Magnepan 3.6/R specs don’t give efficiency, but they give voltage sensitivity. That’s 86 dB @ 500 Hz @ 2.83 V. From this we can determine efficiency. 500 Hz is carried by the midrange panel which has 4.2 Ohm impedance, so 2.83 V drives 2.83/4.2 = 0.674 A of current, which makes 2.83 * 0.674 = 1.907 Watts.
So, 1.9 W of power makes 86 dB SPL at 1 meter. That’s lowish efficiency for a speaker.
The Adcom 5800 is rated at 400 W continuous in each channel with 2.1 dB of headroom. 400 W is 10 * log (400 / 1.9) = 23 dB louder than 1.9 W, which makes 86+23 = 109 dB SPL in each speaker. 2 speakers is twice the power which is +3 dB making 112 dB SPL from both speakers. Plus 2.1 for headroom makes 114 dB SPL peak.
At this power level, what is the voltage and current? We have 400 W into 4 ohms, which is 40 Volts and 10 Amps. That’s per channel.
I’m ignoring distance mainly because (A) dispersion is line source not spherical so it decays less with distance and (B) it’s in a room so some energy is not lost but reflected back, and (C) listener position is close, only about 2 meters from the speakers.
Subjectively, I can say this is VERY loud. Over the 26 years I’ve owned this amp I can count on the fingers of 1 hand the number of times I’ve seen its yellow 1% distortion warning lights briefly flicker during a transient peak.
NOTE: I tested this last night by holding an SPL meter while listening to a test CD. A full scale (0 dB) digital signal, passing through my preamp (Oppo HA-1) with volume at 0 dB measures 104 dB SPL at the listening position. The power amp (Adcom 5800) warning lights do not even flicker. The preamp goes up to +6 dB output, which would be 110 dB SPL. That’s pretty close to the theoretical measurement–within 2 dB.
That 2.1 dB of headroom means peak power is 10^(2.1/10) = 1.62 times higher than continuous, making 400 * 1.62 = 648 Watts.
Also we can sanity check the amp’s overall efficiency. The 5800’s max continuous power draw is rated at 1800 VA (Watts). While delivering 800 W to a pair of speakers (400 per speaker), that’s 44% efficient. It’s actually less efficient at lower volumes because it’s biased to run in symmetric class A up to about 10 Watts output. The max theoretical efficiency of class A is 25%. It’s rated to draw about 250 W when idle.
Next question: if the Adcom 5800 operates in symmetric class A up to 10 W, how loud can it play these speakers while in class A, before transitioning to class AB?
From above, the speakers play at 86 dB SPL when consuming 1.907 watts. 10 watts is 7.2 dB louder, plus 86 = 93.2 dB SPL. That’s per side, so +3 dB makes 96.2 dB SPL. That’s very loud. But most likely, the transition from A to AB depends on voltage not current so the power level will vary depending speaker impedance.
Double-check the answer: 400 watts is 16 dB louder than 10, so add 16 dB to 96.2 and you get 112.2 dB. The math checks: same answer as above.
What Input is Required?
As mentioned above, the Adcom 5800 max continuous power both channels driven into 4 ohms, is 400 W, which is 40 V and 10 A. What voltage output from the preamp is needed to achieve this?
The Adcom 5800 gain is 29 dB with unbalanced inputs and 26 dB with balanced. These are ratios of 28.18:1 and 19.95:1 respectively. So the input voltage needed to achieve those output levels is 40 V divided by these ratios. That is 1.419 V unbalanced, or 2.0 V balanced. This is also known as the Adcom’s input voltage sensitivity.
However, those are continuous ratings and the Adcom has 2.1 dB of headroom, and 2.1 dB is a voltage ratio of 1.274:1. So to really crank it to max rated levels you’d need 1.274 times that voltage, which is 1.78 V unbalanced or 2.55 V balanced.
Let’s do the math the other way to double check. 2.55 V input, times 19.95:1 gain ratio is 50.8 V. Into a 4 ohm load draws 12.7 A, and the product of voltage and current is 50.8 * 12.7 = 645 Watts. This jives with the Adcom 5800’s rated peak power of 648 Watts.