[fiddler]

I was measuring the output impedance of output sources at my disposal, discovering that my laptop headphone jack has a rather high output impedance, explaining why my Porta Pro sounds a good bit boomier than out of other sources, due to its impedance spike at 100hz.
 
I had the idea that it might be useful to the entire headphone enthusiast community if there was a database of output impedance measurements, because product spec sheets usually don't make any mention of this. Only decent dedicated headphone amp manufacturers bother to make a mention of this, even though it can have a significant impact on sound quality. Since a lot of us would like to get decent sound quality straight out of a smartphone, I thought it might be of interest to audiophiles who are looking to purchase a new device, and want to know a little more about the quality of the headphone output.
 
The required equipment for making this measurement is not very much, but it does require a few basic electronics DIY supplies, and a general interest in headphones - which is why I thought it might be a good idea to ask here.
 
Everybody with a multimeter capable of measuring low AC voltage (0-10V, as opposed to 120V/240V mains voltage), a resistor to act as a dummy load, a sine wave test track within the frequency response of your meter, and a 3.5mm-to-3.5mm interconnect cable to faciliate measuring voltages between signal and ground, and some source equipment to test and add to the database is all the equipment required for this.
 
Would there be any interest in taking part in this project? I think it'd be a pretty cool project once we get a few people interested, and start compiling data for most common types of sources. I will write up step-by-step instructions on taking the measurement, which just takes a few minutes of your time.
 
Update: see the tutorial in the next post.

 

[Budgie]

I think it's a interesting idea. You might have to post atutorial on how to make the measurement and calculate the output impedance.

 

[Budgie]

Oh, wait, you said that. ( sheepish grin)

 

[fiddler]

I think it's a interesting idea. You might have to post atutorial on how to make the measurement and calculate the output impedance.

 
Indeed, so here it comes, in pictures:
 
http://imgur.com/a/KJkOJ
 
I only measured the left channel here, a bit less fiddly to measure between ring and tip vs ring and sleeve, and we're not really testing for channel imbalances here. Edit: As tomb mentions below, this isn't a perfect way to attach a resistor to the contacts - please take the precautions to clean the resistor leads in case they're oxidised by wiping it a bit with sandpaper, and use both hands with the meter probes and apply firm pressure to the resistor leads so that you press them against the connector for good measure.
 
Also see my update below - please use a resistor no lower than 100 ohms and keep the volume knob low enough so that the open circuit voltage is no higher than 0.5V. This should prevent your device from clipping no matter how wimpy the current output of your device.
 
EDIT 2:  We ran into a case recently where a user used an HD650 cable to take the measurement, which had a notably high resistance of ~1 ohm each on ground and signal. This is essentially like adding a 2 ohm resistor in series with the output of the amp, so our calculation will be off by that amount too - our end result will be 2 ohms too high. Short lengths of cable like the one I used in the tutorial should only have negligible resistance, but do double check the cable you use here by measuring end-to-end on the signal and ground channels, particularly when using longer cables like the HD650 cable. For example, if you're measuring the left channel, measure between the sleeve connector on both ends, then do the same for the tip connector. Add those values together, and you end up with the total cable resistance that you will need to subtract from your end calculation.
 
You can use this online tool to generate a sine wave (I measured at 1khz in the tutorial, but your meter may not be able to measure that high. 50 hz is a safe choice, but check your meter's datasheet.)

 

As I said earlier, the multimeter has to be able to resolve in the millivolt range, which not all cheap meters do.

 
Once the you have the dummy load resistor's resistance, the open-circuit AC RMS voltage playing a sine wave test tone, and also the voltage with the same test tone with the dummy load resistor attached, you plug it into this calculator:
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm

 

The values I got in the tutorial give a result of 350 ohm output impedance, very high just as expected because that's typical of this type of old integrated amp.

 

[tomb]

This is a great goal and interesting for a lot of folks around here, but there are maybe a few more things you could take into account:
 
1. Very few meters measure Vrms accurately for Vac.  You will find that even Flukes may measure Vac sufficiently accurately at 60 Hz, but do a poor job at 1K and higher.  You've mentioned this issue, but I'm not sure the datasheet for a meter is something we should depend on.  After all, we're a bit familiar around here with unsubstantiated claims when it comes to audio. 

  A scope might be better (required?).
 
2. Cheap meters don't measure Vac at any frequency reliably, much less at higher ones.  Chinese ones will have dial settings for measuring Vac and may even spec the capability, but THEY LIE.
 
3. I'm not so concerned about your fingers' resistance, but attempting to wrap a resistor's leads around a 3.5mm plug is probably not a good idea. I've touched DMM probes directly to the same contacts on a plug and got mega-ohm readings before.  For that matter, you can touch two probes together and often get mega-ohm readings.  Good contact assurance is not a trivial issue.  Alligator clips or mini-probe clips should be used at a minimum on the resistor and jack.  The spring force ensures that you have reasonable contact.  Plus, there's no handling of the connections once this is done (no worries about finger effects).
 
Finally, I would strongly recommend that you attempt to measure an amplifier with an absolutely known output impedance, first.  Then vet your measurement/connection technique accordingly.

 

[fiddler]

You might have a good point about the contact to plug. I'll add a note that my method there was lackadaisical and one should maybe clean the leads with sandpaper to remove oxidation and at least apply good pressure on the contact with the probe leads. I had no problem fwiw with 3 different resistors with this method though.

I think you underestimate the capability of cheap meters. My Cmoy with no output resistor measured under 0.5 ohms consistently. My laptop measured consistently at around 55 ohms 6 times measuring at 1khz,440hz and 50hz,and at 1khz and 100 ohms, 55 ohms and 270 ohm dummy load. I not doubting that the margin of error is bigger here than with sophisticated instruments, but it seems sufficient to give us an idea of whether a source is low or high in impedance.

I can happily change the doc to recommend using only 50 Hz if that would improve universality.

 

[fiddler]

https://www.youtube.com/watch?v=ue0wtlrmCJE
 
Around the 4:30 mark he shows how a 50hz sine wave measures fine with his cheapo meter, but then demonstrates that the square wave measurements will be wonky because the meter assumes a ratio of .707 to peak voltage regardless of waveform, requiring a true RMS meter for an accurate reading. Which is why I said to always use a sine. I'll also edit the documentation to say to use 50hz, I think it's a good idea. edit: or 60 hz, but I assume either must be fine considering those are the AC wall power frequencies of the world.

 

[fiddler]

Quick update: I got some erroneous measurements because I caused my CMOY to hit its current output limit of 40mA using a 50 ohm resistor at max volume. Therefore, I now recommend using a 100 ohm resistor or higher, and keeping the volume low enough so that the open circuit voltage measures around 0.5V. That shouldn't be a problem for the wimpiest of sources.

 

[fiddler]

I've started compiling data that I and a member on Reddit at /r/headphones measured in an Google Docs Spreadsheet.
 
Some clear patterns emerging with smartphones with low output impedances < 2 ohms, on-board audio and soundcards higher, and receivers/integrated amps doing badly.
 
Let's get some more data!

 

[fiddler]

Added a note about double checking the cables you use to measure. We had a case where a HD650 cable added ~2 ohms to the end result.

 

[tomb]

OK - here goes another comment. 

 
You should distinguish whether you're measuring the headphone out on a DAC-amp or the line out.  That also means you shouldn't conflate DAC with Amp or DAC-Amp.  They're all different.  For instance, the "pure" Cambridge DACmagic does not have a headphone out or headphone amplifier.  Thus, it's output impedance of 33 means nothing bad.  It will only be feeding a 10,000 ohm load or higher,* not any kind of headphone.
 
 
* Whatever the volume pot impedance is of the headphone amp or pre-amp to which it's connected.  Most portable sources are 10K.  Most full-size home components are 50K or 100K. 

 

[fiddler]

DacMagic Plus does have a headphone jack, which is what was measured by Ken Rockwell here:
 
http://kenrockwell.com/audio/cambridge/dacmagic-plus.htm#headphone
 
I am only adding sources with jacks designed to plug headphones into. I'm sorry if that wasn't clear.

 

[fiddler]

By the way tomb, it'd be great if you could contribute a few measurements of your own, since you've put so much effort into peer-reviewing this project.

 

[tomb]

  DacMagic Plus does have a headphone jack, which is what was measured by Ken Rockwell here:
 
http://kenrockwell.com/audio/cambridge/dacmagic-plus.htm#headphone
 
I am only adding sources with jacks designed to plug headphones into. I'm sorry if that wasn't clear.

You're right - I missed the "Plus" part.  One has a headphone out, the other does not.

 

[tomb]

  By the way tomb, it'd be great if you could contribute a few measurements of your own, since you've put so much effort into peer-reviewing this project.

Nah, I've already said too much.  I'll bow out, now - before I get myself into further trouble.