High power amplifiers for Heritage speakers

[garyrc]

 

 

I'm no expert, but here's what I have heard:

 

I have read that whether an amplifier will "pass" a peak without clipping depends on the duration of the peak.  Briefer peaks will pass easier.   I have read that the peaks being considered when writing amplifier specifications are 200 milliseconds to 2 milliseconds in duration.

 

I don't think "peaks" in a steady tone (the top of a sine wave) are like peaks in music.  I think that RMS is 0.707 times the height of the sine wave of the steady tone.  The leading edge of a musical peak can be much more powerful.  Someone posted here (maybe 7 or 8 years ago?) that the intensity of a rim shot close up would beyond the capacity of any amp/speaker combination, if it were not for the brevity of the leading edge of the peak.

 

I would think that separate amps would have beefier power supply sections, and should be able to produce more instantaneous power above their rated power than would receivers. 

 

My dealer measured one of my power amps at 171 watts per channel at the point where the top of the sine wave would just begin to flatten.  The manufacturer (NAD) rated them at 150 w.p.c..  If the 0.707 thing is correct, they would be (0.707x171) about 120 wts RMS. 

 

I think that doubling the distance in an anechoic environment, or outside at the top of a flagpole, causes a 6 dB loss, BUT doubling the distance in a room causes only about a 3 dB loss.  In music, for just an instant, I can get 110 dB through 1 Khorn quite easily in my 4,000+ cu.ft. room @ 16 feet (no, I don't subject my ears to that for more than and instant ... the loud passages in the orchestral music I play average about 90 dB).  My room is 1/3 larger than the one PWK was using for the chart.  The 110 dB figure is right at the top of the needle swing ("C," "Fast").  Since this is a needle meter, the true, instantaneous peaks may be 13 dB higher than the meter reads, at more like 123 dB (according to PWK in another paper).  According to the Klipsch chart, 123 dB (120 dB +3 dB, doubling the power needed) may take about 400 watts (or more, because my room is bigger), just for a split second, but I only have 171watts to "spend."  I am familiar with what clipping sounds like, and I hear no clipping.  But then I have a separate amp with a good sized power supply.

 

 

 The .707 RMS is factored in before arriving at the max power of an amplifier (at least it should be). So if they measured 171 watts then that is what your amplifier made. Many manufacturers under rated the max power of amplifiers so they can boast better distortion numbers at full rated power.

 

 

 

I've read that Nakamichi PA-7 amplifiers that are rated at 200 watt RMS actually can put out 370 watts per channel, but have never measured them.

 

Roger

 

 

For how long?  I would think a 50 millisecond peak might be possible at an even higher power out, whereas the un-flattened top of a steady tone sine wave power for 200 RMS would be about 283 watts (283 x .707 = 200 watts).  In the oldest catalogs I've seen, a tube amp "peak power" is typically 2x (3 dB) the RMS power.  Some modern "dynamic power" ratings are about 3 dB higher than RMS, but I've never seen the duration specified. 

 

The other question re: RMS is, "at what frequency?"  Is it at 1KHz? Or 30 Hz (PWK's suggestion)? Or with band limited pink noise, 500Hz to 2KHz?  Over the desired bandwidth (e.g. 20 Hz to 20K hz)?

 

Someone a few years ago posted here that it is an error to talk about RMS power at all. 

[Fjd]

 

 

 

I'm no expert, but here's what I have heard:

 

I have read that whether an amplifier will "pass" a peak without clipping depends on the duration of the peak.  Briefer peaks will pass easier.   I have read that the peaks being considered when writing amplifier specifications are 200 milliseconds to 2 milliseconds in duration.

 

I don't think "peaks" in a steady tone (the top of a sine wave) are like peaks in music.  I think that RMS is 0.707 times the height of the sine wave of the steady tone.  The leading edge of a musical peak can be much more powerful.  Someone posted here (maybe 7 or 8 years ago?) that the intensity of a rim shot close up would beyond the capacity of any amp/speaker combination, if it were not for the brevity of the leading edge of the peak.

 

I would think that separate amps would have beefier power supply sections, and should be able to produce more instantaneous power above their rated power than would receivers. 

 

My dealer measured one of my power amps at 171 watts per channel at the point where the top of the sine wave would just begin to flatten.  The manufacturer (NAD) rated them at 150 w.p.c..  If the 0.707 thing is correct, they would be (0.707x171) about 120 wts RMS. 

 

I think that doubling the distance in an anechoic environment, or outside at the top of a flagpole, causes a 6 dB loss, BUT doubling the distance in a room causes only about a 3 dB loss.  In music, for just an instant, I can get 110 dB through 1 Khorn quite easily in my 4,000+ cu.ft. room @ 16 feet (no, I don't subject my ears to that for more than and instant ... the loud passages in the orchestral music I play average about 90 dB).  My room is 1/3 larger than the one PWK was using for the chart.  The 110 dB figure is right at the top of the needle swing ("C," "Fast").  Since this is a needle meter, the true, instantaneous peaks may be 13 dB higher than the meter reads, at more like 123 dB (according to PWK in another paper).  According to the Klipsch chart, 123 dB (120 dB +3 dB, doubling the power needed) may take about 400 watts (or more, because my room is bigger), just for a split second, but I only have 171watts to "spend."  I am familiar with what clipping sounds like, and I hear no clipping.  But then I have a separate amp with a good sized power supply.

 

 

 The .707 RMS is factored in before arriving at the max power of an amplifier (at least it should be). So if they measured 171 watts then that is what your amplifier made. Many manufacturers under rated the max power of amplifiers so they can boast better distortion numbers at full rated power.

 

 

 

I've read that Nakamichi PA-7 amplifiers that are rated at 200 watt RMS actually can put out 370 watts per channel, but have never measured them.

 

Roger

 

 

For how long?  I would think a 50 millisecond peak might be possible at an even higher power out, whereas the un-flattened top of a steady tone sine wave power for 200 RMS would be about 283 watts (283 x .707 = 200 watts).  In the oldest catalogs I've seen, a tube amp "peak power" is typically 2x (3 dB) the RMS power.  Some modern "dynamic power" ratings are about 3 dB higher than RMS, but I've never seen the duration specified. 

 

The other question re: RMS is, "at what frequency?"  Is it at 1KHz? Or 30 Hz (PWK's suggestion)? Or with band limited pink noise, 500Hz to 2KHz?  Over the desired bandwidth (e.g. 20 Hz to 20K hz)?

 

Someone a few years ago posted here that it is an error to talk about RMS power at all. 

 

 

 

I was actually looking into trying to find more detailed information regarding peaks and better substantiate an average time of a peak or even a range; however, I realized that it seems mostly impossible to get enough information to remotely establish any comparability or reliability in any of the published numbers. 

 

Here one "standard" definition of a "dynamic headroom" test that I considered in some of my responses in this thread along with gathering a few more insights into transformer and capacitor ratings used in power supplies, in addition to power calculations.  I found the Myers Sound paper below meaningful.

 

"The EIA RS-490 (former IHF A-202) amplifier test standard includes a "dynamic headroom" test employing a 20-mS tone-burst. In an informal survey of musical recordings, power bursts were found with durations from a few milliseconds up to several hundred milliseconds, with an apparent clustering in the 80-200-mS range. Since the practical value of an amplifier depends on its ability to reproduce musical dynamics, a more useful power rating would be obtained by amending the dynamic headroom test to employ a 200-millisecond (or similar) tone-burst."

 

http://www.aes.org/e-lib/browse.cfm?elib=4898

 

 

Excerpt from Myers Sound.

 

[in the engineering community, the accepted method of generating a rating of the audio power produced by an amplifier is to connect it to a known load, apply a continuous sine wave signal to its inputs, and monitor its output behavior into the load. This is important to keep in mind as we examine the definition and measurement of “power.”

 

The definition of instantaneous electrical power is quite simple: P = EI, where P = instantaneous power in watts, E = potential difference in volts, and I = current in amperes. However, this definition is minimally useful to us in an audio application because audio sources are not instantaneous pulses, nor are audio loads purely resistive.

 

A sine wave is the building block from which real-world audio signals are built, making it a more appropriate source signal for measurement of an audio system. For a sinusoidal voltage source, power, while still measured in watts, is defined as“average power.” RMS (root mean square) is a method of calculating the voltage and current to obtain the average power.

 

For example, if we look at the sine wave voltage at the output terminals of a power amplifier, we will find the RMS voltage to be the peak voltage (E_peak) divided by the square root of 2. If we measure the RMS sine wave current from the amplifier, we will see that, similarly, it is the peak current (I_peak) divided by the square root of 2.

 

Multiplying the two, we get:

 

(E

peak

/(sq root 2)) * (I

peak

/(sq root 2)) = (E

peak

* I

peak

)/2

 

which is the average power for a sine wave.

 

When an amplifier is rated in RMS watts, this is a shorthand way of saying “average watts obtained by the RMS method.” If you use a signal other than a sine wave, you must use a meter reading ‘true’ RMS voltage to obtain the correct average power.

 

So what about peak power? Peak power is a special case where P_peak = E_peak * I _peak. For a sine wave, this is always twice the average power. A major problem with using this rating, however, is that many power amplifiers cannot maintain peak power for more than a few milliseconds.

The standard method of testing a power amplifier to see if the power supply can maintain continuous peak power is to connect all channels of the amplifier into load resistors, drive the amplifier’s input with a square wave and monitor the peak voltage at the outputs. Almost all power amplifiers will ‘sag’ in output power under this drive condition.

 

Now, having a power amplifier produce twice the continuous sine wave power is hardly necessary for music reproduction, but sometimes music signals produce short-term square wave or large sine wave-like waveforms. So how long should a power amplifier be able to maintain reproduction of a square wave or sine wave at full amplitude?]

 

http://www.meyersound.com/support/papers/amp_power.htm

[garyrc]

200 milliseconds sounds like a good standard to me; 500 milliseconds is even better. 

 

Isn't it true, though, that if an amplifier can reach a certain peak for 500 milliseconds without clipping, that it can reach an even higher peak for a shorter duration, such as 200 milliseconds, or 20 milliseconds?

 

I don't want much; I'd be happy if all respectable amp makers would use the same standard, and publish same.

 

As to what is necessary for music reproduction, I still think that a live feed, with no recording, can present a challenge to any speaker/amp combination.  The minute it is recorded, however, dynamic range is compromised, isn't it?  After recording and taking measurements at symphony concerts, PWK concluded that you need 115 dB peaks at your ears.  Interestingly, reference SPL for movies produces 105 dB peaks at the listeners position from a main speaker, and 115 dB from a sub.  That is called "full scale" (fs).

 

Naturally, pop music today, during the "loudness wars" often has constricted (compressed or limited) dynamic range, because the corporations want it to be "all loud."  But orchestral music, movie music, or a live, uncompressed and unlimited feed of any kind of music could still have extreme peaks that should not be clipped.

Edited April 7, 2016 by garyrc

[svberger]

Some of the best sound that i've experienced has been with a Crown XLS1000 amp through a Decware CSP2 tube pre, pushing my Cornwall II's.  The Crown is completely transparent and neutral, and the CSP provides a  good tube counterpart while not making the overall presentation too syrupy. Headroom galore, but I rarely listen very loudly.  Where it shines is low level listening. The bass on the II's really come alive with very little volume. I like that. 

[chriswhotakesphotos]

I think the cool thing about Klipsch speakers is that you can shop for an amp almost independently of power ratings, you don't really need to worry about what the power output will be as long as it sounds good.

 

I'm gonna get some tubes in my system eventually (I'm really liking the look of those new Schiit preamps) but I've run my Heresies with an Adcom GFA-535 (65w) and liked it, and now with an Emotiva A-300 and I like that even more. I was concerned that an inexpensive 150w power amp would sound ugly on speakers made to sound good with 10w, but it's been a real joy. It's never gotten warm driving them, either.

[JohnA]

On 4/5/2016 at 0:09 AM, KeyOfGee said:

This has probably been talked about more times over the years than it needs to be, but I just want to understand what is going on with power amplifiers and Heritage speakers. Please don't be offended at my ignorance about this. Simple question, why is everyone recommending low powered amps for Heritage speakers for Home Theater when it's virtually impossible to find a high quality low powered 7 channel amplifier to run them?? In many of the post that I have seen, everyone is using saying don't use more than 50w per channel (because you don't have to), but the quality amps are rated from 125w to 250w. My guess is that a McIntosh MC207 or a ATI AT4007 is out of the question for (3) Klipsch LaScala up front and Heresy surrounds or am I just crazy for even thinking about amps like this?? OK, I'm ready for the beating.

 

Unfortunately, that is the market we live in.  Few speakers are anywhere near as efficient as Klipsch.  Most require 100 watts to be as loud as your La Scalas get on 1 watt.  So your listening will frequently use a few milliwatts.  Unfortunately, most amps have their best sound and least distortion at a few percent, say 5% of rated power, but that's ear-splitting from La Scalas.  Five per cent power a 20 watt amp, would likely put you near that amp's best performance.  I once had a pair of 8 watt tube amps; I never lacked for power with them.  For that reason, we recommend low power amps, but, as you know, good ones are few and far between.  Nelson Pass' First Watt amps are some of the few.  Seventy’s era Japanese amps are another, but usually require some restoration.  Then there are tube amps, nearly always low powered. 

 

The good part is that most really good amps are biased so that they operate in Class A until the power output exceeds 1 or 2 watts.  I believe my Acurus A125x5 transitions out of Class A at 1.25 watts, enough that I rarely get out of Class A until The Fallen falls.      Seems like some of the old Aragon monsters ran class A until 10 watts.  They were HOT at idle.  So, yes, it's OK to buy a big amp, but I'd try to stay near 100 watts, to save money for a better Pre/pro, or universal disc player. 

 

What America needs is a good 20 watt amp! 

[Frzninvt]

I had a pair of very rare DBX BX-1 Configurable Amplifiers that could be configured in 2, 3 or 4 Channels.  I ran one in three channel mode 100W + 400W + 100W and the other in four channel (100W X) and headroom was off the charts.  They could handle any speaker load down to essentially a dead short.  The torrodial transformers were huge and bolted in place.  I only parted with them due to age and servicability for a Sherbourn 7/2100A. In retospect I should have kept them.

DBX BX-1 Amplifier Specifications.pdf

BX1.bmp

[richieb]

Do you think the "first watt" theory that it must be a quality watt or the remaining xxx watts are useless applies to digital power amps - Class D, ICE, Hypex, gainclone etc. Or do these contemporary amps produce a first watt that is as pure as the 200th watt. I'm guessing they come closer to this performance level than "standard" transistor powered amps. 

[moray james]

On 6/5/2017 at 3:42 PM, richieb said:

Do you think the "first watt" theory that it must be a quality watt or the remaining xxx watts are useless applies to digital power amps - Class D, ICE, Hypex, gainclone etc. Or do these contemporary amps produce a first watt that is as pure as the 200th watt. I'm guessing they come closer to this performance level than "standard" transistor powered amps. 

 Curious to know if you have listened to the Hypex Ice or any other digital amps out there and what you thought? Since most of your listening is done in the first hand full of watts then those need to be very clean. Big peaks tend to be very short duration for the most part. If you plan on cutting corners it is obvious where that will get done. So your first fraction of a watt up top say ten watts is where you will be spending the vast majority of your listening time. The rules don't change just because you choose a different technology to amplify your signal. Switching amps have to use mosfets or transistors in the output nothing has changed there. Linear or switching supplies can sound good or bad you get what you pay for there are simply different sets of compromises in various designs that's all. In the end it all comes down to you liking the sound and the price just like it always has.

[Schu]

switching... not digital

[ClaudeJ1]

On 4/5/2016 at 7:21 PM, babadono said:

Fjd,

 All I was trying to say is if you play your KHorns at 2 watts (loud) and you want to be able to handle 10dB peaks over this(very loud) you need 20 watts.

 

And that would be 20 watts of unadulterated, unclipped RMS power.

When I mentioned to PWK, when he was 81 years old, half my lifetime ago, that I planned on 10 db headroom, he said "Make it 17 db." So I just round up to 20 db from 1/4 watt, which he said was average in a Khorn, while I was listening to PWK's own Khorns (in false corners) in his huge living room, playing his classical music tapes. He measure typical 17 db peaks in a full scale symphony, which is the ONLY type of music he had in his collection of home made tapes. He hated commercial recordings, referring to them as "dilute stereo."

 

So, 1/4 watt is a negative 6 dbWatts. Adding 17 db headroom to a -6 dbWatts, give us 11 dbWatts, which translates to 12.589 Watts of power per channel.