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Old 09-22-2015, 12:37 AM
 
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Originally Posted by lovingdvd View Post
Good point.



Yes. I ran it with one speaker for the reason you cited. I get 99 dB with 250w that way, when not located near a wall. Basically they give you a +6 dB bonus for being in a baffle wall, which is what I heard previously (actually I heard +3 dB to +6 dB benefit from baffle wall).

So yes, at just 250 watts I can hit 105 dB reference. However my concern is that if I don't go with something higher, like at least 300 or 350w, then there may not be any (or enough) headroom as needed. For example, as pointed out here, speakers can change their impedance at different frequencies. So if the KEFs drop to a 2 Ohm or whatever load then all of the sudden the 250 watts is no longer enough. Maybe it delivers 125 watts for those times which then drops things by 3 dB. So its nice to have at least the piece of mind that there is extra watts available as needed.

That said, even if it drops to a 2 ohm load with 250 watts then becomes 125 watts and the loss is only 3 dB. So essentially - if I have this straight - if I am OK with knowing that I can always play movies just fine at 3 dB below reference, that's acceptable to me. Realistically I won't listen at reference anyway - to my aging ears its painful and makes them ring. I like the idea of knowing I can play perfectly fine at reference because it then means I can play perfectly fine at any level below reference. But if I can be 100% confident that with 250w of power I can play with no problems at -3 dB or -5 dB below reference, I think I'm good to go. I really can't imagine watching a full movie any louder than that anyway.



It would be great to know what numbers they have for power compression as well, which Mike Garret says can account for 3 dB to as much as 9 dB of loss depending on the speaker.
What about a ATI 6012 for all surround channels, with a ATI 6003 for L/C/R.
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Old 09-22-2015, 08:28 AM
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Originally Posted by bigguyca View Post
When compared in dB's, and that seems the relevant comparison, the output of all these ATI amplifiers is similar. It's hard to get excited about a dB or two from an engineering standpoint.
Agreed, from a dB output standpoint when the impedance is around 4 ohms. My concern is about having the headroom and trying to figure out how much is enough. For instance in this case its not so much about "my speakers can play 2 dB louder if I have that extra 200 watts" but more like "for passages where the impedance is lower, and/or for reduced efficiency at the speaker when the coils are hot (power compression), having that extra 200 watts should help me be able to MAINTAIN the same loud listening level".

So IOW its not so much the extra watts for being able to pick up an extra 2 dB, but more about the extra watts to be able to properly maintain loud levels when conditions exist the require more power from the amp, such as peaks or dips in impedance or power compression etc. Does this make sense?

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That said, my personal choice is for big amplifiers. I don't think I ever use their full capability, but I get satisfaction in owning them; who knows why. I don't own a Corvette, but I doubt most owners drive 150 mph very often, if ever, but they like owning the beasts.
The car - that's a great analogy. In my case I'd prefer to have the exact amount of power that I'd ever need, without having wattage in reserve that is wasted. That's the challenging part, because without specific tests and data from the manufacturers (or others that have tested) its very difficult to know. So what I think I've learned in this process is that a) its very hard to predict exactly how much you many watts you may ever need beyond what a SPL calculator tells you, and b) to cover all bases people recommend 1.5-2x the speaker max wattage is to account for this overhead and "unknown" amount of watts really needed - and perhaps a fair or large amount of that 1.5-2x winds up never being used but is there for those times (perhaps very short or specific passages etc) when needed.

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To be clear: I have no experience with high-end Class D amplifiers. Likely my iPhone, and the audio system in my car use them, but that's my limit of experience. I've read about Class D amplifiers and have a basic idea of how they work...
Thanks. It would be really interesting to audition both types of amps in my room. I am almost tempting to get the speakers in place and try a two channel class D and class A/B rated at the same watts in the room to see if I could hear any differences (beyond would could be EQ'ed) and if one seemed to run out of power before the other... I wonder if others have done so. I have to image they have...
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Old 09-22-2015, 08:32 AM
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Originally Posted by fatbottom View Post
What about a ATI 6012 for all surround channels, with a ATI 6003 for L/C/R.
Only 90 watts per channel.
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Old 09-22-2015, 08:39 AM
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How about the BGW VXi8.8. Fully balanced, 200W RMS at 8 ohms and here's the big one----16 channels. Or the VXi8.4. 200W x 8 plus 400W x 4 at 8 ohms.

www.bgw.com

Jeff

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Old 09-22-2015, 08:40 AM
 
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Originally Posted by lovingdvd View Post
Only 90 watts per channel.
Which is plenty for side/surrounds/ceiling. Saying that, I have a 2004 for side/rears which is a bit stupid.

Still if I was going atmos (I'm not) I'd probably get the 6012 for side x2, rears x2, ceiling x 4 maybe front effects, VOG channel, with a decent 3 channel.
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Old 09-22-2015, 10:12 AM
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Originally Posted by Stereojeff View Post
How about the BGW VXi8.8. Fully balanced, 200W RMS at 8 ohms and here's the big one----16 channels. of the VXi8.4. 200W x 8 plus 400W x 4 at 8 ohms.

www.bgw.com

Jeff
Looks like a great amp - they are owned by ATI. It would be nice to have 16 channels of power all in a compact chassis (probably around a 6U) like this. However the VXi is really intended more as a commercial cinema amp and as such it has fans which are rather loud. The AT2000 series puts out similar power and 16 channels of that would cost less than the VXi so I'd prefer the 2000 series.
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Old 09-22-2015, 10:58 AM
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Quote:
Originally Posted by lovingdvd View Post
Agreed, from a dB output standpoint when the impedance is around 4 ohms. My concern is about having the headroom and trying to figure out how much is enough. For instance in this case its not so much about "my speakers can play 2 dB louder if I have that extra 200 watts" but more like "for passages where the impedance is lower, and/or for reduced efficiency at the speaker when the coils are hot (power compression), having that extra 200 watts should help me be able to MAINTAIN the same loud listening level".

So IOW its not so much the extra watts for being able to pick up an extra 2 dB, but more about the extra watts to be able to properly maintain loud levels when conditions exist the require more power from the amp, such as peaks or dips in impedance or power compression etc. Does this make sense?



The car - that's a great analogy. In my case I'd prefer to have the exact amount of power that I'd ever need, without having wattage in reserve that is wasted. That's the challenging part, because without specific tests and data from the manufacturers (or others that have tested) its very difficult to know. So what I think I've learned in this process is that a) its very hard to predict exactly how much you many watts you may ever need beyond what a SPL calculator tells you, and b) to cover all bases people recommend 1.5-2x the speaker max wattage is to account for this overhead and "unknown" amount of watts really needed - and perhaps a fair or large amount of that 1.5-2x winds up never being used but is there for those times (perhaps very short or specific passages etc) when needed.



Thanks. It would be really interesting to audition both types of amps in my room. I am almost tempting to get the speakers in place and try a two channel class D and class A/B rated at the same watts in the room to see if I could hear any differences (beyond would could be EQ'ed) and if one seemed to run out of power before the other... I wonder if others have done so. I have to image they have...
in real content, afaik, there is no passage where the total impedance is significantly lower. There are frequencies at which the impedance is lower. So for a millisecond to (let's imagine) a second, if 20 perceent of the total power requirement is at 2 ohms and the rest is at an average of 6 ohms or more, you'e likely in no trouble. The amp has to push the total impedance, and never, unless you are sitting around listening to sine wave test tones, is anywhere like the majority of the power requirement going to be at the speaker's impedance minimum . . . . onaccounta that's not how sound works.
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Old 09-23-2015, 03:55 PM
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Originally Posted by JHAz View Post
in real content, afaik, there is no passage where the total impedance is significantly lower. There are frequencies at which the impedance is lower. So for a millisecond to (let's imagine) a second, if 20 perceent of the total power requirement is at 2 ohms and the rest is at an average of 6 ohms or more, you'e likely in no trouble. The amp has to push the total impedance, and never, unless you are sitting around listening to sine wave test tones, is anywhere like the majority of the power requirement going to be at the speaker's impedance minimum . . . . onaccounta that's not how sound works.
Although your post makes perfect sense to me, I'm still wondering why reviews in Stereophile and elsewhere sometimes say "this amplifier may have difficulty in driving a low impedance load." Why would those of us who listen to music care about that? I tend to see these comments in Atkinson's technical section of a review. I've also heard that some speakers that are rather efficient may still not deliver much "umph" (usually in the mid bass, for some reason) because of "impedance problems."

Of course, maybe that's for the benefit of people who do sit around and listen to sine waves, instead of music.
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Old 09-23-2015, 09:06 PM
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Although your post makes perfect sense to me, I'm still wondering why reviews in Stereophile and elsewhere sometimes say "this amplifier may have difficulty in driving a low impedance load." Why would those of us who listen to music care about that?...
I am not sure, but I think the answer is that speakers have different levels of impedance at various frequencies. Take for example the KEF R700 - specs here: http://kef.com/html/en/showroom/hi-f...700/index.html . Its nominal impedance is 8 ohms, but it also says minimum 3.2 ohms. I'm still pretty new to this, but I *think* this means that the speaker can dip down to those lower ohms, and that is where amp performance at lower impedance lows can come into play. Perhaps 3.2 ohms can be handled by most quality amps but some speakers can go even lower to 2 ohms or even less which could be problematic for some amps.
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Old 09-24-2015, 06:00 AM
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Most speakers have peaks and valleys in their impedance and "nominal" impedance may mean very little. If there is a broad dip in impedance in the midbass, say to 3 or 4 ohms, and your amplifier has fairly high output impedance and/or limited current capability and you listen loudly, you may notice the lack of bass. Similarly, a lot or planar dynamic and ribbon speakers drop to 3 ohms or lower in impedance at high frequencies, and many ESLs drop below 1 ohm at 20 kHz, so with some amps you might notice the drop in HF energy. I would guess the majority of us would never tell in a blind test using most source material, but there are certainly real cases out there.

Tube amps generally have high output impedance and thus the overall system response depends upon the speaker's impedance.

And the amp and speaker do not care if it is music or movies. If your cymbals or kettle drum are lacking it is just as vexing as when a big explosion fizzles out or car chase sounds wimpy.

Over on WBF I wrote a little article showing how amplifier and speaker impedances interact.

IMO - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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Old 09-24-2015, 03:58 PM
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Originally Posted by lovingdvd View Post
I am not sure, but I think the answer is that speakers have different levels of impedance at various frequencies. Take for example the KEF R700 - specs here: http://kef.com/html/en/showroom/hi-f...700/index.html . Its nominal impedance is 8 ohms, but it also says minimum 3.2 ohms. I'm still pretty new to this, but I *think* this means that the speaker can dip down to those lower ohms, and that is where amp performance at lower impedance lows can come into play. Perhaps 3.2 ohms can be handled by most quality amps but some speakers can go even lower to 2 ohms or even less which could be problematic for some amps.
I agree that they may point it out for exactly the reasons you stated, but I infer from the post by JHAz that, even though at a certain frequency the speaker impedance may be quite low, in music ("real content," as he said), because more than one frequency is presented at once, i.e., one or more (usually more) fundamental, as well as all of the overtones of such fundamentals, the amplifier would only "see" a speaker load as low as 2 Ohms quite rarely, perhaps only for a few milliseconds, and would recover before it knew what hit it. Also, I would think that big climaxes that tax amplifiers are composed (pardon the pun) of quite a gaggle of fundamentals and overtones, therefore a more normal (higher) impedance would be presented, more like the speaker's "nominal" impedance, or many times higher, in some cases.

That being said, I still wouldn't want an amp that would freak out while sending a high intensity sine test tone to a speaker that happened to have a low impedance at the particular frequency of a test tone.

Some people have accused several popular speakers (no names please) of not delivering "punch" with small amplifiers because of impedance problems. Whether this a bald guess, a respectable hypothesis, or a confirmed result, using music as a source, I don't know.

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Old 09-24-2015, 04:21 PM
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Over on WBF I wrote a little article showing how amplifier and speaker impedances interact.

IMO - Don
Can you give us a linc to that article? Thanks.
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Old 09-24-2015, 05:27 PM
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No, the last time I posted a link to another forum I received a warning from an AVS Moderator. Google What's Best Forum then look in the technical section. There is, or was (haven't been there in a while), a sticky at the top with links to all my little babbles. I think you have to join to see the pictures, however.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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Old 10-10-2015, 07:18 PM
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No, the last time I posted a link to another forum I received a warning from an AVS Moderator. Google What's Best Forum then look in the technical section. There is, or was (haven't been there in a while), a sticky at the top with links to all my little babbles. I think you have to join to see the pictures, however.
I'm not sure where to post this.... but does anybody have a recommendation for a dedicated Audio 5.1 or 6.1 Amplifier ?

I use a video processor and don't need any Video capabilities that come with A/V Receivers.
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Old 10-14-2015, 05:14 PM
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I'm not sure where to post this.... but does anybody have a recommendation for a dedicated Audio 5.1 or 6.1 Amplifier ?

I use a video processor and don't need any Video capabilities that come with A/V Receivers.
Search for info on whatever NAD has in multichannel amps. I've had good luck with their amps.

What kind of speakers do you have, what is their sensitivity rating, and how big is your room? How far will you sit from your main front speakers?
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Old 12-15-2015, 05:27 PM
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Just wanted to say thanks for taking the time to write all that up. It was a great read for me and very interesting!
Ditto...

That was a really good ....

I learned from it
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