or Connect
AVS › AVS Forum › Audio › Receivers, Amps, and Processors › Amplifier FAQ
New Posts  All Forums:Forum Nav:

Amplifier FAQ - Page 6

post #151 of 227
Quote:
Originally Posted by MichaelJHuman View Post

Thanks for the comments.

Addressing a few of them...

I admit to disliking the term headroom, because I feel it's misunderstood. Headroom, by common definition, is power beyond what you think you need to deal with unexpected signal peaks. Say you wanted 100 dB SPL at your listening position for each speaker. You factor in speaker sensitivity and loss due to distance. You decide 200 watts / channel will do the trick. Well, maybe you got it a bit wrong. Knowing this is possible, you decide you want headroom. So you double the power. But that gets expensive! Maybe it's cheaper just to reduce volume a bit. Headroom, IMO, is not dynamics. You should factor in dynamics into your planning - for example using THX's stated 20 dB of dynamic range.

Haven't chimed in in a while...here's my take on the gentleman's point on headroom. You can achieve headroom two ways:

1) Brute Force (buy an amp with RMS rating far in excess of what you think your peak power need will be

2) Clever engineering (buy an amp with an adaptive rail (Class G or H) whose RMS rating is matched to your average power need but with significant dynamic headroom to meet your peak power need). Now you'll have a hard time finding an amp wioth 20 dB dynamic HR, so there has to be some degree of "brute force" over specification even with Class G&H to meet a 20dB figure.

The other factor that comes into play and is perhaps more significant is how does your amp perform when driving a load with impedance curves that dip well below 4 ohms and or that exhibits large phase angles? This pertains more to amps with constant rail voltages instead of those described above. Most of us buy an amp based on how it performs in continuous output into an assumed 8 ohm, time and frequency invariant load. A factor I always look for is does the PA "double down" in continuous and peak output when you halve the test load resistance? A PA with a power supply designed to do this will double the current it can supply when the test load is cut from 8 ohms to 4, from 4 ohms to 2, and very few can even continue down to 1 ohm.

Because your "8 ohm" speaker very likely does present a load less than 4 ohms at one or more frequencies, this is a significant consideration. AVR PAs spit the bit on this test. Here's a link to a Stereophile test done on a Parasound PA that has long been out of production, showing how it behaves in terms of peak output when driven with short tone bursts (duration not specified but probably in the 10s of msecs) as the test load is decreased from 8 to 4 to 2 to 1 ohms.

This PA is a friggin arc welder. It delivers bursts of 2200+ watts into a 1 ohm load. I maintain that if you performed this test on all PAs, you'd see the 3 curves corresponding to 4, 2 and 1 ohm loads pushed way to the left, as 90+% of PAs have not been engineered to source this kind of current.
post #152 of 227
Thread Starter 
Right, load seems to matter. As amps are spec'd into 8 ohm dummy loads, and maybe 4 ohms, a clear picture of how they react to real loads seems hard to come up with.

There's an article floating around that attempts to show how some speaker's reactive loads can be seen as an impedance. I believe that depends on the signal they are getting, so it's not trivial.

I think this is it -

http://www.stereophile.com/reference...avy/index.html
post #153 of 227
That's a very intriguing article. While you need an EE background to really understand all of it, the takeaway to me is that 8 ohm and 4 ohm nominal impedance ratings are really two leaps, not one, removed from reality and usefulness.

The 1st leap is yo the complex impedance curve with both the magnitude and phase of the speaker impedance plotted, while the 2nd leap is from there to the analysis provided in the paper.

It really is astonishing that audio testers whose measurements are published in magazines like Stereophile, etc., have not begun to use a figure of merit similar to that published in the article. It tells so much more about the challenge any speaker presents to a PA.

Anyway you slice it though it still gets down to what is the peak power dissipation capability of the transistors in the output stage, and does the power supply have the ability to deliver that much current if called upon.
post #154 of 227
Thread Starter 
There was another article, I can't locate. As I recall, possibly incorrectly, the idea was that amps could, under some conditions, experience some sort of very nasty load condition they would have trouble recovering from immediately.

But I forget the specifics.
post #155 of 227
Quote:
Originally Posted by MichaelJHuman View Post

There was another article, I can't locate. As I recall, possibly incorrectly, the idea was that amps could, under some conditions, experience some sort of very nasty load condition they would have trouble recovering from immediately.

But I forget the specifics.

You're probably referring to oscillation. Any PA (or system for that matter) with global negative feedback can go from its normal state of stability to into a state of oscillation at a given frequency if the system's phase margin is consumed by a phase shift in the load.
post #156 of 227
Thread Starter 
That may be
post #157 of 227
Of I use pro audio amps.. I Will need Atleast 4. My q is how do I get them to turn on or controlled by my processor. Being they don't have a dc voltage control
post #158 of 227
Thread Starter 
I think you can use a switched power strip of some kind. As always I suggest questions NOT related to the FAQ are best posted to the forum. You will get more responses that way.
post #159 of 227
is it not advisable to stack 2 home audio amps on top of eachother?? I got a H & K 5800 5 channel and a Marantz 2 channel wthis wouls save bout 2 inches rack space
post #160 of 227
Thread Starter 
Depends on the amps. Some run cool and that can be done. Done don't.
post #161 of 227
Quote:
Originally Posted by russ_777 View Post

You're probably referring to oscillation. Any PA (or system for that matter) with global negative feedback can go from its normal state of stability to into a state of oscillation at a given frequency if the system's phase margin is consumed by a phase shift in the load.

Not if they're designed properly. But yes, if a loudspeaker looks capacitive or inductive in a certain range of freq, the PA could exhibit a realtively low phase margin (<45deg) which is dangerous. Global feedback in and of itself is not bad.
post #162 of 227
Quote:
Originally Posted by gman76 View Post

Not if they're designed properly. But yes, if a loudspeaker looks capacitive or inductive in a certain range of freq, the PA could exhibit a realtively low phase margin (<45deg) which is dangerous. Global feedback in and of itself is not bad.

A good engineer never assumes something is designed properly.
post #163 of 227
I had posted this in the AVR that I have section before, but now I want to ask the amp pro's

What do you guys think of the Onkyo M-282 as a surround amp for some surround speaker rated 80w rms @ 6 ohms(Monitor Audio RXFX's)?

Actually 2 amps for 2 sets of surrounds.
post #164 of 227
Thread Starter 
Repost your question to amp/receiver forum. Posts in this thread should be about the FAQ (and you will get better responses by posting to the AVR/Amp forum.)
post #165 of 227
Michael,

Thanks for taking the time for writing this very informative article on amplifiers. Your information has allowed me to save a considerable amount of money by allowing me the knowledge to be able to disregard most of the "bigger is better" attitudes that seem prevalent in the high end discussions.

I have been researching amplifier "requirements" for my Aerial Acoustics setup: (3) LR5's across the front. Before I read your article and began to understand the mathematics I was ready to purchase (what I thought were needed) amplifiers with outputs of 700-1000 watts into 4 ohms. I now see how ridiculous this fallacy is.

I now plan to bring my old Parasound HCA-1206 out of the closet and use it with my Aerials. It produces 200 watts/channel into 4 ohms. No need, unless I am totally wrong in my understanding, for anything more.

Thanks again Mike. The forum is lucky to have you!
post #166 of 227
MichaelJHuman,
Don't know why, but I just now got around to looking at this.
Another very nice write up, for those with and without engineering education.
The accurate, yet plain language is easy for anyone to understand.
Writing an atricle like that is not a trivial task, there are a few contributors on the site that go above and beyond, you are one of them.
post #167 of 227
If you run an amp pulling 2800 VA (2800 watts at pf=1) from the wall continuously, a 20 amp breaker will open after 500 seconds or more. This assumes you are at 120 V. There is a time curve for breaker opening and a variance in operation of breakers. Your 20A breaker might not EVER trip with 2800 watts of power coming from it. For 2800 watts of power amp OUTPUT, it is more complicated. You have to take the efficiency of the amp and amp power supply power factor into account. A typical 20 A breaker can supply 30 A of power continuously for about 90 seconds before tripping. Or even more.. 40A for 15 seconds. 200A for one second. This is a pretty much dead reply, but not many people realize that breakers just dont magically trip at 20 or 30.
post #168 of 227
Thread Starter 
Quote:
Originally Posted by goobaah View Post

If you run an amp pulling 2800 VA (2800 watts at pf=1) from the wall continuously, a 20 amp breaker will open after 500 seconds or more. This assumes you are at 120 V. There is a time curve for breaker opening and a variance in operation of breakers. Your 20A breaker might not EVER trip with 2800 watts of power coming from it. For 2800 watts of power amp OUTPUT, it is more complicated. You have to take the efficiency of the amp and amp power supply power factor into account. A typical 20 A breaker can supply 30 A of power continuously for about 90 seconds before tripping. Or even more.. 40A for 15 seconds. 200A for one second. This is a pretty much dead reply, but not many people realize that breakers just dont magically trip at 20 or 30.

It's an important point, thanks for making it.

I suspect that's why more of us don't blow breakers or fuses during peaks in movies. And there's large caps in the power supply of many amps which can supply a lot of power (briefly.)
post #169 of 227
thanks +12345!
post #170 of 227
I have two Anthem MCA 50s amps bi amped across my front channels to create 450 watts. I just purchased two Anthem M1 mono block amps at 1000 watts each and set them up on my Paradigm S8s, my question is, if I have a 50' speaker run to my speakers from my amps, would it be in my best interest to run balanced out of the Anthem Statement D2v 50' to the area where my speakers are and set the amps up there and have a 5' speaker cable run from the amps to speakers?

John
post #171 of 227
Yes. Long balanced interconnects and short speaker cables. Blue Jeans Cable can make you up some nice 50s using Belden 1800F.
post #172 of 227
The amps would look nice in my rack, can you explain the difference...........................in layman's terms? smile.gif
post #173 of 227
Basically it has to do with a voltage signal (the low level connection between amp and preamp) having essentially zero losses, while the high current (relatively speaking) amp-speaker connection incurs greater losses.
post #174 of 227
Thanks for the information whoaru99
post #175 of 227
I hope this is the right thread to ask this question. I have a Pioneer A/V receiver and I am using it for my 5.1 & 2.1 listening. Would it be worth adding an amplifier to it? Most of the amplifiers that I find are 2 channel. so it seems to me that it would only improve the 2ch listening & maybe hurt the balance of the 5.1. Since the center channel wouldn't have the same power to it.. This is just something I was pondering and thought I'd see what people thought.
post #176 of 227
Q. Would it be worth adding an amplifier
A: Impossible to answer without more details of the model, speaker, room size and what SPL you listen at.

Q. it would only improve the 2ch listening & maybe hurt the balance of the 5.1. Since the center channel wouldn't have the same power to it.
A: not necessarily. Improvement is by no means a foregone conclusion. There are mono amps, 3CH, 5CH and 7CH amps so other channels aren't necessarily left out. In any case speaker calibration should take care of the 'balance' whether speakers have ext. power or not.

Having said all that, people do buy ext. power amps even if they don't need them. MJH who wrote this thread has MCH power amps added to his flagship Yamaha Z11 (and noticed no added benefit) IIRC. biggrin.gif

There are two sides of the coin. Some people reckon you hardly ever need power amps as normal listening rarely reach reference levels. One example:

http://www.avsforum.com/t/1447051/is-reference-level-really-that-hard-to-achieve
post #177 of 227
Quote:
Originally Posted by Snafu55 View Post

I hope this is the right thread to ask this question. I have a Pioneer A/V receiver and I am using it for my 5.1 & 2.1 listening. Would it be worth adding an amplifier to it? Most of the amplifiers that I find are 2 channel. so it seems to me that it would only improve the 2ch listening & maybe hurt the balance of the 5.1. Since the center channel wouldn't have the same power to it.. This is just something I was pondering and thought I'd see what people thought.

The only way a outboard amplifier helps sound quality is if the on board amplifier is causing audible distortion, which would mostly likely be due to clipping.

There is an audiophile myth that says that having several times the power you need improves sound quality, but of course this is a myth. It does sell unnecessary amplifiers.

Balancing the channels of an AVR is up to the user, and any automated system tuning facility that the AVR has which in your case would be MCACC. MCACC or the manual adjustements should work well with either an on board or outboard amplifiers if the outboard amplifier's gain is anywhere in the ball park.
post #178 of 227
I believe this thread was created to explain the technical explanations of how an amplifier works and practical applications of using one.
I do not believe it was created to take specific questions about your (general you) personal amplifier and speaker setup.
IMO, comments outside the scope of providing helpful amplifier information just clog this thread.
post #179 of 227
Hello, nice post, good info, just now read all of it and I find that there are several flaws in what you have written, I do not mean to sound condescending but there are HUGE differences in one manufactures prescribed wpc and that of another, ratings do matter depending upon the company and how said amplifier was tested, ect. I got the sense that your article means all amplified power sounds the same at a given wattage/amperage/voltage- I very much disagree.

For example: when comparing ARC/ ATi /Bryston/ B&K (Now ATi)/ Classe/ Krell ect. (USA/Canada built amps) to their off shore counter parts there is generally no comparison in/of sound quality- and yes most of these brands double down into 4ohms from 8ohms and sound extremely better- why? Simply put, far better build quality, and far better components used- even if the wpc are mesured the same.
Quote:
There's also the amusing situation of amplifiers being rated for more power than could be pulled from the wall. Take a 400x7 Watt amplifier. That's 2800 Watts. Even a 20 amp circuit is not going to manage to supply it with enough power to meet it's specification. There's little point to an all channels driven measurement of this sort. The amp is likely limited by wall power. They may as well just list the specification as 'Puts out as much power as you wall outlet can supply minus losses.' Maybe that's overstating the situation, but not by much I suspect.
This is simply not true. I would advise that you might want to have a look at the websites of ATi, Rotel, Krell, Lexicon, and many others. BAT writes a very good exp/white paper of how and why this works, which goes way beyond what I am able to write here. Cinenova, Classe, BAT, Krell, ATi, Cinepro (AB International), Lexicon, Anthem, NAD- all of these companies build massive 7 channel amplifiers that greatly exceed your "wall current" @ 15a or 20a, yet produce MORE then rated power doing such on 15 or 20a wall power. Remember what a speaker sees is not AC wall power.

I personally own an ATi 1805 for my surrounds and an Odyssey Stratos for my front mains along with my NAD T785, I use the NAD to Bi-amp the center and the fronts along w/the Odyssey Using a single 20A circut, I have no problems. Mind you it is a dedicated circut. So that is 165 x 5 plus 120 x 3 plus 185 x2- (all min rated TRUE rms power 8ohms) a lot of power, yet I never pull more then 14-16a off the wall full power from my set up.

Dont mean to seem like a butt head, but I feel the need to perhaps clarify what you were trying to get at......maybe?

EDIT: I would like to also add that the job of a quality amplifier is not to simply amplify a signal, but rather, and more to the point- control the speaker. When in doubt- always choose quality over quanity when it comes to audio/video.
Edited by kodi41 - 2/18/13 at 6:52pm
post #180 of 227
Ok here is the perfect explanation: please read this, it will greatly help anyone curious to understand how audio amplifiers function.

http://www.qscaudio.com/support/library/papers/amptalk.pdf
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Receivers, Amps, and Processors
AVS › AVS Forum › Audio › Receivers, Amps, and Processors › Amplifier FAQ