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How do I measure output impedance?

I'd like to know how to measure output impedance on my (and friend's) devices, specifically for headphone gear, and the headphone output on various receivers. I think I have all of the tools that I would need, but if someone could give the down low on what I would need as far as equipment, and what tools I would need to measure the output impedance as accurately as possible, that would be amazing. Also, I assume that I would need to actually be playing something through the headphone out, so what would be the best sound to play?

Thanks!

How do I measure output impedance?

Reply #1
A quick search for:

measure output impedance
measure output impedance of amplifier

Will give you many sets of complete instructions, some with videos.
Kevin Graf :: aka Speedskater

How do I measure output impedance?

Reply #2
Thank you! I was just a bit worried, as I recently saw another thread here that said that measuring output impedance by hand can be inaccurate, and I'd rather be as accurate as possible.

How do I measure output impedance?

Reply #3
Thank you! I was just a bit worried, as I recently saw another thread here that said that measuring output impedance by hand can be inaccurate, and I'd rather be as accurate as possible.


https://en.wikipedia.org/wiki/Output_impedance

You solve for Zs (output impedance) and Vs (open circuit voltage) in that diagram by using two different load impedances so that you have two knowns and two unknowns and can do the algebra.  FWIW, I'd measure the open circuit voltage (no load at all, which gives you Vs), and then a relatively large load (16 ohm for headphone amp, 4 ohm for power amp).  The accuracy will be limited only by how well you can measure the voltage and impedance, which with even a $25 multimeter should be very accurate.  For best results, use a resistor, not a pair of headphones as the load.

 

How do I measure output impedance?

Reply #4
You need: 60 Hz sine test signal, digital multimeter, dummy load.

Measure the voltage unloaded and with dummy load.
Calculate:
Zout = (Rload * (Vnoload - Vload)) / Vload

Example:
We measure 1V unloaded, 0.9V loaded and our dummy load is 30 ohm ==> (30 * (1 - 0.9)) / 0.9 = 3.33 ohm output impedance at 60 Hz.
"I hear it when I see it."

How do I measure output impedance?

Reply #5
I'd like to know how to measure output impedance on my (and friend's) devices, specifically for headphone gear, and the headphone output on various receivers. I think I have all of the tools that I would need, but if someone could give the down low on what I would need as far as equipment, and what tools I would need to measure the output impedance as accurately as possible, that would be amazing. Also, I assume that I would need to actually be playing something through the headphone out, so what would be the best sound to play?


We just lately covered that here but what thread, what forum?

Here's a good general article:

NWAVGUY headphone amp measurements


and another:

Westhost (Elliot) article on measuring source impedance

How do I measure output impedance?

Reply #6
"By measuring the output voltage with no load, and with a known load, you can  calculate the output impedance. This online  calculator makes it easy. The no load voltage is the "Input Voltage", R2 is  the known load resistance (don't use headphones), the Output Voltage is the  loaded voltage. Click Compute and R1 is the calculated output impedance. This  can be done using a 60 hz sine wave file (Audacity can create such a file), a  Digital Multi Meter (DMM), and a 15 – 33 ohm resistor. Most DMMs are only  accurate around 60 hz. Play the 60 hz sine wave file and adjust the volume for  about 0.5 volts. Then attach the resistor and note the new voltage. For example,  0.5 volts with no load, and 0.38 volts with a 33 ohm load gives an output  impedance of about 10 ohms. The math is: Zout = (Rload * (Vnoload - Vload)) /  Vload "  - NwAvGuy

@Arnold B. Krueger

So Arny, you strike me as the kinda guy who has a 15 - 33 ohm resistor right on hand; might you please do us a big favor and measure your current Yamaha or Denon receiver's headphone output impedance? I know lots of people besides myself would be curious to know the results even though they would only be specific to that particular AVR. Thanks.

How do I measure output impedance?

Reply #7
"By measuring the output voltage with no load, and with a known load, you can  calculate the output impedance. This online  calculator makes it easy. The no load voltage is the "Input Voltage", R2 is  the known load resistance (don't use headphones), the Output Voltage is the  loaded voltage. Click Compute and R1 is the calculated output impedance. This  can be done using a 60 hz sine wave file (Audacity can create such a file), a  Digital Multi Meter (DMM), and a 15 – 33 ohm resistor. Most DMMs are only  accurate around 60 hz. Play the 60 hz sine wave file and adjust the volume for  about 0.5 volts. Then attach the resistor and note the new voltage. For example,  0.5 volts with no load, and 0.38 volts with a 33 ohm load gives an output  impedance of about 10 ohms. The math is: Zout = (Rload * (Vnoload - Vload)) /  Vload "  - NwAvGuy

@Arnold B. Krueger

So Arny, you strike me as the kinda guy who has a 15 - 33 ohm resistor right on hand; might you please do us a big favor and measure your current Yamaha or Denon receiver's headphone output impedance? I know lots of people besides myself would be curious to know the results even though they would only be specific to that particular AVR. Thanks.



Can I cheat?

I have the service manuals for both AVRs, and they both have 470 ohm resistors, 1 per channel running from the power amp outputs to the headphone jack. Bad, bad news.

The good news is that I use neither jack. My headphones are Sennheiser digitals, with line level inputs that are driven by the line level outputs of a dedicated 2-channel Rane equalizer.

How do I measure output impedance?

Reply #8
Sure, cheating is fine, if your known resistor's value translates to a more specific headphone jack output impedance value you can then verbalize to me so I can start doing some numbers cruching on a specific value other than "it's bad". I can't do 1/8th rule calculations using the numerical value of "bad" now can I?

How do I measure output impedance?

Reply #9
https://en.wikipedia.org/wiki/Output_impedance
The accuracy will be limited only by how well you can measure the voltage and impedance, which with even a $25 multimeter should be very accurate.  For best results, use a resistor, not a pair of headphones as the load.

I have an analog multimeter from the 70s, does that count?

I also have a really nice digital oscilloscope. I watched a youtube video where the guy in the video was using an oscilloscope, would that be more accurate than the multimeter, or should I stick with the one I have/buy a digital one at the hardware store?

How do I measure output impedance?

Reply #10
A scope or multimeter will both work. Anything that can measure ac voltage.

How do I measure output impedance?

Reply #11
https://en.wikipedia.org/wiki/Output_impedance
The accuracy will be limited only by how well you can measure the voltage and impedance, which with even a $25 multimeter should be very accurate.  For best results, use a resistor, not a pair of headphones as the load.

I have an analog multimeter from the 70s, does that count?


It should work provided you can get enough of an indication to have reasonable values to work with.

The potential problem is that general purpose multimeters only have accurate response up to 400 or 1,000 Hz. They may have needles or displays that fluctuate below 50 Hz. As long as you work with signals in that range, which can be useful for measuring the source impedance of headphone amps, they are good enough.

However, the seriousness of the potential problem is vastly reduced by the fact that the impedance calculation is based on the ratio of two voltages. If an inaccurate meter causes one voltage to be so  many percent low, it will have the same effect on the other voltage, and the ratio of the two voltages will be correct even though the individual voltages have built-in errors.

Quote
I also have a really nice digital oscilloscope. I watched a youtube video where the guy in the video was using an oscilloscope, would that be more accurate than the multimeter, or should I stick with the one I have/buy a digital one at the hardware store?


I have an Owon USB  digital oscilloscope and its numeric readout features seem to be sufficiently accurate. Yours may be just as good if not better. With response up to 10-20-50-100 MHz, the frequency response problems with using general purpose multimeters to measure audio would appear to be circumvented.

How do I measure output impedance?

Reply #12
Sure, cheating is fine, if your known resistor's value translates to a more specific headphone jack output impedance value you can then verbalize to me so I can start doing some numbers cruching on a specific value other than "it's bad". I can't do 1/8th rule calculations using the numerical value of "bad" now can I?


I can confirm that the 470 series resistors are the overwhelmingly dominant influences relating to the source impedance of these two AVRs. The AVR power amps themselves have  source impedances of under 0.1 ohm, and the source impedance at the headphone jack is the sum of that and the 470 ohm resistor.

Interesting coincidence that they were both the same.

More expensive AVRs may have separate stand-alone headphone amps, but there is no guarantee that their source impedance is down in the 1 ohm or so sweet spot that we like.

How do I measure output impedance?

Reply #13
Isn't it favorable to have a (sine) function generator at hand to measure the impedance as a function of frequency instead of at a single frequency or DC? A single value will never give you the full picture what's happening. That's even more important for headphones.
It's only audiophile if it's inconvenient.

How do I measure output impedance?

Reply #14
Thanks Arny, so as I understand it the 470 ohm resistors are wired in series from the positive terminals of the AVRs speaker outputs and you estimate the output impedance for the headphone jack to be around 470.1 ohms.

Fixing this with an intermediate buffering amp like a Fiio connected to the headphone out to establish a nice low output impedance for one's headphones seems inelegant and like a visual eyesore to me. Have you ever explored building a simple, passive voltage divider network like this guy discusses on the lower half of the page underneath the simplistic method AVRs use which he first discusses, or have comments on it?

How do I measure output impedance?

Reply #15
Thanks Arny, so as I understand it the 470 ohm resistors are wired in series from the positive terminals of the AVRs speaker outputs and you estimate the output impedance for the headphone jack to be around 470.1 ohms.

Fixing this with an intermediate buffering amp like a Fiio connected to the headphone out to establish a nice low output impedance for one's headphones seems inelegant and like a visual eyesore to me. Have you ever explored building a simple, passive voltage divider network like this guy discusses on the lower half of the page underneath the simplistic method AVRs use which he first discusses, or have comments on it?


I think that what we want is a headphone signal source that puts out about 2 volts RMS with a source impedance of no more than 1 ohm.

A 100 wpc AVR puts out about 32 volts RMS into 8 ohms. A simple voltage divider with source of impedance of 1 ohm and an input impedance of 8 ohms would have a gain of 1/8 which would cut the 32 volts down to 4 volts. So this is doable without violating the law of conservation of energy.  The investment in reasonably stable and accurate non-inductive 8 resistors might be in the same range as a good cheap headphone amp. It might get hot if you crank it, but it would not require external power.

How do I measure output impedance?

Reply #16
If you really wanted to use a power amp, you'd have to note that:
- it needs to have a common ground, or else it will blow up
- the noise floor will likely be audible
- resistors as attenuators will essentially turn a lot of power into heat
"I hear it when I see it."

How do I measure output impedance?

Reply #17
Thick film resistors of 1 ohm and 10 ohm, 35W, 1% tolerance can be had for less than $4 each. Note: the 35W rating requires a heat sink.

How do I measure output impedance?

Reply #18
AVRs are a real work horse: they do so many things, for such a reasonable cost, so they are a dream come true for audio buffs who don't fall for the lies of the stereophool magazines, but why oh why must they almost all have this fatal flaw?

Sure we can fix it with an kludge outboard $28 (shipped) Fiio complete with its own dedicated power supply, lithium ion battery [possibly its most expensive part], volume control pot, tone control (bass boost switch), case, etc. which means if we strip it down to just the essential op amp and a few wires the material cost for a Japanese AVR maker is couple of bucks, tops, maybe even under  $1, yet even when we buy a top level $2,000+ receiver from the big brands they won't spring for this for us?! ARGH!

This commonly repeated notion of "Well, some of the top level units use a dedicated op amp for the headphones instead of just the simple resistor method" is effectively untrue with current designs. I challenge anyone reading this to cite a specific, non-British AVR made within the past decade which does, based on manufacturer's claims, not anecdotal forum post claims.

How do I measure output impedance?

Reply #19
What about instead of attenuating the unneeded power into heat, using resistors or pads, we use an autoformer? There are expensive volume control companies which do it this way which I can't justify the cost of, but then again there are also less expensive ways to buy an autoformer meant for tapping the speaker outputs of an audio amp which are more affordable.

How do I measure output impedance?

Reply #20
It's still a clunky device. Audio performance will probably not be as good as a similarly priced headphone amp.
"I hear it when I see it."

How do I measure output impedance?

Reply #21
I see now they even make my autoformer idea as an assembled product.

How do I measure output impedance?

Reply #22
But that's 3x as expensive as the "switch" ... probably just due to the case.
"I hear it when I see it."

How do I measure output impedance?

Reply #23
What about instead of attenuating the unneeded power into heat, using resistors or pads, we use an autoformer? There are expensive volume control companies which do it this way which I can't justify the cost of, but then again there are also less expensive ways to buy an autoformer meant for tapping the speaker outputs of an audio amp which are more affordable.



Transformers require careful engineering to avoid adding audible nonlinear distortion (particularly at low frequencies) and frequency response aberrations. 

The cut sheet that you linked neatly avoids dealing with both issues, right? ;-)

Ditto for the Niles volume control, right?

Nice thing about resistors is that engineering products with zero nonlinear distortion and negligible frequency response aberrations is pretty simple.

How do I measure output impedance?

Reply #24
Transformers require careful engineering to avoid adding audible nonlinear distortion (particularly at low frequencies) and frequency response aberrations.

Which is why it is so difficult to design a tube amplifier with very low distortion and flat response.

 
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