A couple of power questions (1 Viewer)

[GabrielC]

1) If you have speakers that take a max of 250W, does that mean that if you have a preout going to a 500W amp you could turn the amp to halfway and keep it there all the time? Or if you had a 250W amp you could turn it all the way up (assuming the signal doesn't start clipping I think)? For the sake of argument, let's just say the impedances are matching
2) The Paradigm site gives two numbers for the Studio 60's (among others):
Suitable Power Amplifier Range: 15-200W
Maximum Input Power: 150W
What does this mean for the kind of amp you can power it with? Does the amp have to be a maximum of 200W or 150W?

 

[ColinM]

That's just a guide to keep you from frying the speakers.

If the Para's are 90db efficient, at one meter you need 1 watt to get 90 db's (LOUD).

Then it goes like this - -

watts / DB

1 - 90

2 - 93

4 - 96

8 - 99

16 - 102 (Minimum rec.)

32 - 105

64 - 108

128 - 111

256 - 114 (Max rec)

So you see, the power needed increases dramatically, but in real world situations you may never need more than 15 - 30 watts.

Anything less than that and the amp will clip and fry the speaker, eventually or immediately.

 

[GabrielC]

Thanks Colin, a bit more...how many dB are lost each meter further from that 1 meter?

Anybody have an answer for the first question?

 

[ColinM]

-6db every time you double the distance.

 

[TimothyW]

Ideally, sound intensity follows the Inverse Square Law and will be 1/4 the intensity at twice the distance. This translates to a 6db decrease.
Here's a link if you're interested in this stuff:
http://hyperphysics.phy-astr.gsu.edu.../roomi.html#c1

 

[ColinM]

.....head...hurts....too...much....math....

 

[Phil Iturralde]

FYI: For a more precise & easy maximum SPL (loudness) @ your listening position (distance) vs. AMP power vs. speakers (number) vs. speaker sensitivity, use C. M. Collins 'excellent' SPL Calculator @ his website = How Loud Will it Go?
Phil

 

[GabrielC]

Cool calculator....thanks for the info guys! Knowing how things work is always interesting to me
Umm....anybody have an idea on the first question though?

 

[GabrielC]

Anybody want to try the first question?

1) If you have speakers that take a max of 250W, does that mean that if you have a preout going to a 500W amp you could turn the amp to halfway and keep it there all the time? Or if you had a 250W amp you could turn it all the way up (assuming the signal doesn't start clipping I think)? For the sake of argument, let's just say the impedances are matching

 

[Harold_C]

1) If you have speakers that take a max of 250W, does that mean that if you have a preout going to a 500W amp you could turn the amp to halfway and keep it there all the time? Or if you had a 250W amp you could turn it all the way up (assuming the signal doesn't start clipping I think)?

No. I don't believe that either assumption would be necessarily be valid.

Speaker power ratings are not that precise. The best case assumption is that the stated power handling for a speaker represents a safe range of amplifier sizes, given normal "non-abusive" operation. It's just a guide.

Beyond that, "halfway up" on the volume with a 500 watt amplifier probably doesn't result in a 250 watt output. Just like "all the way up" on the volume probably doesn't result in a 500 watt output. It depends on the signal level from the preamp and the sensitivity of the amplifier.

Let's say, for the sake of argument that a setting of -10 on your digital preamp produced a 500 watt output with a sine wave test tone. A setting 3dB lower (-13 dB) on the preamp would produce a 250 watt output. The full 500 watts would be a little louder than the 250 watt output, but not by a whole lot. Either of these tests would blow up virtually any speaker ever made within seconds -- and I mean "blow up" quite literally with smoke pouring from the drivers.

It's all rather academic since we very seldom listen to sine wave test tones, but rather dynamic recordings whose power demands vary instantaneously from one moment to the next. Typical listening would only require 20 to 30 watts on average. The reason you would use a 250 watt or 500 watt amplifier is to avoid any chance of clipping, even on the loudest instantaneous peaks.

 

[GabrielC]

Harold:

I think I sort of understand, thanks for the explanation... so if I follow that at all, does that mean that I *could* set the amp to a full 500W if the preamp signal was very low?

You also said:

Let's say, for the sake of argument that a setting of -10 on your digital preamp produced a 500 watt output with a sine wave test tone. A setting 3dB lower (-13 dB) on the preamp would produce a 250 watt output.

I know the SoA disclaimer, but does a digital preamp normally produce that much power? I thought they generally had very low signals and relied on an amp to boost them to those kind of outputs? (just asking to make sure)

 

[Harold_C]

I think I sort of understand, thanks for the explanation... so if I follow that at all, does that mean that I *could* set the amp to a full 500W if the preamp signal was very low?

I'm having trouble with that question because I don't know how you set an amplfier to "500 watts" except on a test bench driving a load resistor with an sine wave input. If your amp has gain controls for each channel, then you can set the input sensitivity of the amplfier, but the amount of power any particular sensitivity setting depends on the signal level coming from the preamp, the impedance of the speakers, and the content of the program material.

Perhaps it would be more useful to move the discussion out of the hypothetical and into to the real world.

None of my current amplifiers have input sensitivity adjustments (gain controls). In fact, I can't recall that I have have ever owned a home amplifier (Dynaco, Phase Linear, Hafler, Conrad Johnson, GAS, Audio Research, etc.) that did have sensitivity adjustments. So all of the amps I've ever owned were essentially set to run wide open.

Barring some special circumstances or the need to balance channels with the amplifiers, I can't really imagine the need to ever set a home amplifier to less than full sensitivity. Most preamps provide somewhere in the neighborhood of 1 V at the preamp outputs at full volume with a typical input signal (some a little more, some a little less). For optimum gain structure, you would be perfectly happy with an amp that clipped with a .3 to .5 volt signal. Most amps aren't that sensitive even with their gain controls cranked wide open -- most of them clip at somewhere around 1 volt. So my general recommendation would be to run any gain controls on the power amp wide open.

Now, I'm NOT saying that you can then peg the volume knob on the preamp. In fact, that would generally be pretty bad thing to do.

As a general rule, I think you you are less likely to blow up speakers with a larger amp than a smaller amp. However, I would hesitate to give you any assurances about your amp and speakers without knowing what they are.

One of the really nice things about calibrating a Dolby Digital home theater system with the Radio Shack SPL meter is that you can figure out exactly what volume level on the preamp corresponds to what peak SPL at your listening position. If you have an accurate 1w/1meter sensitivity spec for your speakers and the distance from the speakers to your chair, you can also make a pretty fair estimate of what average and peak amplifier power requirements will be for Dolby Digital recordings. For example, my system with an honest 80 watts per channel is probably capable of about 102 dB from a full range main channel with absolutely no clipping. This would be peak power of 80 watts per channel and average dialog in a movie would probably requiring something on the order of 1 or 2 watts continuous.

In reality, because I run the speakers as SMALL, because the amp has massive power supplies and no current limiting, and because Dolby soundtracks would very rarely have a full scale digital peak, my system will probably achieve Dolby reference levels with a minimal and likely inaudible amount of clipping.

Again because I run the speaker as SMALL, I have never heard any audible signs of distress from the speakers and I don't expect that I would be likely to blow them up, particularly since I find full Dolby reference levels to be uncomfortably loud and generally listen some 6 db below that which cuts my power demands by 75%. In my fairly small room, I could have a 1000 watt per channel amplfier and never blow up a speaker as long as I stay at or below Dolby reference levels.

Sorry to ramble, but I'm trying to give you some sense of how amplfier power works. The key things to understand are that averge power demands are very small (heck, you can make the little 5 watt amplifiers in your TV play dialog fairly loud), peak power requirements may be 40 or 50 times more than average power requirements with Dolby Digital recordings, and that every time you reduce your volume by 3dB, you are cutting your power demands (both average and peak) in half (500 watts is barely louder than 250 watts).

 

[GabrielC]

After that first sentence, I realised my mistake. All this time I've been thinking that the gain control increased the amount of watts being put in linearly. Now that I look at it, it has -dB ratings on it.

Right now I'm using a Samson S1000 with my Acoustic Research S50 mains which can handle 250W. With all of these questions, I've been wondering how to set my amp gain control knobs without blowing my speakers up. Since the amp puts out around 380W/channel into 8ohms, and the speakers are rated at 250W, I've always thought that if I turn the knobs all the way up, they would kill my speakers.

How should I setup this system?

 

[Harold_C]

How should I setup this system?

Really, you should set it up the same as any other Dolby Digital system. Assuming that you have a powered subwoofer, set your AR's and all the other speakers in your system to SMALL and the subwoofer set to ON, with either the 80 or 100 Hz crossover in the receiver.

Use the Avia calibration disk or your surround sound receiver's test tones. Get a Radio Shack SPL meter for $40, and find out what position on your volume control it takes to get the Dolby reference level. As long as you play at or below that volume setting, I don't think you will be likely to blow up the ARs.

Your amp will deliver about 300 watts per channel into an 8 ohm load. I think the 250 watt power handling number for the AR's is probably optimistic, but you won't need to dump anywhere near that amount of continuous power into those speakers to get to Dolby reference levels.

Your amp requires a 0.775v input to reach full rated power. Most A/V receivers won't deliver much more than that at the pre amp outputs, so I would just leave the gain controls on the amp wide open, although you could probably turn them down 3 dB or so and be fine, too. It will really depend, in part, on what it takes to get equal SPL from all of your speakers when you do the SPL meter calibration since the Samson's sensitivity needs to roughly match the sensitivity of whatever is driving your surround and center channels.

Not blowing up the system is going to be a function of what you do with the pre-amp's volume control -- not the sensitivity controls on the amplifier.

You should be fine up to Dolby reference levels (105 dB peaks from any one speaker). I think you'll find that is plenty loud.