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Author Topic: Why 2 different rating for amp specs??  (Read 7387 times)

Debbie Dunkley

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Why 2 different rating for amp specs??
« on: January 17, 2013, 11:46:58 AM »

I was wondering if someone could help me out with this question....
When i check out the specs on my amp for running at 8ohm bridge, it has 2 listed sets of figures. first is 1300 watts @ 8 ohms, 20 hz- 20 khz, 0.1% THD and the second shows 1500 watts @ 8 ohms, 1 khz, 0.1% THD. How do I know which rating is correct for my application - 1300w or 1500 watts???

Thank you in advance............
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Scott Harris

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Re: Why 2 different rating for amp specs??
« Reply #1 on: January 17, 2013, 03:56:21 PM »

What is the amp & model?  I wouldn't trust the spec too much unless I knew the brand name.  Many ratings are stretched to meet marketing goals.  (I will take a guess at this being a qsc rmx2450 in which case the rating is believable.)

The difference between the 2 ratings you have listed are related to the bandwidth.  It is telling you that it has 1,300 watts across the full audible spectrum while it can reach 1,500 watts at 1,000 Hz which is a marketing number.  (If you are using this to power a sub you care about the 10-10Khz rating and the 1khz number is useless as you want the power output from 20-200Hz or where ever you plan to use it.)  The good thing about what your spec is the 0.1% THD portion which means it is measured at no audible distortion.  The difference from 1,300 watts to 1,500 watts is irrelevant in the real world.
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John Roberts {JR}

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Re: Why 2 different rating for amp specs??
« Reply #2 on: January 17, 2013, 04:03:00 PM »

I was wondering if someone could help me out with this question....
When i check out the specs on my amp for running at 8ohm bridge, it has 2 listed sets of figures. first is 1300 watts @ 8 ohms, 20 hz- 20 khz, 0.1% THD and the second shows 1500 watts @ 8 ohms, 1 khz, 0.1% THD. How do I know which rating is correct for my application - 1300w or 1500 watts???

Thank you in advance............
Are you operating the amp at 1kHz only or 20-20kHz... ?

This is a common question that has been answered many time before, with more detail.

JR
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #3 on: January 17, 2013, 04:08:45 PM »

What is the amp & model?  I wouldn't trust the spec too much unless I knew the brand name.  Many ratings are stretched to meet marketing goals.  (I will take a guess at this being a qsc rmx2450 in which case the rating is believable.)

The difference between the 2 ratings you have listed are related to the bandwidth.  It is telling you that it has 1,300 watts across the full audible spectrum while it can reach 1,500 watts at 1,000 Hz which is a marketing number.  (If you are using this to power a sub you care about the 10-10Khz rating and the 1khz number is useless as you want the power output from 20-200Hz or where ever you plan to use it.)  The good thing about what your spec is the 0.1% THD portion which means it is measured at no audible distortion.  The difference from 1,300 watts to 1,500 watts is irrelevant in the real world.
Thank you for the response Scott.......this is more of a question for me to learn something. The amp in question is an older Behringer EP2500 that I got used cheap. I know Behringer is well known to inflate their numbers. It is my back up amp - all the amps in my rack are Crowns. It was simply something I had come across before but did not know the difference and it seemed a good time to ask. If I use it, it will be for stage monitors OR if one of my Crowns goes down and I will drop in the Behringer to use bridged at 8 ohms.
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #4 on: January 17, 2013, 04:10:40 PM »

Are you operating the amp at 1kHz only or 20-20kHz... ?

This is a common question that has been answered many time before, with more detail.

JR
Hi John...this is just a back-up amp which will only get used if one of my other amps goes down so it could be for sub, mid or horn use.
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David Parker

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Re: Why 2 different rating for amp specs??
« Reply #5 on: January 17, 2013, 04:14:26 PM »

I was wondering if someone could help me out with this question....
When i check out the specs on my amp for running at 8ohm bridge, it has 2 listed sets of figures. first is 1300 watts @ 8 ohms, 20 hz- 20 khz, 0.1% THD and the second shows 1500 watts @ 8 ohms, 1 khz, 0.1% THD. How do I know which rating is correct for my application - 1300w or 1500 watts???

Thank you in advance............

here's the big deal. compare apples to apples. If you are comparing two amps, make sure the specs are the same. If the specs are not identified, they are useless, especially some cheap amps that give "peak" power. Another thing, you have to double the watts to get 3db increase in volume, which is generally considered to be the least increase that matters. So if one amp is 10 or 20 percent less than another, that usually is not a meaningful difference. I'd suggest buying only amps from reputable manufacturers, such as crown, QSC, peavey, and then there are others that are a lot more expensive. Many have had good luck with behringer amps, but you have to watch their specs, they use their own ratings, and they make their amps look a lot more powerful than they are.
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Don Boomer

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Re: Why 2 different rating for amp specs??
« Reply #6 on: January 17, 2013, 05:11:41 PM »

How do I know which rating is correct for my application - 1300w or 1500 watts???


Not that you asked ... but even though 200W might seem like a substantial difference, the two numbers are only about .6 dB different.
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Don Boomer
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #7 on: January 17, 2013, 09:05:03 PM »

Not that you asked ... but even though 200W might seem like a substantial difference, the two numbers are only about .6 dB different.

Hi Don...yes it's really not the extra power that I am interested in as much as the difference between the 2 stated ratings but thanks for reminding me that the 200w difference would be inaudible in real terms....
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Tim Perry

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Re: Why 2 different rating for amp specs??
« Reply #8 on: January 19, 2013, 01:00:22 AM »

The1 kHz spec is easiest for a service tech to use in testing. you can simply calculate the peak-to-peak expected voltage, operate channel into a nonreactive known load measure with an oscilloscope (and maybe a distortion meter)

As most music does not consist of continuous single frequency sine waves, the marketing department insists on a variety of confusing and sometimes fraudulent ratings.

As amplifiers have grown so large these days, obtaining a non-reactive load has become a non trivial matter. (think if you had to service equipment all day in the summer with a several thousand watt space heater nearby with a loud cooling blower running)
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John Roberts {JR}

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Re: Why 2 different rating for amp specs??
« Reply #9 on: January 19, 2013, 10:37:25 AM »

Hi John...this is just a back-up amp which will only get used if one of my other amps goes down so it could be for sub, mid or horn use.

As Boomer pointed out that is less than 1dB difference between those two power levels so no big yup.

To give you more information than you need, amplifiers require much larger capacitors in the power supply to make the same power at 20 Hz as 1 kHz, because they are only getting topped off at 2x the mains frequency rate. 

Audio signals that are higher frequency than the rate that the PS caps are getting topped off, pull roughly half power from each the + and - rail, because they are AC signal. Very low frequency audio, stays high or low for multiple recharging cycles, so draw 2x the current of the HF signal while mostly high or mostly low.

This only makes a very small difference at very low frequency. There are much more important things to worry about. Amps are pretty mature technology and well sorted.

JR
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #10 on: January 19, 2013, 11:30:58 AM »

As Boomer pointed out that is less than 1dB difference between those two power levels so no big yup.

To give you more information than you need, amplifiers require much larger capacitors in the power supply to make the same power at 20 Hz as 1 kHz, because they are only getting topped off at 2x the mains frequency rate. 

Audio signals that are higher frequency than the rate that the PS caps are getting topped off, pull roughly half power from each the + and - rail, because they are AC signal. Very low frequency audio, stays high or low for multiple recharging cycles, so draw 2x the current of the HF signal while mostly high or mostly low.

This only makes a very small difference at very low frequency. There are much more important things to worry about. Amps are pretty mature technology and well sorted.

JR

So to a less knowledgeable person like myself, this all means that my srx718 subs will draw more power from the amp than my srx715's will   even though the speaker power ratings are the same because of the lower frequencies?
I had read that subs require more and it is not advisable to run an amp bridged at 4 ohms unless you can help it so I have been using 2 x Crown XLS2000 - one each side (bridged 8ohms) for the subs,  a XTI4002 (2 channel) for the mids and a Carvin HT760 (2 channel) for the horns.
I use a DRPA and have it  set to the appropriate JBL tunings. It does sound good. I recently used it outside at a huge neighborhood BBQ for a couple of bands and I had adequate  headroom.
I know I said I purchased the Behringer as back up but I was wondering whether there would be any benefit (as I now own it anyway) to adding it to the existing system for more head room. Maybe run the XTI4002 8ohm bridged one side and the Behringer 8ohm bridged the other side (for the subs or the mids) - The Behringer has a lower output than the Crown XTI but the XTI is a bit too much anyway so adjust to suit??- At 8 ohms bridge-Behringer 1300/1500 w, Crown 2400w.
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Ivan Beaver

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Re: Why 2 different rating for amp specs??
« Reply #11 on: January 19, 2013, 12:10:59 PM »

So to a less knowledgeable person like myself, this all means that my srx718 subs will draw more power from the amp than my srx715's will   even though the speaker power ratings are the same because of the lower frequencies?

Speakers don't "draw" power.  They just present a load (that is NOT the rated impedance)  It may be at some freq-but as a general rule-the ACTUAL load is both higher and lower than the speced load-depending on what freq you look at.

The lower freq response of the driver usually has nothing to do with the power it "draws".  The only way to tell is to look at the IMPEDANCE CURVE of the loudspeaker.  They are not the same.  You will see all kinds of peaks and valleys in the curve.  This is an indication of the tuning of the cabinet.  So it is very freq specific as to what the impedance is.

As a "general rule", given 2 bass reflex cabinets and one goes lower than the other-at the "lower freq" (you have to decide what is "lower"), the cabinet with the lower tuning will have a higher impedance.  This is a function of the tuning.  But at a higher freq-the one with the lower tuning will have a lower impedance than the one with the higher tuning-because the one with the higher tuning will have one of its peaks at a higher freq.

The amp does not "deliver" wattage.  It delivers VOLTAGE.  The actual wattage is a combination of the voltage applied and the complex impedance (NOT RESISTANCE) of the loudspeaker.

How much the loudspeaker can "handle" is a combination of the voltage that can be applied and the impedance of the loudspeaker.

NOTE: This impedance changes with input voltage-once the loudspeaker gets hot.  This is called power compression-and the actual impedance rises as the voice coil gets hot.

The "wattage" as presented on the spec sheet (from all manufacturers I suspect) is also wrong.  It is NOT the point of maximum input in terms of watts-even though it says so.

It is the VOLTAGE that can be applied to the driver without damage-and the rated impedance.  Since the impedance rises when the driver gets hot-the wattage can actually go DOWN.

But the thing to consider is-what is the number trying to tell you?  It is a suggestion of the size of the amplifier than can be used with a particular loudspeaker.

I know to some people it doesn't matter-but it is important to understand what is really going on.

Take a look at the following example-it is a single 12"sub (bottom) and a single 18" sub (top).  The 18" sub is tuned much lower.  So let's look at just 2 freq  11Hz and 26Hz.

At 11Hz the 18 sub has a much higher impedance than the 12", and at 26Hz, the 12" has a much higher impedance than the 18". So if you say that the one that goes lower "draws more"- what is low?

The question that HAS to be asked when anybody presents a "spec" is AT WHAT FREQ?  This involves all sorts of topics-coverage pattern-power capacity input and output)-impedance-SPL-noise.

A simple answer is GOING to be wrong-at at best-incomplete.

You cannot use simple numbers to describe something in a dynamic environment-such as audio.  With a light bulb on a standard line it is MUCH easier-the voltage remains the same-the freq remains the same-the heating remains pretty constant and so forth.  So it is real easy to come up with simple numbers that describe it.

But when all sorts of variables are "moving around" (as in audio) it becomes much harder.  And if you want to "pinpoint" a specific number-you HAVE to put a lot of variables around it to qualify the answer.

Even something as what would seem simple SPL.  How loud is it?  What scale is the meter using?  (that can account for a 20-30 dB difference.  What is the speed of the measurement-slow-fast-impulse etc?  that can account for possibly another 10-20dB.

So you could use the same meter-at the same distance-measuring the same source and get a 30-40dB difference in level.  So how loud was it?  It depends-on the parameters you set up in the measurement.

Sorry to rant-but "simple numbers" get me going.
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #12 on: January 21, 2013, 11:55:59 PM »

Speakers don't "draw" power.  They just present a load (that is NOT the rated impedance)  It may be at some freq-but as a general rule-the ACTUAL load is both higher and lower than the speced load-depending on what freq you look at.

The lower freq response of the driver usually has nothing to do with the power it "draws".  The only way to tell is to look at the IMPEDANCE CURVE of the loudspeaker.  They are not the same.  You will see all kinds of peaks and valleys in the curve.  This is an indication of the tuning of the cabinet.  So it is very freq specific as to what the impedance is.

As a "general rule", given 2 bass reflex cabinets and one goes lower than the other-at the "lower freq" (you have to decide what is "lower"), the cabinet with the lower tuning will have a higher impedance.  This is a function of the tuning.  But at a higher freq-the one with the lower tuning will have a lower impedance than the one with the higher tuning-because the one with the higher tuning will have one of its peaks at a higher freq.

The amp does not "deliver" wattage.  It delivers VOLTAGE.  The actual wattage is a combination of the voltage applied and the complex impedance (NOT RESISTANCE) of the loudspeaker.

How much the loudspeaker can "handle" is a combination of the voltage that can be applied and the impedance of the loudspeaker.

NOTE: This impedance changes with input voltage-once the loudspeaker gets hot.  This is called power compression-and the actual impedance rises as the voice coil gets hot.

The "wattage" as presented on the spec sheet (from all manufacturers I suspect) is also wrong.  It is NOT the point of maximum input in terms of watts-even though it says so.

It is the VOLTAGE that can be applied to the driver without damage-and the rated impedance.  Since the impedance rises when the driver gets hot-the wattage can actually go DOWN.

But the thing to consider is-what is the number trying to tell you?  It is a suggestion of the size of the amplifier than can be used with a particular loudspeaker.

I know to some people it doesn't matter-but it is important to understand what is really going on.

Take a look at the following example-it is a single 12"sub (bottom) and a single 18" sub (top).  The 18" sub is tuned much lower.  So let's look at just 2 freq  11Hz and 26Hz.

At 11Hz the 18 sub has a much higher impedance than the 12", and at 26Hz, the 12" has a much higher impedance than the 18". So if you say that the one that goes lower "draws more"- what is low?

The question that HAS to be asked when anybody presents a "spec" is AT WHAT FREQ?  This involves all sorts of topics-coverage pattern-power capacity input and output)-impedance-SPL-noise.

A simple answer is GOING to be wrong-at at best-incomplete.

You cannot use simple numbers to describe something in a dynamic environment-such as audio.  With a light bulb on a standard line it is MUCH easier-the voltage remains the same-the freq remains the same-the heating remains pretty constant and so forth.  So it is real easy to come up with simple numbers that describe it.

But when all sorts of variables are "moving around" (as in audio) it becomes much harder.  And if you want to "pinpoint" a specific number-you HAVE to put a lot of variables around it to qualify the answer.

Even something as what would seem simple SPL.  How loud is it?  What scale is the meter using?  (that can account for a 20-30 dB difference.  What is the speed of the measurement-slow-fast-impulse etc?  that can account for possibly another 10-20dB.

So you could use the same meter-at the same distance-measuring the same source and get a 30-40dB difference in level.  So how loud was it?  It depends-on the parameters you set up in the measurement.

Sorry to rant-but "simple numbers" get me going.

I appreciate all the time everyone has put into educating me. I still struggle with a lot of these terms  but I'm getting there slowly. I'll just keep listening and learning!!!!
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Ivan Beaver

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Re: Why 2 different rating for amp specs??
« Reply #13 on: January 22, 2013, 07:34:44 AM »

I appreciate all the time everyone has put into educating me. I still struggle with a lot of these terms  but I'm getting there slowly. I'll just keep listening and learning!!!!
Don't worry-the more you learn-the more you realize how little you actually know and how much more there is to learn.

It like the deeper you dig-the more rock you hit.

It was sooooo much simplier years ago when I didn't know anything.  We just "did it".  But now I think a lot more about what I am doing.  Of course the end results are better :)
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #14 on: January 22, 2013, 09:20:26 AM »

Don't worry-the more you learn-the more you realize how little you actually know and how much more there is to learn.

It like the deeper you dig-the more rock you hit.

It was sooooo much simplier years ago when I didn't know anything.  We just "did it".  But now I think a lot more about what I am doing.  Of course the end results are better :)
That is EXACTLY it Beaver...it was simpler years ago and now I put much more thought into it all, it really is so much more rewarding!!!
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Debbie Dunkley

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Re: Why 2 different rating for amp specs??
« Reply #15 on: January 22, 2013, 09:22:02 AM »

That is EXACTLY it Beaver...it was simpler years ago and now I put much more thought into it all, it really is so much more rewarding!!!

Sorry...I meant Ivan - didn't mean to be rude and use your last name alone
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John Roberts {JR}

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Re: Why 2 different rating for amp specs??
« Reply #16 on: January 22, 2013, 11:49:55 AM »

So to a less knowledgeable person like myself, this all means that my srx718 subs will draw more power from the amp than my srx715's will   even though the speaker power ratings are the same because of the lower frequencies?
No not what I was saying at all. HF and LF use the same power over all (all other things equal), it's just that they use it slightly differently. Perhaps it will make more sense if I put numbers to it.

A 20 Hz sine wave will have a 50mSec period. It repeats every 50 mSec, spending 25 mSec swinging positive, then 25 mSec swinging negative. The Power supply charged by the 60 Hz (in US 50 Hz europe) mains power, gets full wave rectified and charged (2x) for each mains power cycle, or each 8.3mSec. (Full wave rectification convert both positive and negative mains voltage swings into charging pulses).

Both the + and - power supplies are getting charged every 8.3 mSec, but the 20 Hz audio signal is only pulling from one for 25 mSec, then the other for 25 mSec, then back to the first.  So to the power supply only one rail is getting all the current draw at a time. 

At higher audio frequency, like 1 kHz, the cycle repeats every 1 mSec, spending 1/2 mSec pulling from the + supply and 1/2 mSec pulling from the - supply.  To the power supply a 1 kHz sine wave looks like half the load pulling from both supplies at roughly the same time, while the 20 Hz sine wave looks like the full load drawing alternately from one supply then the other.

I do not want to make a bigger deal of this than it deserves. This is well known to amp designers and they could make an amp put out the same power for 20 Hz as 1 KHz by significantly oversizing the power supply capacitors. But generally it isn't worth the trouble. Also some modern amps use switching PS (that charge up at faster rate) or even power factor correction with regulation so this gets simpler or more complicated depending on your POV.

Try not to lose any sleep over this.

Quote
I had read that subs require more and it is not advisable to run an amp bridged at 4 ohms unless you can help it so I have been using 2 x Crown XLS2000 - one each side (bridged 8ohms) for the subs,  a XTI4002 (2 channel) for the mids and a Carvin HT760 (2 channel) for the horns.
LF music can involve longer sustained periods of loud output (Think bass lines, drum rolls, etc).

JR
Quote
I use a DRPA and have it  set to the appropriate JBL tunings. It does sound good. I recently used it outside at a huge neighborhood BBQ for a couple of bands and I had adequate  headroom.
I know I said I purchased the Behringer as back up but I was wondering whether there would be any benefit (as I now own it anyway) to adding it to the existing system for more head room. Maybe run the XTI4002 8ohm bridged one side and the Behringer 8ohm bridged the other side (for the subs or the mids) - The Behringer has a lower output than the Crown XTI but the XTI is a bit too much anyway so adjust to suit??- At 8 ohms bridge-Behringer 1300/1500 w, Crown 2400w.
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Re: Why 2 different rating for amp specs??
« Reply #16 on: January 22, 2013, 11:49:55 AM »


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