Measuring Output Impedance

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Measuring Output Impedance

Рет қаралды 1,657

6 ай бұрын

In this video I follow up on a viewer’s comment that was posted on my “Impedance Matching” video (link below).
• Impedance Matching
The comment asked to demonstrate how you would go about measuring the Output Impedance of a laptop’s headphone jack.
Using a standard Multimeter, a resistor and a bit of ohms law, I demonstrate how this is achievable.
If you are uncertain about what impedance is, then you may find it useful to watch my “Impedance Explained” video here:-
• Impedance Explained.
I also make reference to Voltage Divider circuits, which were previously explained in my “What Are Voltage Dividers” video here:-
• What Are Voltage Divid...
For 3.5mm TRS Audio Jack breakout adaptor used in this video:-
amzn.to/3SUQyzQ
I also highly recommend the UNI-T UT61E Multimeter used in this video:-
amzn.to/49y6xt5
Topic Covered in this video:-
----------------------------------------------
Measuring Output Impedance
Impedance Matching
Ohms Law
Output Impedance
Input Impedance
Maximising output power
Multimeter and Multimeter Bandwidth.

Пікірлер: 34

@petergunn551 29 күн бұрын

i've found a more direct way of measuring audio output impedance. you feed an audio signal into the output of an idle audio amplifier with a known resistance in series with the UUT output and the test signal output.measuring with an oscope across the resistor yields a trace that shows the behavior of the output stage in all parts of the waveform, as well as measuring directly the output impedance. you can use this method in stereo amplifiers for instance for measuring the output impedance of a channel that has just been repaired. one way of doing this would be to connect a 5 ohm resistor between the two channel outputs, drive one channel to 10Vp-p, and measure the residual signal at the output of the second channel with the oscope. the resulting p-p voltage is equivalent directly to the output impedance, so, one mV=one milliohm. the reason for using a 5 ohm resistor in this case is it corrects for errors that could occur due to the output impedance of the amplifier driving the signal. this works with any amplifier with negative feedback (not with open loop amplifiers that can't control their output impedance), so the comparative resistor method may be more appropriate for open loop (cheap PWM like in laptop PCs) amplifiers. i've written a paper on the subject of measuring the output impedance of audio amplifiers... PM me if you are interested.

@electrocat9 6 ай бұрын

This video is good so we know when laptop's output is set to line or headphones, I'm always not sure how it is set but i know it can change from software.

@GKB-vn3ns 6 ай бұрын

Excellent explanation Thanks

@paulpkae 6 ай бұрын

Many thanks for your comment. It's very much appreciated.

@paulperano9236 5 ай бұрын

So if the impedance is a complete mystery you could measure the output with you DMM. Then get a low-med value potentiometer, (500r-5k) across the output. Start at max resistance and reduce it until it has half the voltage across it. Disconnect and read the resistance value.

@paulpkae 5 ай бұрын

Hi. Yes that's an alternative method. I would say that it's best to know what sort of load impedance is typical and keep your test load within close range to that.

@Growler-F18 6 ай бұрын

Maximum power transfer. 😊

@csak3532 4 ай бұрын

I enjoyed the video! Could you explain in a video why they use percentage of impedance instead of just henrys. Thanks!

@paulpkae 4 ай бұрын

Thanks for your comments. I will add that to my growing list of requests. I assume you deal or work with transformers?

@csak3532 4 ай бұрын

@@paulpkae Yes! I deal with 3 phrase transformers.

@bob_mosavo 6 ай бұрын

Thanks 👍

@bob_mosavo 6 ай бұрын

I'm a bit skeptical about the use of the splitter. Perhaps at 1kHz, it may not matter much. In transmission lines, if you have that configuration at RF, you would likely get different values, since you've added a long transmission line that is essentially open (very high impedance) and, thus, would reflect the signal back. Anyway, this could be the subject of another video 😉 I'd be interested to see if the method used in this video differs much from the method used in the video "#138: How to Measure Output Impedance" from the channel "w2aew". Let's get nerdy 🤓

@bob_mosavo 6 ай бұрын

@@jim9930 Excellent! Let's do it!

@bob_mosavo 6 ай бұрын

@@jim9930 I would love to and have time, but I live several states away ☹

@paulpkae 6 ай бұрын

@bob_mosavo Thanks for your comment, I may indeed do a further follow up. In the meantime I can confirm the splitter cable makes no noticeable difference in the results. I have also measured the dummy load with and without the splitter cable using an impedance meter @1khz and get the exact same readings. It also shows 0 degrees phase angle suggesting there's no detectable parasitic capacitance or inductance imposed by the short 100mm cable (or 4inches in American money) at this frequency.

@bob_mosavo 6 ай бұрын

@@paulpkae Yeah, I was thinking that 1kHz might have a wavelength too long to be noticeable using that length of cables. I haven't done any work in this area in a while. And as an American, I wished we would switch to the metric system, but I don't think that will occur in our lifetimes. Folks like their base-two fractions too much 😉

@TR3A 5 ай бұрын

This was very informative and helpful to me. Thank-you.

@electrocat9 6 ай бұрын

Thank you!

@electrocat9 6 ай бұрын

My take would have been more empirical then logical. I would have connected a potentiometer to the output I want to measure its impedance, I would have started the music, I would have connected an oscilloscope and I would have adjusted the potentiometer until I saw a maximum and then I would have measured the potentiometer value.

@paulpkae 6 ай бұрын

@@electrocat9 That's certainly another method. You would need to be careful about your potentiometer low resistance range, also for those that don't have an oscilloscope to hand, most standard multimeters are suitable for measuring RMS @ 1khz. Using music as a test source would be very difficult to gauge, you really need a constant level output.

@electrocat9 6 ай бұрын

@@paulpkaeYou just need a voltmeter, that's so cool. I'm not aware if analogic ones can measure 1khz.

@electrocat9 6 ай бұрын

It seems that impedance varies with freq, then at 16khz you need another headphone or the difference can be negligible, just thinking

@paulpkae 6 ай бұрын

@@electrocat9 impedance does indeed vary with frequency. Frequency directly relates to the "Reactance" caused by inductance and capacitance. Speaker and amplifier manufacturers go to great lengths to try and make their products deliver a flat frequency response so that the sound reproduction is as faithful to the original source as possible (fidelity).

@Paulscole3 3 ай бұрын

Ok, another video for you. The number of blown home theater subwoofers out there is insane, and a majority of them it's the amplifier board or switching power supply that's gone bad. Online, there are a ton of amplifier boards that people are itching to try, but they all require different power requirements coming from the pre-amp board on the subwoofer. How do you measure the pre-amp output when there's no power going to it because the power usually goes through the power amp, into the pre-amp, and then back to the power amp and then to the sub. Most subwoofers have a 5 to 8 pin connector that runs between the pre-amp and power amp, and I know a ton of people would love to know how much their pre-amp puts out so they can buy the power amp that will go with it.

@newmonengineering Ай бұрын

You can't really measure it unless it is running. To measure it you need to have a wire at the preamp. You can measure a 1khz wave voltage on the preamp line. Then use a potentiometer on the preamp line to ground and measure the voltage until it's 1/2 the original voltage. Then turn it off, and disconnect the potentiometer without moving the setting and measure the resistance of it. Whatever the potentiometer ohms is, the preamp is the same. It's basically a voltage divider. And when the voltage is exactly 1/2 the ohms are equal on both sides. Hope that helps.

@debojitacharjee Ай бұрын

*BUT HOW TO FIND THE OUTPUT IMPEDANCE WITHOUT KNOWING THE INPUT?*---I need to know how I can find the matching speaker impedance for an audio amplifier? If I measure the voltage of the output then how I will find out the ohms of the matching speakers?

@paulpkae Ай бұрын

Use any resistor value that is higher than what you anticipate the normal load impedance would be. Using a higher value will have less chance of causing any damage. If its a domestic amplifier, then the load impedance is usually expected to be either 4 or 8ohms.

@debojitacharjee Ай бұрын

@@paulpkae But what if I don't know the output impedance of an amplifier? BTW, how manufacturers find the impedance of their amplifiers while designing them? If I meanure the resistance across the output connectors of an amplifier using a multimeter, then would that be it's impedance?

@paulpkae Ай бұрын

@@debojitacharjee No, it would not, and don't ever put your mustimeter, set to ohms, on an output of anything. An example method to measure output impedance is shown in the video. If you are uncertain, do not attempt it. Manufacturers would know in advance the expected output impedance of their device. It's part of the design process. And, they would have their own in house mechanisms and procedures for measuring.

@debojitacharjee Ай бұрын

@@paulpkae So, you mean to say that there is no way one can find out the output impedance of an amplifier (if we can't guess the load resistor for testing)?

@paulpkae Ай бұрын

@@debojitacharjee using the method in the video you should try and use a load that's representative of the normal load conditions so as not to cause damage to the device under test. As a fail safe, go higher impedance, i.e. less load. My apologies, but there are many more methods, but not something to be described in a comment response. If you have no idea about the order of magnitude of the load, then I would suggest you don't play around.

@matthewcooke7182 3 ай бұрын

would this work for measuring guitar pedal output impedance?

@paulpkae 3 ай бұрын

Hi. I am no expert on Guitars or pedals, but the principles of measuring output impedance using the method I demonstrated can be applied to most devices. Ensure you have a rough idea of what the impedance is expected to be before you go adding a dummy load. I would suggest you make the dummy load to be at least one order of magnitude higher (i.e. 10X) than the expected output impedance. Also, bear in mind that the impedance may vary through the frequency range and/or any gain controls you have.