **wa10** » 21 May 2008, 01:02

btman if your looking at the usa custom amp ratings it often means diddlysquat,
i know plenty of the builders and they dont seem to agree on what they mean by peak,

average or the misused term rms power is what you would see usually on a bird43 avg reading meter,
when it comes to peak, some mean peak swing in avg reading mode and some mean pep measured with an active peak reading board again usually in a bird43 or 43p,

you will also see 500w bird 10,000w bird 75,000w bird, usually they mean the most swing you will see on a bird43 in avg reading mode or the larger bird couplers if your above 10,000w,

the 80w rms 240w peak rating??

@100% modulation an am transmitter will give an average power of 1.5x the unmodulated resting carrier and a pep power of 4x the resting carrier,
90w avg 240w peak would be the average power and pep power from an am transmission with an unmodulated resting carrier ( deadkey ) of 60w modulated to 100% modulation,
80wavg 240wpeak would indicate overmodulation,

most of the amp builders and virtually all of the bigger stations use bird43 meters or the larger size birds leading to the term bird watts, its the industry standard meter, since they mainly use the same meter you can kind of make comparisons from one maker to another but its still guesswork as they dont have a standard modulation depth they use for testing,

for ssb and am, measurements should be taken using a quality active peak reading meter or a scope and callibrated load otherwise it means nothing outside of the world of cb radio,

the term rms power when refering to the output of modulated rf amps is the wrong term,
the correct term is average power, the bird 43 measures avg power and peak envelope power if you add the active peak board,

none of the ratings relate to what you can safely get out of them on fm on a regular basis,

theres more to it than that but as usual i have said too much already

.

hope this helps.

Post by **crusty** » 21 May 2008, 02:04

Put simply as a rule-of-thumb RMS equates to fm/am output with full audio, PEP equates to max output on SSB with full audio. However as wa10 points out you will get less than the manufacturer claims and the terms they use are misleading to the home user. There are several ways and types of equipment for measuring power output, hence the reason industry has adopted bird43 measurments as a standard. This of course means bugger-all to the home user.

Ask a big question, you'll get a big answer

Sorry for barging in wa10.. Emergency aid, I think BTMan was about to let smoke out

ATB
Crusty.

Post by **wa10** » 21 May 2008, 03:01

crusty letting off steam is ok, its the 10ohm smoke you dont want to be letting out,

note to self,
check your maths before you hit the submit button

http://www.youtube.com/user/theBTman · Post by **thebtman** » 21 May 2008, 06:21

Thanks guys - I am much more enlightened...some seriously knowledgable people on this forum, its impressive...and crusty thanks for the smoke prevention hehe

jazzsinger · Post by **jazzsinger** » 21 May 2008, 13:25

would 80w rms not indicate undermodulation rather than overmodulation bob if 90w rms is 100% modulation,lol.

i would also like to add that even a bird 43 is innacurate if not used with a purely resistive dummy load,you cant measure power accurately on a reactive antenna which some people just don't seem to realise.

Post by **wa10** » 21 May 2008, 16:29

george, its the peak to avg ratio,

53.3w deadkey = 80w avg and 213.3w pep @ 100% modulation

60w deadkey = 90w avg and 240w pep @ 100% modulation

they get the low avg to pep ratios by removing the modulation limiter,
deadkeying low and swinging high into none linear amps ( overmodulated )

a few smart guys get over 100% modulation using asymetric modulation in the same way am broadcast stations get over 100% with acceptable levels of distortion through negative cycle loading, they limit the negative excursions to just under 100% but allow the positive peaks to rise above 100%, this stops the carrier cutting off on negative peaks causing splatter and makes you sound larger than life at the expense of some distortion,

have a look at my buddies website, hes not a big station but onair he has wicked audio, fighting well above your weight is his speciality,
http://www.motormouthmaul.com/

yes you need a none reactive dummyload to make accurate measurements, keep spreading the word george

jazzsinger · Post by **jazzsinger** » 21 May 2008, 17:22

i think the 80w rms 240w pep is a bit misleading because for a 240w pep signal modulated at 100% the rms power would be 90w.not sure where bt man got those figures from but either the 80w rms is wrong or the 240w pep is wrong,although i would imagine those readings were taken on a meter with two scales with differing accuracies.surmising that the deadkey was 60w rms then an 80w rms reading would indicate undermodulation below 100%.

i think too many people are obsessed with power at the expense of the quality of their audio and the strain that listening too distorted audio puts on the listeners ears,not to mention the interference a badly adjusted rf amplifier causes to other radio users.power must be the single most misunderstood concept in radio with transmission lines not far behind.

a tip for those who like reading radio mod sites if the site mentions removing modulation limiters,or defeating/cranking up the alc on ssb avoid it!!! those circuits are there for a good reason.to put it simply they reduce harmonics/distortion.

i find the best way to get decent audio on a budget is to use a dynamic microphone with a speech processor with a sensible amount of clipping so as to minimise distortion whilst maximising punch and average output power.cheap but very effective.far better investment than power mikes or linears,unless of course you use a proper biased linear amp with decent filtering which costs big bucks,although processing is practically useless on fm the difference on ssb is awesome.on fm generally using a decent quality dynamic microphone and setting the deviation properly gives plenty of kick.power mikes are a bit like atu's,pretty much useless,all they do is amplify everything including the background noise.

i can see why he has wicked audio Bob but a bit extreme for an average cb setup,he's certainly splashed the dollars to achieve it.very interesting reading though.

Post by **wa10** » 21 May 2008, 23:38

the amplifier thing is not so much misunderstood george its more like mislead by the people that make them or have a monetary interest in selling them, building/selling class c amps with ssb delay to be used on ssb is taking the ****,
commercial makers dont/cannot bullshit their customers about output capabilities and mode suitability like they can in the cb arena,

feedlines lol im still getting my head round that topic, i think i have a reasonable grasp of it but theres plenty more to learn,

yep motor is a smart man, hes a broadcast engineer so he is able to pickup gear when it becomes obsolete, hes a genuinely nice bloke,

i agree a processsor does the trick for dx especially rf clippers, i used to build a few different types of audio based processors for locals in the 80's and a couple of rf based processors for my own use followed, the designs came from mags like pw and radcom, i just modified them to suite my own ears,
i dont have any of my homebrew units anymore but i still have a datong asp that worked fantastic on the old yaesu ft767 , i dont know if it will work ok in a mobile rf environment but i will be testing it when we get some strong dx from the usa.

http://www.youtube.com/user/theBTman · Post by **thebtman** » 22 May 2008, 17:41

I am genuinely amazed by the knowledge and skill of some of the people replying here. I can wire a mic or replace a diode or something but its obvious that there are some really intelligent radio enthusiasts here. How brilliant is that. Its a shame though that with all that experience noone offered to build me my 2kw amp hehe