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.\u00a0 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.<\/p>\n
So, 1.9 W of power makes 86 dB SPL at 1 meter. That’s lowish efficiency for a speaker.<\/p>\n
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<\/strong> from both speakers. Plus 2.1 for headroom makes 114 dB SPL peak.<\/p>\n 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n NOTE:<\/strong> 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.<\/p>\n 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.<\/p>\n 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\u00a0 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.<\/p>\n 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?<\/strong><\/p>\n 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<\/strong>. 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.<\/p>\n 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.<\/p>\n 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?<\/p>\n 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.<\/p>\n But the digital source is often attenuated, for example to apply EQ. If it’s attenuated by 6 dB this halves the voltage, so now the preamp must produce twice the voltage: 2.84 V unbalancd or 4.0 V balanced.<\/p>\n 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.<\/p>\n 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.<\/p>\n The Soul has a maximum output of 8 V RMS balanced for a full scale digital signal (if that is based on a sin wave, it is 8 * sqrt(2) = 11.3 V peak). My Room EQ (July 2025) applies -3 dB, which is a ratio of 0.71, which makes about 5.6 V. This is more than enough to drive the Adcom into clipping.<\/p>\n At what volume setting is max output achieved? 2.55 V input to the Adcom 5800 produces peak power into 4 ohms: 51 Volts \/ 12.75 amps \/ 650 watts. We have 5.6 V at max volume, so reducing it to 2.55 is -6.8 dB. This is the 49th of 64 volume positions, about 2:30 clock position.<\/p>\n Max continuous power would be at 40 Volts \/ 10 amps \/ 400 watts. Dividing by the Adcom gain ratio 19.95 gives 2 Volts. With 5.6 V at max volume, this is -8.9 dB. Sanity check: this is 2.1 dB less than peak, and the Adcom 5800 has 2.1 dB of headroom. This is the 43rd of 64 volume positions, about 1:30 clock position.<\/p>\n In reality, the Soul can be cranked up louder without audible distortion on the speakers. The digital domain test CD has a dynamic range test track and it can go up all the way to max, peaking at -1.3 dB. That would be 5.6 V * 0.89 * 19.95 = 100 Volts \/ 25 amps \/ 2,500 watts. Of course there’s no way it’s actually producing this. So either the Soul doesn’t produce as much voltage as this, or the Adcom 5800’s gain ratio is less than the specified 26 dB.<\/p>\n","protected":false},"excerpt":{"rendered":" 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.\u00a0 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 = … Continue reading How Loud Does it Get?<\/span> What Input is Required?<\/h3>\n
Example: Corda Soul<\/h3>\n