Does anyone care to ask an amplifier designer a technical question? My door is open.

Ramtubes 11-26-2018:

HI, Roger here with a question.

I would like to hear how each of you figured out how much power you needed to buy in watts?

I would appreciate the following information in your response.

Your listening level LOUD SPL (preferably measured at 1 meter from the speaker with a REAL SPL meter. Your low listening level. If you are using a cell phone app then you have confirmed it?

Your speaker sensitivity?

Listening Distance from speakers?

How many watts at your load is the amp is rated to supply?

As I mentioned earlier in the thread I listen to a lot of classical symphonic music that has been engineered with minimal or no dynamic compression. Two such recordings, which I believe have just about the widest dynamic range in my collection, are Stravinsky’s “Firebird Suite” on Telarc (Robert Shaw conducting the Atlanta Symphony), and Prokofiev’s “Romeo and Juliet” (excerpts) on Sheffield Lab (Erich Leinsdorf conducting the Los Angeles Philharmonic). I have examined the waveforms of those recordings using a professional audio editing program (Sound Forge Pro), and by doing so I have found the difference in volume between their loudest notes and their softest notes to be approximately 55 db, which is (to me) amazing.

Correspondingly, at my 12 foot listening distance I have measured peak SPLs on those recordings of close to 105 db, with the softest notes being in the vicinity of 50 db. I used a Radio Shack digital SPL meter for these measurements, set for C-weighting and fast response.

My speakers (Daedalus Ulysses) are rated at 97.5 db/1w/1m, and have a very flat impedance curve with a specified nominal impedance of 6 ohms. My 12 foot listening distance corresponds to 3.66 meters. Putting aside room effects for the moment I assume that SPL produced by a box-type dynamic speaker such as those falls off at 6 db per doubling of distance, which means an 11 db reduction going from 1 meter to 12 feet. I conservatively add in 3 db to reflect the presence of two speakers (as I understand it that figure will actually be closer to 6 db at my centered listening position when both speakers are producing similar signals), and I add in perhaps 3 db for “room gain.”

97.5 -11 + 3 + 3 = 92.5 db at the listening position for 1 watt per channel. Let’s call it 93 db.

I add in about 3 db of margin to the 105 db I want my amp/speaker combination to be able to produce at the listening position. So the required minimum amplifier power (into 6 ohms) is:

105 + 3 - 93 = 15 dbW (decibels above 1 watt)

15 dbW = 32 watts.

To answer your question about my amplifier, for several years prior to just recently I was using a VAC Renaissance 70/70 MkIII, rated at 70 wpc. I recently changed to a Pass XA25, which is specified as a class A amplifier rated at 25 wpc into 8 ohms and 50 wpc into 4 ohms. Per JA’s measurements, though, it is capable of 80 and 130 wpc into those impedances. I presume that most of that increase represents the amp’s capability after leaving class A, although per JA’s comments some of the increase apparently reflects differences in the distortion percentages the ratings and measurements are based on.

Best regards,

-- Al

Thanks again, Roger.

Cary has had notoriously bad measuring amplifiers for as long as I can remember. They have long been the poster-boy amps when people want to argue the "measures awful, but sounds great" stance. Personally, it’s been far too long since I last heard a Cary amp for me to have an opinion on the sonic performance.

I just read the Stereophile review of your amp. Congratulations, it really does impress upon the reader that you know what you are doing!To see JA actually impressed with the measurements of a tube amp is really something.

As for this comment you made to someone else:

I would like someone to tell me why spend big bucks on cone speakers when there are better technologies.

I’m wondering which "better technologies" you are referring to.

Do I infer correctly from your comment to me about the QUAD 57 vs the Alan Jones monitor, that you are speaking of, for instance, electrostatic speakers?

If so, I can tell you why I prefer cone speakers.

I first fell in love with electrostatics in the 90’s and first owned the Quad ESL 63s. As did another audio pal. The shock of not hearing any box sound, and that amazing transparency and "hearing in to the recording" sensation were at first intoxicating. But after a while I found the sound too disembodied, too ghostly, like peering through a window in to another room where the music was happening, but it wasn’t "moving air" in the room I occupied. When I’d play my little old Thiel 02 monitors there was such a difference in palpability, aliveness and "thereness" that it just re-enforced what I was missing. That was the case even after I added the Gradient dipole subwoofers to the Quads. Still among the best sub/panel matches I’ve heard.


I still love to "visit" electrostatics (the Quad ESL 57s being my favorite), for their unique qualities. But every time I listen to an electrostatic, of any make, I come away happy to have moved on to cone speakers.That includes every hybrid I’ve ever heard: The cones seem to add some body, but only within their frequency range. As the frequencies climb up to where they are handled by the panel, the sound character changes to my ears to that ghostly quality, so I am always aware of this discontinuity.

I’m not sure if this problem is solvable. Though, one brand I find intriguing is the JansZen speakers, using the electrostatic panels in more of a sealed box design. I wonder if those might maintain the palpabiity and body in the sound, but I’ve never heard them.

Ramtubes,

My bad. I rarely go to shows. I meant showrooms. Audio store showrooms I presume are set up to show the gear at some approximation of optimability.

By “more power” I’m just going by where I set the volume control before and after. It was a surprise, as I thought there would be less, and I was pleased because in my layman’s view it meant less stress on the tubes and circuitry.

There wasn’t much more volume, as the goal was simplicity, as you say; stability, as I have observed; and even more transparency in the mids, as I have also observed, though the Jensen caps also had something to do with the transparency eventually.

So, if you are building OTL triodes too, Jon is not correct. There are more than three. But are you using the same design approach? You guys should talk to each other!

Also, I’m not sure what you mean by better technology than cone speakers. First off, the Altec is a horn/cone hybrid. That aside, while electrostatics have their virtues, they don’t move air the way a cone does, as someone here also observed. That’s what I mean by the dynamic punch of the Altecs. I’ve heard some great horn speakers, with prices in the stratosphere. 

The serial number on my Altec dates them as 1954, the year I was born, lol. If it ain’t broke...

All OTL amps like high impedance because they have lots of voltage but limited current. Since current is the limit use the forumla

Power = current squared x impedance. The amplifier max current is the same for both speakers but 16 ohms gives you twice the power of 8.

@ramtubes
Roger, I think if you revisit the above comments you will find them to be incorrect. An OTL has to be able to drive real world loudspeakers and so can produce the same currents at the output as any other amplifier. FWIW, the output tubes in most OTLs can easily blow a 10 amp fuse in certain situations without damage to the tubes.

Your use of the formula is not accurate. As an example (I think you have clio9's M-60s on hand) look at the output power at clipping into 4, 8 and 16 ohms. Now this is a smaller OTL, but I think you can see that its output power does not behave as you stated above. The half power of 16 ohms occurs at 4 ohms, not 8.

Two versions of the Futterman circuit. East coast/West coast. Harvey was a hoot. I visited him and his gang around this time. Sadly or not, he folded too. Is there a Futterman curse?

There most certainly is! Every manufacturer that has made a Futterman amplifier has had to go out of business. This is because there is more to the circuit than meets the eye, and no-one was able to do the execution such that a reliable amplifier resulted. The reliability problem is what drove them out. Of course, there are notable exceptions- the Fourier company simply under-rated parts and used sloppy construction, to the point where it would not have mattered what they made- they would have failed and gone out of business anyway. Harvey used surplus and unreliable capacitors in his amps (unreliable because they were not used in the right application).

Since I dont believe wire has a sound I prefer colors. I use all 9 colors in my amplifiers. With colors you can actually start to see the circuit without a schematic.

If you look carefully you will find some long wires repeated so you could simplify things there.

Our  wire is custom-built. Because of that we have to buy a lot of wire at any one time. The long wires repeated is so that all the power tubes have exactly the same series resistance involved (although the cathode resistors dominate that aspect) and transmission line effects are minimized (the output section has bandwidth to several MHz so stability is important). Once you understand how the dielectric behaves you find no need for Teflon.

If my amps are being run at levels that do not bring on distortion, why do they still have that classic "tube-amp" character even at those low listening levels? If it’s not the clipping characteristics that are coming in to play...what is it that produces that classic tube sound as I described it?

@prof
The fact of the matter is that the amps make audible distortion, which is the coloration you hear. Below a certain low power level they can often be making more distortion than at higher levels!

I have a 3 disc LP set of Theodorakis’s “Canto General” that Atmosphere produced, and I found the engineering to be disappointing. It has been muddy, and there has been no bass to speak of.

@unreceivedogma
There is plenty of bass on that recording! We had the biggest bass drum in the state at the time. But it is very deep, and some systems don't play it very well. Since I recorded it, I can use it as a reference and I can tell you that many tone arms don't play that bass right either.