How Amplifier Output Power is Measured

well greetings out there in YouTube land in today's video I'm going to try to explain to you how the output power of amplifiers is measured I've had numerous requests to show how this is done so I thought today's the day hey rusty I see you lurking there in the workshop want to play chase the tennis ball I can tell you're anxious here it comes that's it boy fetch good dog oh now you're supposed to bring it to me Oh guess that's the end of the tennis ball although it is a fairly simple procedure it does take some special equipment that a lot of people may not have but regardless I will go through the process and explain it step by step and hopefully you from this day forth will always know how amplifier power ratings are determined step 1 since amplifier output powers measured at a specific frequency you have to have a frequency generator like this to create an input a say 1000 cycle per second signal into the input of our amplifier the amplifier in this case will be a new compander model e 20 with 2 6 bq5 output tubes which we expect to produce around 15 to 17 watts of output since you have to have some sort of load attached to the output of the amplifier and we don't want to use a speaker because a 1000 cycle per second tone at full volume would not be a pleasant experience we're going to use what is called a dummy load this is an 8 ohm 100 watt heat dissipating resistor so the output of the amplifier is going to go to this instead of to a speaker so we're going to have three different things attached here to the terminals of this 8 ohm load number one will be the output from the fire number two will be our DC voltmeter and number three will be our oscilloscope channel one input of probes and finally we will need an oscilloscope to monitor the output signal from the amplifier since we're putting in a 1000 cycle per second signal we expect to see the same frequency of signal on our scope except that the amplitude will be increased by the amount of amplification that occurs in the Neukom pathfinder circuit we're going to crank the volume up on the app until the output signal begins to clip at the top and bottom showing that significant distortion is occurring then we're going to back off a little bit until we get a nice smooth on output signal with no clipping and then we will measure what the output power is at that volume setting well that makes sense then let's get started and even if it doesn't make sense I think it will after you've seen the procedure performed before your very eyes hey rusty I see you lurking there in the workshop you want to play chase the tennis ball okay here it comes that's it boy fetch go get it okay don't go get it just take it easy and see you're out of breath from all that exertion okay let's turn on the app and get it started warming up at a volume level right now zero let's turn on our signal generator and set it to 100 times 10 which will be a thousand cycles per second that it will input into the microphone input of the Neukom amplifier ok we've turned on the oscilloscope and it's warming up we see that we've got our baseline set right here on the x-axis right to the horizontal center of the screen I've set the horizontal axis at 2 milliseconds per division that means that from here to here represents 2 4 6 8 10 milliseconds or 1/100 of a second since it's 1/100 we have to multiply the number of Peaks we see here times 100 to find out the frequency and you'll find that there are 10 Peaks this now is 1,000 cycles per second here is 1,000 cycles per second so we have our input generator doing its job and the amp is putting out a nice clean 1,000 cycle per second output now as I turn up the volume on the amplifier we'll see that the amplitude will increase dramatically on the scope now we're going to crank the volume of the amplifier up to the highest setting we can before distortion starts to sit in you can see that the curve is starting to act up right at the midpoint so we're going to back off a little bit until we have a nice smooth output of our 1000 cycle per second signal we also see that the volume control is pretty close to maximum output and when we look at our AC voltmeter we see that there's 11.7 three volts of rms output power being applied to the 8 ohm load which is getting very very warm and we can read the peak voltage output on our oscilloscope screen at around say 16 a shade over 16 volts peak but I believe the more accurate measurement is going to be made in this case with the AC voltmeter now using Ohm's law we know that power and watts is equal to voltage squared divided by resistance the power then is equal to eleven point seven five volts RMS squared divided by eight that's one thirty eight point zero six divided by eight or seventeen point two six watts of RMS output which is very impressive I think for a pair of 6bq fives now let's look at the calculation if we'd use the oscilloscope measurement of sixteen point five volts peak first off you have to multiply times 0.707 to reduce it to its RMS value of eleven point six seven then square it divide it by eight and we get 17 watts RMS output using the oscilloscope measurement or 17 point two six watts RMS output using what I think is probably more accurate which was the voltmeter one final point and this is very important what if I had used the peak output from the scope of sixteen point five volts and not corrected it okay I squared that number and it's two seventy two point two five divided by 8 ohms thirty-four watts of peak output it's exactly double so if you're ever buying an amplifier be sure that you understand whether they're quoting you RMS output values or peak output values if it's pink you can divide it by 2 to get the rms output which is really the normal and most ethical way to report the power output from an amplifier and it should be also noted that these computations are only correct at 1,000 cycles per second at 2000 or 3000 cycles per second the output of may differ just for fun let's see what the output is at 2000 cycles per second well it's a wee bit lower remember it was eleven point seven four or seventy-five at 1000 now I changed the seconds per division here but this is 5000 cycles per second let's see what sort of output we get well see it's dropped down even more so now you I think see why when you're quoted a power output value for your amplifier you should know whether it's rms or peak and also at what frequency the higher the frequency the lower the output and finally because I can't resist here's ten thousand cycles per second you see we dropped way down so there's no doubt about it the power output is indirectly proportional to the frequency okay it's time for another rusty taste test already down here on the ground I have placed some of his regular food and now over here I'm going to place some extremely expensive gourmet food it costs about three or four times as much let's see which he prefers well it appears it's going to be the gourmet food unfortunately hey rusty how about that cheap food how about this stuff down here the cheap food rusty come on Oh Lord well that's about it for this video I hope you enjoyed it and that it was informative I was curious about the lack of distortion in the Pathfinder output even at extremely high volumes until I remembered the it has an ultra linear output transformer if you check back in my video list I have already posted a video in which we go through the Pathfinder from one end to the other and listen to it and I think you'll find an explanation of the ultra linear output transformer to be interesting and informative too so please check it out if you get a chance meanwhile thanks so much for watching and bye for now hey rusty I see you lurking there in the workshop door you ready to play chase the tennis ball okay here it comes get it boy that's it or not well I guess that's it for chasing the tennis ball

Channel: Uncle Doug

Views: 30,628

Rating: 4.9673023 out of 5

Keywords: Amplifier Output Power, RMS, Peak, Frequency

Id: 38bj6-MSx58

Length: 11min 16sec (676 seconds)

Published: Thu May 19 2016