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Author Topic: Why 2 different rating for amp specs??  (Read 1743 times)

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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A young child says to his mother, "Mom, when I grow up I'm going to be a musician." She replies, "Well honey, you know you can't do both."

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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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

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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Ivan Beaver
Danley Sound Labs

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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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A young child says to his mother, "Mom, when I grow up I'm going to be a musician." She replies, "Well honey, you know you can't do both."

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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Tune it or don't play it... please
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