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Author Topic: Amp + Sub performance  (Read 5232 times)

Raj Sookraj

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Amp + Sub performance
« on: February 11, 2007, 07:08:47 pm »

Is it harder for an amp to produce 40hz or 100hz, or is there no difference at all?
Does 40hz or 100hz generate more heat in a subwoofer, or no difference?
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Ivan Beaver

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Re: Amp + Sub performance
« Reply #1 on: February 11, 2007, 07:21:01 pm »

The freq does not matter in an amplifier-at least in the normal audio band.

In a loudspeaker, there are several things to consider.

1: The impedance of the loudspeaker at the particular freq of interest.  Let's assume a flat amplitude signal being applied to the input of the amplifier.  That will produce a equal output voltage across the freq of interest.  The freq that has the lower impedance will be producing more heat and conversely the one with the higher impedance will be producing less heat.

2: How is the cabinet tuned?  if the loudspeaker moves a different amount at different freq (as they all do) there will less heating going on at freq that cause the loudspeaker to move more.  Now this is assuming that the impedance is equal (which it is not) at different freq.  This less heat is due to the cooling effect of the movement.

Now this excursion which helps reduce the heat puts an additional strain on the loudspeaker physically.

As in all things audio, it depends on many different factors, many of which interact with others, so there is no easy answer.
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Raj Sookraj

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Re: Amp + Sub performance
« Reply #2 on: February 11, 2007, 07:55:42 pm »

Ivan Beaver wrote on Mon, 12 February 2007 00:21

The freq that has the lower impedance will be producing more heat and conversely the one with the higher impedance will be producing less heat.

Is that because the lower impedance freq is drawing more power from the amp?

BTW, thanks for answering all of my questions.
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Andy Peters

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Re: Amp + Sub performance
« Reply #3 on: February 11, 2007, 09:01:10 pm »

raj sookraj wrote on Sun, 11 February 2007 17:55

Ivan Beaver wrote on Mon, 12 February 2007 00:21

The freq that has the lower impedance will be producing more heat and conversely the one with the higher impedance will be producing less heat.

Is that because the lower impedance freq is drawing more power from the amp?


Sorta; you don't "Draw power."  The lower the impedance, the more current the amp sources into the load.  More current, more power, assuming the voltage doesn't sag, too.

-a
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Ivan Beaver

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Re: Amp + Sub performance
« Reply #4 on: February 11, 2007, 09:21:26 pm »

As Andy said. Basic ohms law.  Power is the end result of a particular voltage producing a particular current through a particualr load.

Kinda of like Miles per gallon.  You don't produce it, but it is the result of a particular amount of fuel used to propel a particular load (vehicle) over a specific distance.
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Pascal Pincosy

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Re: Amp + Sub performance
« Reply #5 on: February 12, 2007, 08:10:32 pm »

If you take a look at Bink's amplifier shootout,

  http://srforums.prosoundweb.com/index.php/m/102841/9721/?src h=amplifier+shootout#msg_102841

you'll see that many amps are not capable of putting out their rated power at the extremes of their bandwidth. For example, a Crest 8001 will put out only 800watts @ 20Hz (rated for 1200 watts), and a Lab Gruppen fp6400 will only put out 1327watts @ 20 Hz (rated for 2200watts!) On the other hand, a Crest 8002 will put out rated power @ 20Hz, as will a QSC PL 9.0.

The shootout shows clearly that many amplifiers will have a different max output depending on frequency. The 8001 will put out 1728watts (728 over rated output) and an fp6400 will only put out 648watts (1152 below rated output) @ 20Hz. And a QSC PL9.0 will put out a massive 4804watts @ 20Hz while they are rated for 3400 watts.

In theory an amp should put out the same power throughout its power band. Reality is a different story.
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Michael 'Bink' Knowles

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Re: Amp + Sub performance
« Reply #6 on: February 12, 2007, 08:25:33 pm »

Pascal Pincosy wrote on Mon, 12 February 2007 17:10

If you take a look at Bink's amplifier shootout,

   http://srforums.prosoundweb.com/index.php/m/102841/9721/?src h=amplifier+shootout#msg_102841

you'll see that many amps are not capable of putting out their rated power at the extremes of their bandwidth. For example, a Crest 8001 will put out only 800watts @ 20Hz (rated for 1200 watts), and a Lab Gruppen fp6400 will only put out 1327watts @ 20 Hz (rated for 2200watts!) On the other hand, a Crest 8002 will put out rated power @ 20Hz, as will a QSC PL 9.0.

The shootout shows clearly that many amplifiers will have a different max output depending on frequency. The 8001 will put out 1728watts (728 over rated output) and an fp6400 will only put out 648watts (1152 below rated output) @ 20Hz. And a QSC PL9.0 will put out a massive 4804watts @ 20Hz while they are rated for 3400 watts.

In theory an amp should put out the same power throughout its power band. Reality is a different story.


Yeah, like he said.   Cool   Good points, Pascal. If I had had a whole week for that Shootout I could have delved deeper into the more interesting subwoofer performance aspects of amplifiers such as low impedance protection, brownout AC voltages, which amp was the most efficient with wall power, 30Hz performance, 40Hz performance, etc. Oh well! If I ever hit the Powerball jackpot I'll just test shit all day long and issue brutally honest opinions from the protection of my little cave.  Twisted Evil

-Bink
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John Roberts {JR}

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Re: Amp + Sub performance
« Reply #7 on: February 12, 2007, 08:59:36 pm »

raj sookraj wrote on Sun, 11 February 2007 18:08

Is it harder for an amp to produce 40hz or 100hz, or is there no difference at all?
Does 40hz or 100hz generate more heat in a subwoofer, or no difference?


Since nobody has answered your question wrt the amp I'll try. In power amps with conventional power supplies yes, it is harder at very low frequency to put out max power.

The reason is related to how the mains power charges up the power supply capacitors, and how a sinewave signal draws power from those power supplies.

To make this easier to visualize I would like to ignore your 100 Hz choice of a higher frequency and substitute 200 Hz. You'll see why in a minute.

The mains power AC waveform typically charges the power supply reservoir capacitor(s) at either 1x or 2x the mains frequency depending upon specific PS configuration.

Output signals that are much higher frequency than the PS charging refresh rate pull power half the time (positive half cycle) from the plus supply, and the other half the time (negative half cycle) pull from the minus supply.

On the other hand, signal frequencies that are lower frequency than the mains power supply charging refresh rate will pull from one supply near continuously for one or more full charging cycles before alternating to the opposite supply for a similar number of charging cycles.

What this means is that the effective power that must be supplied by a given PS rail, per charging cycle will be approximately twice as much for frequencies much lower than charging rate than for frequencies higher than charging rate.

OK, here is how it becomes a cost issue. The power supply's ability to deliver power between charging cycles is a simple function of how much capacitance is present. It take a lot more capacitance to deliver the same output power at 20 Hz as 1k.

Note: amps like the PFC with regulated rails will hang in easily. I would expect value amps to sag more at LF than their premium brothers and sisters.

JR    
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Mike {AB} Butler

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Re: Amp + Sub performance
« Reply #8 on: February 13, 2007, 07:57:18 am »

Well said, John.
I would add that in nearly all "linear" supply amplifiers, the cost of the transformer is a huge component as well. Transformers that can drop a lot of current require more iron (Core) and thicker gauge wire on both primary and secondary (read larger size gauge = even larger transformer = more expensive as well). In most designs, there is a balance between max current, max voltage, size, and resultant cost.
Regards,
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John Roberts {JR}

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Re: Amp + Sub performance
« Reply #9 on: February 13, 2007, 10:29:38 am »

Mike {AB} Butler wrote on Tue, 13 February 2007 06:57

Well said, John.
I would add that in nearly all "linear" supply amplifiers, the cost of the transformer is a huge component as well. Transformers that can drop a lot of current require more iron (Core) and thicker gauge wire on both primary and secondary (read larger size gauge = even larger transformer = more expensive as well). In most designs, there is a balance between max current, max voltage, size, and resultant cost.
Regards,


True, but for the output frequency specific power drop off mechanism it isn't because of transformer iron but simple reservoir capacitance. The amps put out full power above a couple hundred Hz.

There may be a secondary music content mechanism that impacts amp design decisions related to the higher typical LF energy than HF, but that is not the topic today.

JR
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