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Author Topic: Why do Class D amps seem to pull less power than they make?  (Read 20638 times)

Alex Berry

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Why do Class D amps seem to pull less power than they make?
« on: January 23, 2015, 08:44:20 pm »

So first, if this is in the wrong place or has already been answered, I apologize. I've been reading some stuff on these forums recently but am a brand new member. Anyways, on with my question.

Exactly that which the title says. Why do Class D amps seem to pull less power than they make? For instance, a Powersoft K10 is rated to pull 1250 watts from the wall in order to make 1500 watts. Whereas a Class H amp, like a QSC PLX3402 takes around 1400 watts to make around 400 watts. I understand this is simply due to Class H not being nearly as efficient as Class D, but here's where I start to get even more confused.

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?

I'm hoping I can get an in depth answer on this as I love learning about this subject, and for a long time it has confused the crap out of me. Thanks in advance for any replies!

Alex
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #1 on: January 23, 2015, 09:02:07 pm »

So first, if this is in the wrong place or has already been answered, I apologize. I've been reading some stuff on these forums recently but am a brand new member. Anyways, on with my question.

Exactly that which the title says. Why do Class D amps seem to pull less power than they make? For instance, a Powersoft K10 is rated to pull 1250 watts from the wall in order to make 1500 watts. Whereas a Class H amp, like a QSC PLX3402 takes around 1400 watts to make around 400 watts. I understand this is simply due to Class H not being nearly as efficient as Class D, but here's where I start to get even more confused.

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?

I'm hoping I can get an in depth answer on this as I love learning about this subject, and for a long time it has confused the crap out of me. Thanks in advance for any replies!

Alex
THEY DON'T

They will ALWAYS pull more power than they can put out.

The reason is that they can't deliver full power for any length of time.

Typically you get full power for around 0.080seconds before dropping the power down-sometimes as much as 6dB or 1/4 of the peak or burst power.

Amps of years past could deliver their rated power for very long periods of time (an eternity in term of todays amps)

However music is of a burst nature anyway-so there is no need to deliver constant power.

Many years ago one of the "audio gods" said "What we need is an amp that can produce 100 watts continuous and reproduce 20dB peaks".

We are almost getting to that stage -20dB over 100 watts is 10,000 watts.

HOWEVER that can be an issue with some forms of music (EDM-rap-Dance etc) that have sustained tones that are MUCH longer than the normal 0.08 seconds burst tones.

Some manufacturers can only produce full power for 0.02 seconds (20ms)  That is ONE CYCLE at 20Hz.  Much music has tones that are longer than that.

If the amps could deliver more power than they pulled-then the power companies would be all over them :)
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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #2 on: January 23, 2015, 09:07:06 pm »

THEY DON'T

They will ALWAYS pull more power than they can put out.

The reason is that they can't deliver full power for any length of time.

Typically you get full power for around 0.080seconds before dropping the power down-sometimes as much as 6dB or 1/4 of the peak or burst power.

Amps of years past could deliver their rated power for very long periods of time (an eternity in term of todays amps)

However music is of a burst nature anyway-so there is no need to deliver constant power.

Many years ago one of the "audio gods" said "What we need is an amp that can produce 100 watts continuous and reproduce 20dB peaks".

We are almost getting to that stage -20dB over 100 watts is 10,000 watts.

HOWEVER that can be an issue with some forms of music (EDM-rap-Dance etc) that have sustained tones that are MUCH longer than the normal 0.08 seconds burst tones.

Some manufacturers can only produce full power for 0.02 seconds (20ms)  That is ONE CYCLE at 20Hz.  Much music has tones that are longer than that.

If the amps could deliver more power than they pulled-then the power companies would be all over them :)

Thanks for the reply! I kind of figured, these sort of specs listed by the manufacturer just confused me. I should have specified what music was and will be played. 95% of it will be rap/EDM, the the big, long sustained bass notes. So I guess what I'm asking now is, does this fact change anything? And will I be safe running the Powersoft amps at a total of 4000wrms output combined? I ran those QSC amps harder than that and the Powersoft amps should be much more efficient.
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Tim McCulloch

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #3 on: January 23, 2015, 09:11:34 pm »

Thanks for the reply! I kind of figured, these sort of specs listed by the manufacturer just confused me. I should have specified what music was and will be played. 95% of it will be rap/EDM, the the big, long sustained bass notes. So I guess what I'm asking now is, does this fact change anything? And will I be safe running the Powersoft amps at a total of 4000wrms output combined? I ran those QSC amps harder than that and the Powersoft amps should be much more efficient.

Hook it up at the shop on a pair of 20's  and see what happens.
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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #4 on: January 23, 2015, 09:15:54 pm »

Hook it up at the shop on a pair of 20's  and see what happens.

Unfortunately, this isn't an option as I'm renting the gear and do not own it. In theory though, I should be fine. At this point I'm basically just looking for someone with more experience than me to tell me whether I'm right or I'm wrong. Knowing how those QSC amps worked on two 20A's would be the cherry on top.
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #5 on: January 23, 2015, 09:31:35 pm »

Unfortunately, this isn't an option as I'm renting the gear and do not own it. In theory though, I should be fine. At this point I'm basically just looking for someone with more experience than me to tell me whether I'm right or I'm wrong. Knowing how those QSC amps worked on two 20A's would be the cherry on top.
Since music is DYNAMIC- it depends on how lour for how long with music of a certain dynamic range.

One style of music will run just fine without tripping a breaker-and a different style of music will trip them easily.

It is POWER OVER TIME-not just power or peak power.

It is a VERY COMPLICATED set of parameters and there is no way of knowing some exact answer.

It ALSO depends on the type of breakers (some can handle peaks for longer times than others), how many times the breaker has been tripped (the more they are tripped the less they can handle) and there is a normal tolerance for the current rating.

The only way to give an exact answer is to know WHAT SONG (you can't play a different song because of a different dynamic range) , how hot the speakers are (if they are into power compression there will be less power delivered by the amp-even though the voltage will be the same) and so forth and so on

The BEST thing is to try to oversize the breakers and the system and run it all "easy".  But that is A LOT easier said than done-believe me!
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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #6 on: January 23, 2015, 09:42:00 pm »

Since music is DYNAMIC- it depends on how lour for how long with music of a certain dynamic range.

One style of music will run just fine without tripping a breaker-and a different style of music will trip them easily.

It is POWER OVER TIME-not just power or peak power.

It is a VERY COMPLICATED set of parameters and there is no way of knowing some exact answer.

It ALSO depends on the type of breakers (some can handle peaks for longer times than others), how many times the breaker has been tripped (the more they are tripped the less they can handle) and there is a normal tolerance for the current rating.

The only way to give an exact answer is to know WHAT SONG (you can't play a different song because of a different dynamic range) , how hot the speakers are (if they are into power compression there will be less power delivered by the amp-even though the voltage will be the same) and so forth and so on

The BEST thing is to try to oversize the breakers and the system and run it all "easy".  But that is A LOT easier said than done-believe me!

More than 90% of the music played on the QSC amps was highly compressed EDM and rap and there were 0 problems over 3 hours of continuous music. I don't think the subs were in power compression as they were being run slightly beneath their continuous rating. None of the circuits have ever been tripped while I was using them, and the circuits are located in a gymnasium that's only a couple years old.  Also, I actually am totally able to oversize the system and run it easy. All of the eight Sound Bridge subs I'm planning on running at 1/4 continuous power. Although this time the system is gonna be run for 12 hours of almost continuous music. Thanks again for all the info! Would you say I have a decent chance of being fine or should I be concerned?
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #7 on: January 23, 2015, 09:46:45 pm »

More than 90% of the music played on the QSC amps was highly compressed EDM and rap and there were 0 problems over 3 hours of continuous music. I don't think the subs were in power compression as they were being run slightly beneath their continuous rating. None of the circuits have ever been tripped while I was using them, and the circuits are located in a gymnasium that's only a couple years old.  Also, I actually am totally able to oversize the system and run it easy. All of the eight Sound Bridge subs I'm planning on running at 1/4 continuous power. Although this time the system is gonna be run for 12 hours of almost continuous music. Thanks again for all the info! Would you say I have a decent chance of being fine or should I be concerned?
There is no way of saying.

When you say " I'm planning on running at 1/4 continuous power." what does that exactly mean?  How are you "planning" on doing that?

If the breakers blow-then turn it down!
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Ivan Beaver
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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #8 on: January 23, 2015, 09:56:07 pm »

There is no way of saying.

When you say " I'm planning on running at 1/4 continuous power." what does that exactly mean?  How are you "planning" on doing that?

If the breakers blow-then turn it down!

It means I'm planning on running each amp to only output 2000 watts. According to the Powersoft K10 documentation, the amps have a "User selectable maximum output power for each channel" feature and I plan on using it. The subs are rated for 2000wrms so sending around 500 to each box should be at a quarter of the potential continuous power. Sorry for not being clear enough about that.
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Lee Buckalew

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #9 on: January 23, 2015, 09:58:28 pm »

It means I'm planning on running each amp to only output 2000 watts. According to the Powersoft K10 documentation, the amps have a "User selectable maximum output power for each channel" feature and I plan on using it. The subs are rated for 2000wrms so sending around 500 to each box should be at a quarter of the potential continuous power. Sorry for not being clear enough about that.

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

Lee
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Lee Buckalew
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Re: Why do Class D amps seem to pull less power than they make?
« Reply #9 on: January 23, 2015, 09:58:28 pm »


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