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
