What's With These Inflated Power Ratings For PA Amps? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 16th January 2007, 03:05 AM   #11
diyAudio Member
Join Date: Sep 2006
According to my friend who worked at Rockford Acoustics (and is now buying out their equipment from the GR, MI facility), there are set standards when it comes to measuring output power. However, there are no regulations requiring companies to label per specifications. Essentially, an audio company can put whatever number they want on the box, though I'm sure they wouldn't since they'd be sued in a heartbeat for false advertising if anyone with a signal generator and multimeter took a look at it.
  Reply With Quote
Old 16th January 2007, 04:17 AM   #12
Wizard of Kelts
diyAudio Moderator Emeritus
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State

Well, it might be possible to sue them if they say it puts out 150 watts RMS and it doesn't come anywhere near that.

If they say an amp that actually is rated at 30 watts RMS puts out 150 watts "music power", "max power" or that oldie but goodie, "PMPO", they probably would not be sued because the amp can put out out that power for a fraction of a second.

Fact is, in the seventies the real cheapie amps with unrecognizable names did list wattages way, way over the RMS rating.

I'm just amazed that decades after the power ratings were standardized in the hifi world, the related PA amp world is still messing around with these silly non RMS ratings.
"A friend will help you move. A really good friend will help you move a body."
  Reply With Quote
Old 16th January 2007, 04:34 AM   #13
Wizard of Kelts
diyAudio Moderator Emeritus
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Originally posted by AndrewT
I think Grollins is alluding to the fact that virtually no amplifier can double it's maximum power output into half load impedance.

This applies when both impedances are within specified range.
Well, I have not designed any large amps like Andrew T, but my understanding is that total wattage is the max available voltage times max available current.

So if the max voltage is eight times the max current, you have an amp that delivers most of it's power into an eight ohm load.

If the max voltage is four times the max current, your amp delivers most of it's power into a four ohm load. So if you hook an 8 ohm speaker up to an amp whose max voltage is four times it's max current, I would imagine you would deliver half the power that you would get if you used a 4 ohm speaker. I think most amps seem to be somewhere in between though, the max voltage is less than eight times but more than four times the max current. So yes, you don't get twice the power if you halve the load.

Incidentally, while we are on the subject, back in the seventies amp manufacturers spoke of "headroom", which meant the amp had extra voltage beyond that required for an 8 ohm load. A friend's big Phase Linear, for example, was really more of a 14 ohm amp than an 8 ohm amp.

I notice that headroom number is not listed these days, so I guess they don't have that extra voltage. Am I correct on this?
"A friend will help you move. A really good friend will help you move a body."
  Reply With Quote
Old 17th January 2007, 06:54 AM   #14
diyAudio Member
unclejed613's Avatar
Join Date: Dec 2006
actually "dynamic headroom" is a real number expressed in db above continuous rated RMS power, and is measured with a special test signal (which was also standardized by the IHF) where a 1khz signal is applied to the amplifier, sufficient to run the amplifier at 50% power. the input signal is amplitude modulated to produce an input envelope that is constant for 80ms, and increased a variable amount for 20ms. the signal "spikes" are then increased in amplitude until the amplifier clips during the spikes. the RMS power at clipping of this test is compared to the continuous rated power of the amp, and the result is expressed in db.

for instance the dynamic headroom rating of the Apt 1 amp was 3db at 8 ohms and 4 ohms. it was 2db at 2 ohms, and 1db at 16 ohms. dynamic headroom has a lot to do with power supply design, and not so much with the output stage.

the unscupulous take the same test signal, and instead of using the rms value of the clipping point, probably use the peak value (or even worse the peak to peak wattage) and use it as the basis for their "wattage" rating. they may even modify the standard test signal by shortening the spikes to 10 or even 5 ms to get even higher numbers
Attached Images
File Type: jpg dynhdrm.jpg (12.0 KB, 99 views)
Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net
spammer trap: spammers must die
  Reply With Quote
Old 17th January 2007, 09:11 AM   #15
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
I believe that dynamic headroom was invented by designers at the behest of unscrupulous salespersons to make their cheap/underspecified amplifiers seem more powerful.

I further believe dynamic headroom is a measure of the "badness" of an amplifier. i.e. more headroom = more sag in the supply rails when asked to work hard.

It is misused by advertising departments to seem an asset. If you believe in my opinion then low dynamic headroom is the better selection criteria.

I appreciate your contribution on how DH was measured.
Can you reference a way that us DIYers could mimic such a test?
Could a computer generate the envelope for instance?
Is there a software package available as freeware that has such a facility in it?

your diagram show a psuedo scope view.
Scaling vertically from that posting, I see a voltage output at the low level of about 25% of maximum, that implies a low power signal of about 10 to 15% of maximum. Similarly the horizontal scale is nowhere near the 20:80 split, it looks more like 6:94. Can you confirm if other ratios are actually specified in the IHF test procedure?
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 17th January 2007, 09:16 PM   #16
diyAudio Member
Join Date: Sep 2006
Hi all

Here's how I think the old 70's amp power ratings went:

unregulated supplies with small caps = higher voltage than sustained on load

say Vcc (quiescent)=20% more than Vcc (average)

music power = clipping level for short burst (peak)

power rating compared with RMS=1.2 *1.2 * 2 = at least 3x higher.

I think sometimes the clipping level (10% distortion) figure for output power was also used as this allowed the peak output to increase even more.

Peak music power out = peak misleading power out.

  Reply With Quote
Old 18th January 2007, 01:45 AM   #17
diyAudio Member
Join Date: Dec 2006
Send a message via MSN to NVMDSTEvil
Looks familiar to power output claims vs. actual RMS ratings in the late 90's to me.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Hammond PT power ratings dsavitsk Tubes / Valves 4 23rd August 2009 08:23 PM
Power supply resistors power ratings Nihilist Tubes / Valves 13 31st July 2009 01:47 AM
Tube power ratings Brian Donaldson Tubes / Valves 19 20th November 2005 02:59 PM
power ratings of resistors alecwek Parts 17 4th August 2004 09:54 PM
Power Ratings BAM Multi-Way 1 13th October 2001 11:05 PM

New To Site? Need Help?

All times are GMT. The time now is 02:56 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2