ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2] 3 4 ... 7   Go Down

Author Topic: Why do Class D amps seem to pull less power than they make?  (Read 20686 times)

Alex Berry

  • Newbie
  • *
  • Offline Offline
  • Posts: 41
Re: Why do Class D amps seem to pull less power than they make?
« Reply #10 on: January 23, 2015, 10:00:29 pm »

The K series amps can also be set to draw no more than a given line current if you wanted to do that.

Lee

I saw that as well and plan on using that too. Such awesome features. My current inventory consists of EV ZLX and ETX gear with the only outboard amp I own being a Crown XLS1000. The kind of features the Powersoft amps have are the kind of things dreams are made of.
Logged

Jelmer de Jong

  • Full Member
  • ***
  • Offline Offline
  • Posts: 168
Re: Why do Class D amps seem to pull less power than they make?
« Reply #11 on: January 24, 2015, 06:07:10 am »


I have used 5 QSC PLX3402 amps in a school before, all of them run off of just two 20A 120V circuits. Two were wired in 4 ohm stereo each, to feed a total of four JBL SR4719X subs and we're being run just beneath clip/limit. The other three were being used to bi-amp four JBL 4873 tops, two amps run to the 15" and mid frequency horns, and the last amp ran to the high frequency horns. As you can imagine, these amps we being stressed nowhere near as much as the sub amps. Now this whole system, as previously stated, ran off of two 20A circuits without any problems. How is this even remotely possible? In theory, just the sub amps alone should have overloaded both circuits? I'm wondering not just out of curiosity but also because in February I'm going to be running two Powersoft K10 amps off of these same two 20A circuits, powering eight Sound Bridge 7218SWX subs. Now I'll only be running the subs at about 500wrms per cabinet, but considering the ability to run the QSC amps off the same circuits, I should be more than fine to run these amps at this capacity, correct?

120V/20A is about 2400W. Two sockets makes 4800W. The PLX amp delivers 2200W total in 4ohm clip. If your music is a pure sine wave a clipping amp will actually use half the power you thing because a sine's peak is 3dB higher than its avarage power. So your 4 ohm dual channel amp will not draw 2200W from the wall but 1100. But music is not a pure sine wave(mostly), so your actual power usage will be even less if the clipping occours at peaks that are 10-15dB above the avarage power, that's 10-30 times less continuous power than you will think.
Logged

Steve M Smith

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3213
  • Isle of Wight - England
Re: Why do Class D amps seem to pull less power than they make?
« Reply #12 on: January 24, 2015, 07:46:22 am »

Why do Class D amps seem to pull less power than they make?


It's an old salesman's trick known as lying!


Steve.
Logged

Alex Berry

  • Newbie
  • *
  • Offline Offline
  • Posts: 41
Re: Why do Class D amps seem to pull less power than they make?
« Reply #13 on: January 24, 2015, 08:17:20 am »

120V/20A is about 2400W. Two sockets makes 4800W. The PLX amp delivers 2200W total in 4ohm clip. If your music is a pure sine wave a clipping amp will actually use half the power you thing because a sine's peak is 3dB higher than its avarage power. So your 4 ohm dual channel amp will not draw 2200W from the wall but 1100. But music is not a pure sine wave(mostly), so your actual power usage will be even less if the clipping occours at peaks that are 10-15dB above the avarage power, that's 10-30 times less continuous power than you will think.

This is only true assuming that the QSC amps are 100% efficient. They're actually more like 30% efficient. So with that in mind, it would take, for example, around 1000 watts just to make roughly 300 watts. Considering this, and the fact most of the music played through the amps was highly compressed EDM and rap with copious amounts of long bass tones and almost no transients, we're looking at some serious power draws. I'm really starting to understand the fact that because I was able to run such an inefficient system off the pair of circuits, I'll be easily able to run a much more efficient system that is less powerful off of the same pair of circuits. But thanks for the input!


It's an old salesman's trick known as lying!


Steve.

Then how can Powersoft put in the specifications that the K10 is specifically rated to pull 1250 watts from the wall to output 1500 watts? I don't understand how this form of blatant lying could be tolerable.
« Last Edit: January 24, 2015, 08:20:51 am by Alex Berry »
Logged

Steve M Smith

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3213
  • Isle of Wight - England
Re: Why do Class D amps seem to pull less power than they make?
« Reply #14 on: January 24, 2015, 08:34:47 am »

It's the way the measurements are made.  If you look at various manufacturers' specifications, you will see that power can be measured in RMS (Root Mean Square), PMPO (Peak Music Power Output) and various other Instantaneous peak measurements where the output can only be sustained for a small amount of time.

An even better demonstration of this would be the flash for a camera.  It might take ten seconds to charge up then the flash of light lasts only for about 1/10,000 of a second.

The energy put into the flash tube is huge but it doesn't last long. The current taken from the batteries is relatively low but over a fairly long time.

In my opinion, the only honest measurement is RMS.  However, when salesmen realised that a peak power measurement would allow them to advertise something as twice as much power, that is exactly what they did.  I first noticed this being done in the 1980s with lower end hi-fi equipment.


Steve.
Logged

Alex Berry

  • Newbie
  • *
  • Offline Offline
  • Posts: 41
Re: Why do Class D amps seem to pull less power than they make?
« Reply #15 on: January 24, 2015, 08:45:27 am »

It's the way the measurements are made.  If you look at various manufacturers' specifications, you will see that power can be measured in RMS (Root Mean Square), PMPO (Peak Music Power Output) and various other Instantaneous peak measurements where the output can only be sustained for a small amount of time.

An even better demonstration of this would be the flash for a camera.  It might take ten seconds to charge up then the flash of light lasts only for about 1/10,000 of a second.

The energy put into the flash tube is huge but it doesn't last long. The current taken from the batteries is relatively low but over a fairly long time.

In my opinion, the only honest measurement is RMS.  However, when salesmen realised that a peak power measurement would allow them to advertise something as twice as much power, that is exactly what they did.  I first noticed this being done in the 1980s with lower end hi-fi equipment.


Steve.

I'm actually already pretty familiar with all of the information you have mentioned. Imagine if all the manufacturers would just be honest and rate their equipment according to real world values, and not something never attainable in practical use. Imagine how cool it would be if we could take specs at face value and not with a grain of salt. But anyways, thanks for your response.

Although, if you can be specific, how is Powersoft able to make this claim? Are they saying something along the lines of "it takes X amount of RMS current to make Y amount of peak current" without specifying? Otherwise I just cant understand how they're able to say that their amps are 120% efficient, when it's an obvious lie. It takes power to make power, simple as that.
Logged

Ivan Beaver

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 8867
  • Atlanta GA
Re: Why do Class D amps seem to pull less power than they make?
« Reply #16 on: January 24, 2015, 08:51:15 am »

120V/20A is about 2400W. Two sockets makes 4800W. The PLX amp delivers 2200W total in 4ohm clip. If your music is a pure sine wave a clipping amp will actually use half the power you thing because a sine's peak is 3dB higher than its avarage power. So your 4 ohm dual channel amp will not draw 2200W from the wall but 1100. But music is not a pure sine wave(mostly), so your actual power usage will be even less if the clipping occours at peaks that are 10-15dB above the avarage power, that's 10-30 times less continuous power than you will think.
You can't change "scales" in the middle of talking about things that are compared.

The wall power is measured in RMS voltage-like the output power.

You can't talk about the "peak" of one waveform and the RMS of another when trying to make a point.

The voltage is the measurement of a particular point on a waveform-the most common being: Average=63.6% of the peak, RMS=70.7% of the peak and of course the peak voltage which is 100%

The power is the area "under the curve".

Of course you can "change scales"-but it does not make sense and only serves to confuse others.

Which is the idea of course with a lot of audio specs--------------
Logged
A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Ivan Beaver

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 8867
  • Atlanta GA
Re: Why do Class D amps seem to pull less power than they make?
« Reply #17 on: January 24, 2015, 08:57:10 am »


Although, if you can be specific, how is Powersoft able to make this claim? Are they saying something along the lines of "it takes X amount of RMS current to make Y amount of peak current" without specifying? Otherwise I just cant understand how they're able to say that their amps are 120% efficient, when it's an obvious lie. It takes power to make power, simple as that.
Modern amps can provide the power as speced FOR ( as I stated earlier) FOR VERY BRIEF periods of time.

THere is no "lying" going on-they are not talking about continuous power-but rather peaks.

And if you think this is bad-take any loudspeaker and look at the "output SPL" specs and see if you can actually measure that SPL with a normal SPL meter with music.

You will not be anywhere near close to that.  But it is how the "average SPL meter" is reading (time wise) vs what the speaker can produce on peaks.

When you "change scales" all sorts of things don't "make sense".
Logged
A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Alex Berry

  • Newbie
  • *
  • Offline Offline
  • Posts: 41
Re: Why do Class D amps seem to pull less power than they make?
« Reply #18 on: January 24, 2015, 09:08:56 am »

Modern amps can provide the power as speced FOR ( as I stated earlier) FOR VERY BRIEF periods of time.

THere is no "lying" going on-they are not talking about continuous power-but rather peaks.

And if you think this is bad-take any loudspeaker and look at the "output SPL" specs and see if you can actually measure that SPL with a normal SPL meter with music.

You will not be anywhere near close to that.  But it is how the "average SPL meter" is reading (time wise) vs what the speaker can produce on peaks.

When you "change scales" all sorts of things don't "make sense".

I completely agree about the loudspeaker analogy. I've actually done this before. EV claims to have measured the ETX-18SP to output 135db peak with broadband pink noise, whereas I measured around a 130db peak at 50hz.

Anyways, I didn't mean to sound like I had anything against Powersoft or their products, I'm just still confused how they can make a claim that their amps make more power than they consume. If they are indeed referring to peaks in the rating, it still doesn't make any sense that the amp could pull a very very brief peak of 1250 watts, and then somehow turn that brief peak of 1250 watts into a brief peak of 1500 watts and send it to the speaker. I am basing this all off of their specs, which dictate that at 1/8 max output power at 4 ohms (max output power at that resistance being 12000 watts, 1/8 being 1500 watts) the amp is only pulling 1250 watts. At 1/4 max output power at 4 ohms its rated to pull 2500 watts, with 1/4 max output power at that resistance being 3000 watts. Hopefully you can point out my error in this and I can learn from it and stop asking these stupid questions, lol!
Logged

Jelmer de Jong

  • Full Member
  • ***
  • Offline Offline
  • Posts: 168
Re: Why do Class D amps seem to pull less power than they make?
« Reply #19 on: January 24, 2015, 10:19:33 am »

You can't change "scales" in the middle of talking about things that are compared.
You're right. My physics teacher will slap me if he sees this.  :-[
Quote
Of course you can "change scales"-but it does not make sense and only serves to confuse others.

Which is the idea of course with a lot of audio specs--------------
This sucks big time. Two amps with built in processing are limited to 1200W@8ohm, both have a RMS and separate peak limiter. The first amp needs 98V entered in the peak limiter to deliver 1200W, the second needs 138V according to manufacturer software. With these settings both amps deliver the same power. None of the manufacturers talk about Vpk or VRMS, just V. Maybe thats where my thoughts went south... :(
Logged

ProSoundWeb Community

Re: Why do Class D amps seem to pull less power than they make?
« Reply #19 on: January 24, 2015, 10:19:33 am »


Pages: 1 [2] 3 4 ... 7   Go Up
 



Page created in 0.103 seconds with 24 queries.