In theory, you're correct. In practice, though, its very difficult to do things that way. Power ratings on amps are not consistent at all. Every manufacturer has their own standards for how they measure power. For example, one company could rate an amp at 100 watts/8ohms. A different company that has higher standards may rate the exact same amp at 75 watts. And another may rate the same amp at 150 watts. If you are new and are having trouble selecting an amp, you'll need to gain some experience through experimentation. Also, talk to the people that make your speakers. They should be able to recommend something that will work for you.
For most music most of the time, a considerable majority of the power that is required will be required at frequencies greater than the crossover frequency that is likely to be used with a powered sub, e.g., 40 or 50 or 60 Hz or thereabouts. (I'm assuming you are referring to a powered sub). As a rough approximation, I believe that 300 Hz is a frequency at which equal amounts of power will often be required at higher and lower frequencies. So in typical circumstances I wouldn't count on a significant reduction of the amplifier power requirement as a result of using a powered sub, crossed over or not.
Also, if the crossover function is being performed "after" the power amplifier, as would be the case if it is performed by the sub itself (with the power amp being connected to the sub, and a high pass filtered output of the sub being connected to the main speakers), the power amp will still have to output a voltage range corresponding to the full frequency range of the signal. So if you were to go to a lower powered amp based on the expectation that it would not have to supply as much power as the full range signal would otherwise require, you still might find yourself clipping the amp if its voltage swing capability is less than what a higher powered amp might be capable of.
ZD makes good points as well. Regards,
Zd542 is right about power ratings of amplifiers, they're not always a reliable indicator of true performance.Some amps rated at 50 watts per channel will clearly out drive some amps that claim 100-150 watts per channel.There are numerous factors that determine actual speaker load drive ability.Power supply "quality"(and output transformer quality) isn't always reflected in the measurements. Yet these two variables are very important.
Per Al's remark, my understanding is that the sensitivity specification is usually for a 300hz or 1khz input signal, so a sub wouldn't affect that (either way).
Moreover, even if that weren't the case, I'd think that a properly designed speaker shouldn't be affected by filtering bass from its input signal. (Whether you filter by crossing to a subwoofer or by filtering the output of the source, for example, shouldn't be relevant. Either way, from the main speaker's point of view, you're just removing bass.)
A "flat" speaker system would take a wide band input signal and produce a flat output across its bandwidth. In a three way system, for example, it's probably true that the woofer will require more power to produce a given SPL than the mid-driver. However, the speaker system should be tuned so that both drivers produce the same SPL when fed the same signal. The output of the mid driver may need some attenuation (possibly within the crossover) to achieve that result, but flat system response requires it.
If that holds true, then adding a sub will increase overall system sensitivity if that sub is more sensitive than the main speaker or decrease system sensitivity if the main speaker is more sensitive (provided the same input signal is used). The biggest difference in the combined sub-speaker should be in maximum clean output capability, not increased sensitivity.
Also, bear in mind that the sensitivity spec is probably quoted for an anechoic environment and you will get room gain in your listening room.
Most powered subs don't have an active high pass for the main speakers although they do have an active low pass for the subs. That means that the main speakers still get the full range signal and the sub supplements the low frequencies.
If you do add an active high pass for the main speakers the benefit has more to do with intermodulation and other distortions. End result... you'll be able to play louder with less distortion. Without the high pass and just a sub, some of those distortions get masked.
Adding a sub will help with power needs, especially if you use the sub crossover.
Keep in mind that doubling the amp's power will only get you 3db increase. You need to increase db by 10db to get a perceived doubling in volume. ie 1 watt=90db, 2 watts get you 93db, 4 watts=96db,8 watts=99db and so on. It gets tough when you have 84 db Maggies and need to go from 200wpc to 400 wpc just to gain 3db volume....
So, I take it that even though my processor is crossing over at 80hz to the powered subs, I gain nothing as far as watts that I would still need for a given db level.
I was hoping it would help.
On a side note. For rear speakers is surround sound... do I need to follow the same amp needs as a front speaker? Or can I get away with a lower watt amp for them?
So, I take it that even though my processor is crossing over at 80hz to the powered subs, I gain nothing as far as watts that I would still need for a given db level."
There's more than 1 way to do this. Can you list your system? Also, about how much are you looking to spend on an amp for the surrounds?
I currently run...
B&W 802d fronts.
Anthem statement p5 amp
Marantz 8801 processor
I am looking to add
And a pair of monitor 600 series.
So I was making sure that...
1. My anthem amp can make the 105 db for thx specs.
2. What kind of head room do I want to try and acheive.
3. What amp do I need for the 6,7,8,9, channels... I eventually only want to use 804 speakers for the remainder of the system.
This last info was very helpful. With all those extra speakers, I can see now why you are being so careful. I think I have a solution for you that won't cost that much money. You'll want the amp that has the best sound quality on your B&W's, so the Anthem will definitely power them. The real question is what you are going to power your rear and surround speakers with. For them, you need a good quality amp with power, but it doesn't have to be anything overly expensive.
I'm currently testing an amp made by QSC (Model GX3). Its a 2 channel pro audio amp that sells for $300. I don't think I would recommend you use it on you 802D's. Power wise, it would be OK, but they demand a better sounding amp like your P5. But for sides, rear, unpowered subs and maybe a center channel, this amp should be more than up to the task. Its got separate gain controls for both channels, balanced, SE and 1/4in connections, its bridgeable and has some crossover features for a sub. It also looks nice and its built like a tank. I don't see where you can go wrong. For your application, its perfect. Also, thats their entry level amp. They make ones that have more power, but I really doubt you would need it. They sell them at Guitar Center, and they have a very good return policy if you don't like it.
There's also something else you can do with your 802D's to make them sound better. If you got more than 1 amp like the QSC I mention above, you can free up some channels on your P5, and use 4 channels on your 802's instead of 2. The 5th channel can go to your HTM. It's not just a power upgrade, it will improve sound quality as well. Its just an idea, and you can always test it at any time because you already have the equipment.
Hope this info helps. If you do something similar to the above, you'll have no issues at all being underpowered.
Thank you for your input...
For clarification, and I missed putting this in last post.
I have 3 subs.. 1 paradigm servo 15 and 2 Rythmik 15's...
Also my anthem powers my Fronts center and rear channels. I do not bi amp or anything of the sort.
The new amp (s) would be just for the 804's
Sorry for the confusion if there was some.
"Also my anthem powers my Fronts center and rear channels. I do not bi amp or anything of the sort."
No confusion there. That's how I thought your system was setup. I just meant that if you were able to free up 2 more channels on the Anthem, you can use 4 of the 5 channels just on the 802D's. The 802's have 2 sets of binding posts on each speaker. That would allow you to use 2 channels of your amp to each speaker. The benefit is that you get more power to your front main speakers, and well as a boost in sound quality. I brought it as just an option that you may want to consider.
"The new amp (s) would be just for the 804's"
The QSC amp I brought up in my last post would be a very good match for the 804's. If the 804's are going to be used as surrounds or side channels, I don't think its necessary to spend more money on something else.
As far as the subs go, I'll have a look at them and see what options they offer. I'll post back a little later.