High power amplifiers for Heritage speakers

[pzannucci]

 

Yes, the upperend AVR would drive the system and sound fine, but separate power amps would almost certainly sound better, and the better the speakers, the easier it is to hear the difference.

This is what I don't understand--If a power amp delivers enough current to drive the speakers efficiently and within the speaker's specs how can it or the listener change/differentiate the sound at a certain SPL that is also below clipping?    

 

The control and the way the amplifier reacts to the crazy load of a Klipsch speaker, especially with auto-formers, will determine it's difference in sound one to another.  Not easily seen on spec sheets but when you first hook it up, especially without a subwoofer handling the low end, you will notice.  Also the glassy, hard, etched highs will give away what the more inexpensive amplifiers can't do, though the specs are the same.  

 

I have used a number of amplifiers from a Rogue 88 in triode mode, to a GFA-555, to a Bel Canto REF-1000 (and lots in-between) on speakers almost all higher end Klipsch outside of Jubes.  They all have their attributes good and bad.

[JJkizak]

The older I get the more I think PWK was right. My Denon X-4000 seems to love SPL's around 70-75 and if I go over about 3 db the result is a more muddled/jumbled sound from the terrible acoustics in my living room.

I can still crank the crap out of it to 98 SPL but it's difficult for the ears to unscramble it.

JJK

[babadono]

Loudness ratio 2 (two times (twice) the loudness) changes the sound loudness level by 10 dB

I was not going to bring this up but since FJD did: Most would say an increase of sound intensity of 10 dB is "yea that's twice as loud" but it is subjective like FJD stated.

[babadono]

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.

And yet this is still not even 3 db of "headroom"

[Schu]

All this talk about power and not one mention of signal to noise ratio?

[moray james]

have a SET 2A3 stereo block sounds very nice on a set of upgraded H3. Also have two rebuilt from the ground up Quad 405 with much improved circuitry 2x100 watts. run them one stereo block per H3 vertical bi amp one channel for woofer the other channel for the mid/hi. Guess which amps are always in the system. The Quad 405 do everything better. I think that a really solid 50 watt per channel stereo block would work well too but I would not go lower than that. when I say solid I mean with a supply that allows the amp to double its output in into four ohms from eight and at least half as much again into two ohms. the amp should be happy to drive a 2 ohm load without clipping or shutting down thermally.

[NOSValves]

have a SET 2A3 stereo block sounds very nice on a set of upgraded H3. Also have two rebuilt from the ground up Quad 405 with much improved circuitry 2x100 watts. run them one stereo block per H3 vertical bi amp one channel for woofer the other channel for the mid/hi. Guess which amps are always in the system. The Quad 405 do everything better. I think that a really solid 50 watt per channel stereo block would work well too but I would not go lower than that. when I say solid I mean with a supply that allows the amp to double its output in into four ohms from eight and at least half as much again into two ohms. the amp should be happy to drive a 2 ohm load without clipping or shutting down thermally.

 

 

you mean the 2A3 can't shake pictures off your walls and vibrate the foundation?

[babadono]

I want the signal loud enough that I don't hear the noise

[Fjd]

10 db peaks you say over average? Just remember 10 db = 10X the power.

1 Watt + 10dB = 10 Watts

10 Watts + 10 dB = 100 Watts

50 Watts + 10 dB = 500 Watts

etc.....etc......

 

 

In case it wasn't clear in the earlier posts, I fully agree with the logarithmic formula that 10 db = 10X the power.  Although, after giving the issue some more thought, I actually do not believe that PWK was trying to state that a 2 watt amplifier is enough for 10dB music peaks over the average listening level of 100dB.  I believe that PWK was talking about how well-engineered his speakers were.

 

Note that in the text of the article PWK indicates that “the larger KLIPSCH Systems are all capable of reproducing the original levels of live rock music (average 100 to 115 dB SPL with peaks 10 dB higher) in a typical listening room, all with a modest amplifier of about 50 watts per channel.”   PWK goes on to state that; “the loudspeaker's short-term (50 ms or less) power handling capacity is much higher (by roughly 10 to 15 dB) than its long term continuous rating.”

 

Therefore, I actually believe that PWK may have been referencing the "general" capabilities of his loudspeakers when he discusses the 10dB peaks before clipping.

 

Given the variability in amplifier ratings there is no way to determine the actual capabilities between amplifiers and how much power one amplifier can actually produce compared to another amplifier with the same general "watt output" rating.  For example, amplifier power ratings are typically established by driving the device to the onset of clipping for a predetermined distortion level that is often different between manufacturers and even can be different with the same manufacturer for a given product line.

 

Driving an amplifier to 1% distortion levels will yield a higher rating than driving it to 0.01% distortion levels.  In addition, testing an amplifier at a single mid-range frequency, or testing just one of two channels, will yield a higher rating than if it is tested throughout its intended frequency range with both channels being driven.

 

Why the variability in amplifier ratings you may ask?  Of course, manufacturers tend to use the method that best helps it to market amplifiers full well knowing that the published maximum power output may include some amount of clipping in order to show higher numbers.

 

Regarding amplifiers, the primary aspect that we do know is that when an amplifier is pushed to create a signal with more power than its power supply can produce, it will amplify the signal only up to its maximum capacity, at which point the signal can be amplified no further.

 

Essentially, "clipping" is the “cut off” portion of the music signal that is beyond the capability of the amplifier resulting in a sine wave becoming a distorted waveform.

 

In power amplifiers, the signal from an amplifier operating in clipping has two characteristics that could cause damage to a connected loudspeaker; (1) because the clipped waveform has more area underneath it than the smaller maximum unclipped waveform, the amplifier produces more output power and this extra power can cause damage to loudspeaker components, including the woofer, tweeter, or crossover, via overheating; and (2) in the frequency domain, clipping produces harmonics at higher frequencies than the unclipped signal would produce where this additional high frequency energy has the potential to damage a loudspeaker via, again, overheating.

 

In another tangent that I took and note that this next part has nothing to do with PWK’s table, but in relation to situations where I use a 2 watt SET amplifier, I did think about an example where I would be listening to music at 100dB (SPL).  My Klipschorn’s are rated at 104dB/m/W, so with two of them, the “effective” sound pressure (at 1 meter) would seem to be 107dB SPL with 1W into each channel.  

 

Although, there would be an impact of a drop of maybe 6dB or more at the listening position, let’s not factor in that impact of the listening position, which will be some distance away from the speakers and we all fully realize that the level would be lower at the listening position vs. the measurement position.

 

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel.  Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

 

Edited April 6, 2016 by Fjd

[Don Richard]

I for one think PWK chart is hogwash... PWK is one of my all time hero's but that chart IMHO is way off. 3000 cubic foot room is a large room... no way could one speaker fill a room of that size to those levels with that kind of power let alone be able to reach 10 db peaks... Think about it a Khorn can and does hit 104db 3 feet away with one watt....now start backing away from that speaker in a 3000 cubic foot room, that 104 db is going to drop dramatically and fast. 

 

Look at the chart he has 20 watts to get 110db but to get just 5 db more takes 43 more watts...  

 

Those numbers were run by Keele, IIRC, and look about right. Many years ago we took one Khorn to the high school gym powered by 50 watts and got 100 dB SPL 30 feet away.

Edited April 6, 2016 by Don Richard

[babadono]

Yes the table claimed "reverberant field" not an anechoic chamber.

[babadono]

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel. Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

The following assumes your 2 watt amplifier has a maximum RMS output (unclipped, undistorted) of 2 watts. It also recognizes that with KHorns at 1 watt it is already LOUD:

You are listening to a 2 watt amplifier at 1 watt average....then a 3 db peak comes along and your amplifier puts out 2 watts for the duration of the peak. And a 3 db increase of sound intensity has you say

( subjectively)  'Oh it got a little louder there for a second'.

Now you are listening to a 2 watt amplifier at 1 watt average....then a 10 dB peak comes along and your amplifier attempts to put out 10 watts and clips the waveform. Your amplifier ran out of gas. You say something like 'well that sounded a little rough perhaps I better turn it down a bit'.

To have 10 db of headroom above 1 watt you need a 10 watt amplifier.

I do not understand where you get "so to get the extra 3 db for our 10 db swing", to get a 10 dB swing you need 10 dB.

[mustang guy]

 

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel. Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

The following assumes your 2 watt amplifier has a maximum RMS output (unclipped, undistorted) of 2 watts. It also recognizes that with KHorns at 1 watt it is already LOUD:

You are listening to a 2 watt amplifier at 1 watt average....then a 3 db peak comes along and your amplifier puts out 2 watts for the duration of the peak. And a 3 db increase of sound intensity has you say

( subjectively)  'Oh it got a little louder there for a second'.

Now you are listening to a 2 watt amplifier at 1 watt average....then a 10 dB peak comes along and your amplifier attempts to put out 10 watts and clips the waveform. Your amplifier ran out of gas. You say something like 'well that sounded a little rough perhaps I better turn it down a bit'.

To have 10 db of headroom above 1 watt you need a 10 watt amplifier.

I do not understand where you get "so to get the extra 3 db for our 10 db swing", to get a 10 dB swing you need 10 dB.

 

Please read post https://community.klipsch.com/index.php?/topic/163148-high-power-amplifiers-for-heritage-speakers/?p=2004025. 

 

When I first read the Dope/Hope article, I had to pause and ask myself "why a typical 3,000 cu ft listening room?" Then it came to me. Back when PWK was designing the khorn, the ceilings were much higher than today. Furthermore, a well respected peer, RH Bolt, came up with what could be considered the best listening room layout calculations in 1946. I believe the chart is talking about 2 speakers in mono. I believe that if you measured from the critical distance of 2.23' in a 20'x15'x10' room, that 1 watts into a pair of (.5 watts each) LaScalas would result in substantially higher than 104 db. Moving the mic further would reduce the measured db by inverse sq law based on the critical position measurement. I cited an audio blogger claiming 20db or higher gains from the CLP, but I don't know the calculations and other caveats for instance frequencies.

 

What I think is that in order to see what PWK is talking about, we have to consider a 20x15x10 room. We have to calculate how .2 watts shared into a pair of 104db efficient speakers can yield 100db. I hope someone will dust off thier slide rule and give that a go. 

Edited April 6, 2016 by mustang guy

[Fjd]

 

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel. Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

 

The following assumes your 2 watt amplifier has a maximum RMS output (unclipped, undistorted) of 2 watts. It also recognizes that with KHorns at 1 watt it is already LOUD:

You are listening to a 2 watt amplifier at 1 watt average....then a 3 db peak comes along and your amplifier puts out 2 watts for the duration of the peak. And a 3 db increase of sound intensity has you say

( subjectively)  'Oh it got a little louder there for a second'.

Now you are listening to a 2 watt amplifier at 1 watt average....then a 10 dB peak comes along and your amplifier attempts to put out 10 watts and clips the waveform. Your amplifier ran out of gas. You say something like 'well that sounded a little rough perhaps I better turn it down a bit'.

To have 10 db of headroom above 1 watt you need a 10 watt amplifier.

I do not understand where you get "so to get the extra 3 db for our 10 db swing", to get a 10 dB swing you need 10 dB.

 

 

 

It's basically a trick question as I’m probably mixing terminology on you somewhat and looking more at the “dynamic range” of a piece of music (e.g., difference between the softest passage and the loudest) in the context of a type of “peak to average capability” of the “in room” amplifier and loudspeaker combination thinking about acoustic SPL output that can be measured with a simple Radio Shack SPL meter rather than something like an amplifier test of “dynamic headroom” regarding how many watts were generated or needed before clipping to cover a 10dB peak over RMS power output using an 80-200 millisecond (or similar) tone-burst.

  

For example, while it may be slightly unrealistic, I’m assuming that the 104dB sensitivity of the Klipschorn before any room gain and one watt of amplifier power will give me 104dB of sound electrically; however, I will probably have 3dB of room gain for SPL output that I “hear” from placing the loudspeaker in a room where the corners, walls, ceiling, and floor of a room all reinforce the loudspeaker’s output.

 

There would be an additional 3dB output that I “hear” that is due to having two Klipschorn loudspeakers in the room rather than one loudspeaker and I was careful to keep considering the odd aspect of “loudness” in relation to “decibels” where one loudspeaker is producing a level of 104dB SPL at one watt of power, adding a second loudspeaker playing at the same level using one watt of power only increases the overall “loudness” by 3dB SPL (e.g., loudness does not double with two loudspeakers).  Essentially, the two Klipschorn loudspeakers in stereo produce a “loudness” level of 107dB SPL in the room with each being driven by one watt amplifiers.  Of course, I realize that I have technically doubled my power from one watt by now using a total of two watts for the two loudspeakers combined.

 

Now rather than a 10dB tone-burst, I want to produce a “realistic” peak sound level for some type of music that I’m familiar with.  For example, let’s say that I’m picking from a solo grand piano playing crescendos that can reach peak levels of 109dB SPL, I believe that a full orchestra in a concert hall measures peak levels of about 106dB SPL, and a rock group may have peak levels around 120dB SPL.  

 

Of course, I fully realize that my Klipschorn loudspeakers with a 2 watt per channel stereo amplifier, even factoring in the “room gain” doesn’t stand a snowball’s chance on a hot day in San Antonio of surviving realistic peaks of a rock concert and that is why I have other amplifiers.  However, it seems that I should be fine with the 106 dB peaks in relation to a full orchestra in a concert hall electrically and acoustically; and maybe I can experience the feeling of 109dB peak level of the grand piano crescendo with my Klipschorn loudspeakers and 2 watt per channel SET amplifier.   

 

I fully realize that my 2 watt per channel amplifier will only electrically drive the Klipschorn loudspeakers to 107dB (104dB plus 3dB for doubling the power from one watt per channel to two watts per channel) and most likely any attempt above the 107dB will result in clipping assuming that my amplifier does not have any “head room” of power to handle a very short 20 or 30 millisecond burst of 2dB in excess of the rated power; however, that 107dB should still give me the “perceived” loudness of the 109dB I wanted to experience in the room given the aspects of room gain that I have outlined above.

[moray james]

tell me does anybody know down to what frequency do Klipsch for example claim their rated efficiency to be accurate to with a fudged room gain? the mid horns and up might be good with two watts pushing them but the woofer is what sets the pace and the horns are adjusted to the efficiency of the woofer(s).

[Fjd]

tell me does anybody know down to what frequency do Klipsch for example claim their rated efficiency to be accurate to with a fudged room gain? the mid horns and up might be good with two watts pushing them but the woofer is what sets the pace and the horns are adjusted to the efficiency of the woofer(s).

 

 

I don't know the impact of horn loading on the woofer and I suspect it may be somewhat different for the Klipschorn vs the La Scala where a chunk of the La Scala frequency response is direct radiator vs horn loading, but I seem to remember that somewhere I had seen posted that 1W is ~ 97.7 dB SPL and 2.83V is ~ 101.7 dB SPL for the woofer.  Although, I don't know if it was in reference to the pre 1984 version with the lower fs or the post version with the higher fs.

[moray james]

so is it reasonable to assume that these figures are given for the crossover point between the woofer and the mid?

[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

 

 

 

The Nakamichi sure has turned into one of the classics that seems to have no problems with a 2 ohm load.  The following is from Tone Audio Publications.

 

 

Circuit Overview

 

The PA-7 delivers 200 watts per channel, uses no overall negative feedback (a Pass trademark), and boasts the Stasis section, which consists of a low-power voltage amplifier coupled to the current mirror bootstrap output stage to do all the heavy lifting required for high-power output.

 

Back in 1989, when Stereo Review featured the PA-7, writer Julian Hirsch measured the PA-7’s output at 253 watts into an 8-ohm load, 400 watts into a 4-ohm load, and 650 watts into two ohms. A quick look under the hood reveals why. A large 700 VA toroidal transformer and a bank of power supply capacitors totaling 132,000 µf proves the PA-7 means business. Such a setup made the PA-7 a perfect choice for demanding loudspeakers and, like the Threshold Stasis amplifiers, it ended up in many systems based around electrostatic speakers or Magnepans.

 

 

.

[Fjd]

All this technical talk is good for those that like to read these things all I truly know is that the "Little Sweetie" of Maynard's with it's 1.3 watts of SET power sounds plenty loud enough with my LaScala's. Much like when I asked an employee of mine years ago the difference of being drunk and high. His answer was "when you're drunk you're drunk and when you're high you're high. More educated responses would be many more words on the subject. 

 

 

It has been a good mental exercise and now I  think I finally did a better job of outlining the math in the post below as to why the low power SET amplifiers can drive Klipschorn, La Scala and Belle speakers to reference-type levels simulating the live event for certain types of music without clipping the amplifier.  All I can say is, "Klipsch Heritage – so much more than the sum of the parts."  

 

 

 

 

 

 

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel. Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

 

The following assumes your 2 watt amplifier has a maximum RMS output (unclipped, undistorted) of 2 watts. It also recognizes that with KHorns at 1 watt it is already LOUD:

You are listening to a 2 watt amplifier at 1 watt average....then a 3 db peak comes along and your amplifier puts out 2 watts for the duration of the peak. And a 3 db increase of sound intensity has you say

( subjectively)  'Oh it got a little louder there for a second'.

Now you are listening to a 2 watt amplifier at 1 watt average....then a 10 dB peak comes along and your amplifier attempts to put out 10 watts and clips the waveform. Your amplifier ran out of gas. You say something like 'well that sounded a little rough perhaps I better turn it down a bit'.

To have 10 db of headroom above 1 watt you need a 10 watt amplifier.

I do not understand where you get "so to get the extra 3 db for our 10 db swing", to get a 10 dB swing you need 10 dB.

 

 

 

It's basically a trick question as I’m probably mixing terminology on you somewhat and looking more at the “dynamic range” of a piece of music (e.g., difference between the softest passage and the loudest) in the context of a type of “peak to average capability” of the “in room” amplifier and loudspeaker combination thinking about acoustic SPL output that can be measured with a simple Radio Shack SPL meter rather than something like an amplifier test of “dynamic headroom” regarding how many watts were generated or needed before clipping to cover a 10dB peak over RMS power output using an 80-200 millisecond (or similar) tone-burst.

  

For example, while it may be slightly unrealistic, I’m assuming that the 104dB sensitivity of the Klipschorn before any room gain and one watt of amplifier power will give me 104dB of sound electrically; however, I will probably have 3dB of room gain for SPL output that I “hear” from placing the loudspeaker in a room where the corners, walls, ceiling, and floor of a room all reinforce the loudspeaker’s output.

 

There would be an additional 3dB output that I “hear” that is due to having two Klipschorn loudspeakers in the room rather than one loudspeaker and I was careful to keep considering the odd aspect of “loudness” in relation to “decibels” where one loudspeaker is producing a level of 104dB SPL at one watt of power, adding a second loudspeaker playing at the same level using one watt of power only increases the overall “loudness” by 3dB SPL (e.g., loudness does not double with two loudspeakers).  Essentially, the two Klipschorn loudspeakers in stereo produce a “loudness” level of 107dB SPL in the room with each being driven by one watt amplifiers.  Of course, I realize that I have technically doubled my power from one watt by now using a total of two watts for the two loudspeakers combined.

 

Now rather than a 10dB tone-burst, I want to produce a “realistic” peak sound level for some type of music that I’m familiar with.  For example, let’s say that I’m picking from a solo grand piano playing crescendos that can reach peak levels of 109dB SPL, I believe that a full orchestra in a concert hall measures peak levels of about 106dB SPL, and a rock group may have peak levels around 120dB SPL.  

 

Of course, I fully realize that my Klipschorn loudspeakers with a 2 watt per channel stereo amplifier, even factoring in the “room gain” doesn’t stand a snowball’s chance on a hot day in San Antonio of surviving realistic peaks of a rock concert and that is why I have other amplifiers.  However, it seems that I should be fine with the 106 dB peaks in relation to a full orchestra in a concert hall electrically and acoustically; and maybe I can experience the feeling of 109dB peak level of the grand piano crescendo with my Klipschorn loudspeakers and 2 watt per channel SET amplifier.   

 

I fully realize that my 2 watt per channel amplifier will only electrically drive the Klipschorn loudspeakers to 107dB (104dB plus 3dB for doubling the power from one watt per channel to two watts per channel) and most likely any attempt above the 107dB will result in clipping assuming that my amplifier does not have any “head room” of power to handle a very short 20 or 30 millisecond burst of 2dB in excess of the rated power; however, that 107dB should still give me the “perceived” loudness of the 109dB I wanted to experience in the room given the aspects of room gain that I have outlined above.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Circuit Overview

 

The PA-7 delivers 200 watts per channel, uses no overall negative feedback (a Pass trademark), and boasts the Stasis section, which consists of a low-power voltage amplifier coupled to the current mirror bootstrap output stage to do all the heavy lifting required for high-power output.

 

Back in 1989, when Stereo Review featured the PA-7, writer Julian Hirsch measured the PA-7’s output at 253 watts into an 8-ohm load, 400 watts into a 4-ohm load, and 650 watts into two ohms. A quick look under the hood reveals why. A large 700 VA toroidal transformer and a bank of power supply capacitors totaling 132,000 µf proves the PA-7 means business. Such a setup made the PA-7 a perfect choice for demanding loudspeakers and, like the Threshold Stasis amplifiers, it ended up in many systems based around electrostatic speakers or Magnepans.

 

 

.

 

 

Nelson Pass at his best. 

 

 

 

 

Nelson Pass sure has developed a nice legacy of amplifiers over the years and it doesn't matter whether we are talking about flea-watt power or gigantic brute force power, he certainly has done it all.  One of my early experiences was with using his white paper from 1981 for the mosfet modification to the old Harman Kardon Citation 12 amplifier and I was hooked.  I liked it so much that I also have a second mosfet modified Citation 12 that now has a complete restoration internally and cosmetically.  In addition, it sure seems like the Sony VFET part 2 project is coming close to fruition.   

 

 

 

 

 

 

.

Edited April 7, 2016 by Fjd

[Fjd]

 

 

Given the above, for 1W per channel for the stereo set-up, the "effective" SPL will be about 107dB SPL for the stereo application, so to get the extra 3dB for our 10dB swing from the 100dB SPL, the power would be doubled under the logarithmic formula to 2W per channel. Can the low watt SET owners now breath a sign of relief or does the analysis contain flaws?

 

The following assumes your 2 watt amplifier has a maximum RMS output (unclipped, undistorted) of 2 watts. It also recognizes that with KHorns at 1 watt it is already LOUD:

You are listening to a 2 watt amplifier at 1 watt average....then a 3 db peak comes along and your amplifier puts out 2 watts for the duration of the peak. And a 3 db increase of sound intensity has you say

( subjectively)  'Oh it got a little louder there for a second'.

Now you are listening to a 2 watt amplifier at 1 watt average....then a 10 dB peak comes along and your amplifier attempts to put out 10 watts and clips the waveform. Your amplifier ran out of gas. You say something like 'well that sounded a little rough perhaps I better turn it down a bit'.

To have 10 db of headroom above 1 watt you need a 10 watt amplifier.

I do not understand where you get "so to get the extra 3 db for our 10 db swing", to get a 10 dB swing you need 10 dB.

 

 

Please read post https://community.klipsch.com/index.php?/topic/163148-high-power-amplifiers-for-heritage-speakers/?p=2004025. 

 

When I first read the Dope/Hope article, I had to pause and ask myself "why a typical 3,000 cu ft listening room?" Then it came to me. Back when PWK was designing the khorn, the ceilings were much higher than today. Furthermore, a well respected peer, RH Bolt, came up with what could be considered the best listening room layout calculations in 1946. I believe the chart is talking about 2 speakers in mono. I believe that if you measured from the critical distance of 2.23' in a 20'x15'x10' room, that 1 watts into a pair of (.5 watts each) LaScalas would result in substantially higher than 104 db. Moving the mic further would reduce the measured db by inverse sq law based on the critical position measurement. I cited an audio blogger claiming 20db or higher gains from the CLP, but I don't know the calculations and other caveats for instance frequencies.

 

What I think is that in order to see what PWK is talking about, we have to consider a 20x15x10 room. We have to calculate how .2 watts shared into a pair of 104db efficient speakers can yield 100db.

 

I hope someone will dust off their slide rule and give that a go. 

 

 

 

 

I'm finding that there is a rational basis for most everything that PWK has written, but it seems to take a lot of searching and background reading to discern exactly what he was looking at.  Although, I just happen to know where we can find a slide rule.

 

 

 

 

 

 

 

 

 

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Edited April 7, 2016 by Fjd