# How do I calculate expected output voltage of car audio amplifier with given conditions?

I have a 4-Channel Amplifier rated at 800 Watt RMS per channel @ 2 Ohm load. Channel 3 and 4 are preset to operate a subwoofer speaker while Ch 1 and 2 are set to operate mid-range speakers at reasonable operating frequencies:

• Ch 1: Has a 2 ohm load (2 speakers @ 200 Watts RMS each wired in parallel) which is less than 800 Watt max for the channel
• Ch 2: Has a 2 ohm load (2 speakers @ 200 Watts RMS each wired in parallel) " "
• Ch 3: 2 ohm load (One 4-Ohm Dual Voice Coil Subwoofer rated at 800 Watts RMS and wired in parallel) - amp should be able to produce ~800 Watts RMS of power to this Subwoofer speaker that is also rated at 800 Watts RMS
• Ch 4: Currently has nothing connected to it

First, is this the right way to calculate expected maximum output voltage per channel? (Assume no speakers are connected)

• Ch 1: Power=(V^2)/Resistance -> V = sqrt(Power*Resistance) = sqrt(800*2) = sqrt(1600) = 40 V (I am expecting 40 V at the output terminal of this channel)
• Ch 2: Is wired the same way as Ch. 1
• Ch 3: Is wired the same way as Ch. 1

When I ACTUALLY measure the AC voltage output I get the following on a voltmeter that is set to AC Voltage and speakers are connected:

• Ch1 reads [0V-6V] as I tune its gain from [nothing to maximum] at 1000 Hz tone
• Ch2 reads [0V-6V] as I tune its gain from [nothing to maximum] at 1000 Hz test tone
• Ch3 reads [0V-27V] as I tune its gain from [nothing to maximum] at 50 Hz test tone

As you can see, all of the voltages read are less than what is expected if I am correct. I am in fact getting sound from my entire system but I feel that I am not getting the maximum output before distortion for my subwoofer. In other words, I feel that I am not getting 400 Watts out of the subwoofer @ 50 Hz test tone. All power ratings given are in RMS.

• Is the value read from AC voltage on the multimeter Voltage peak to Peak, Amplitude, or RMS Voltage?
• Is my method for calculating expected voltage per channel correct?

I can't find any resources online to help exactly help me figure this out. Any help would be much appreciated. Please let me know where I could be going wrong with the mathematics, wiring or what the right way to set this system up might be.

Thanks.

• The word "RMS" means you missed a $\sqrt{2}$ factor. It's not $40\:\text{V}$ but instead closer to $57\:\text{V}$. And that's just the peak, not the peak-to-peak.
– jonk
Apr 17, 2019 at 2:21
• Which amp, and how are you powering it? What model is your multimeter? How do you know that 'maximum' is the actual maximum 'before distortion'? Apr 17, 2019 at 2:48
• @jonk - when I read all of specifications of all of the speakers, subwoofers and amp, I used all of the RMS values in my calculations. I knew that the max power each device would produce would not be close to that indicated max power on the box. My question further down explains how the range of values I am getting at certain test tones, which are no where near what I am expecting (40V). Apr 17, 2019 at 3:34
• @Bruce Abbott - what do you mean which amp? I provided all of the specifications at the top. The maximums I have been calculation we’re “expected voltages” I am not quite sure if those are before distortion because I do not have an oscilloscope to see the sine wave. I go based on my ear hearing the test tone with no clipping at max gain and I obtain the voltage ranges specified above. I am trying to operate the speakers and subwoofers just before they distort by tuning the gain as high as possible just before hearing the clipping. Apr 17, 2019 at 3:36
• @supermario Your post was tl;dr for me. I just wanted to point out something simple, which is that $V=\pm\sqrt{2\cdot R\cdot P}$, where $P$ is the maximum power and $R$ is the load. Normally, because of the probability distribution function for typical music, you buy an amplifier that is rated for perhaps 5 or 6 (or even 10 times, perhaps) the power you actually will use, in practice. I don't have much to add as I haven't actually read carefully everything you wrote. So just be aware of my ignorance about your situation. I did see your 40 V calc, so wanted to comment on that alone.
– jonk
Apr 17, 2019 at 3:43