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Little Dot Mk V Output Impedance?

post #1 of 25
Thread Starter 

Hi evereybody!

 

Do any of you know the headphone output impedance on the Little Dot Mk V?

 

I've tried to search the web and this forum, but it seems that people only tend to be annoyed by the same question, without anyone having any clues. Does anyone have any ideas/actually know?

 

Hope to hear from anyone! :-)

post #2 of 25

Well, if no one knows, maybe someone should measure it normal_smile%20.gif

 

post #3 of 25
Quote:
Originally Posted by Labbetuss View Post

Hi evereybody!

 

Do any of you know the headphone output impedance on the Little Dot Mk V?

 

I've tried to search the web and this forum, but it seems that people only tend to be annoyed by the same question, without anyone having any clues. Does anyone have any ideas/actually know?

 

Hope to hear from anyone! :-)


Haha....just a week or two ago I started a thread asking the very same question....no one seems to know. I have no idea how to measure it myself, but it would be awesome if some mkV owner who does, would do it. It's likely pretty high, or the manufacturer would have released the specs on that. He was asked that very question about the output impedance on the LD forum but he never replied.

 

post #4 of 25
Quote:
Originally Posted by lejaz View Post

 

I have no idea how to measure it myself, but it would be awesome if some mkV owner who does, would do it.

 

It is not very complicated in principle, the output voltage (using a simple test tone as input) needs to be measured with and without a load (resistors, or even a headphone with a splitter like this one if there is nothing better), and from the voltage drop and load impedance the output impedance can be calculated. For the voltage measurements a DMM or even a sound card (or onboard audio) can be used.

 

post #5 of 25

As mentioned in the post above, it is fairly simple to measure the output impedance. I use the method suggested by the designer of the O2 amp. It consists of using a 60hz sine wave file generated from Audacity and play it through the amp to measure the voltage with no load and a dummy load (15 to 30ohm resistor). After you get the measurements, plug the numbers into the following formula:

 

Zout = (Rload * (Vnoload - Vload)) / Vload

 

Zout - output impedance

Rload - dummy resister load

Vnoload - voltage with no load

Vload - voltage with dummy load

post #6 of 25

If anyone is interested, here is a description of measuring the output impedance of a sound card, using only a splitter, headphones, and freely available software. An amplifier can also be added to the loop to measure that instead, but you have to be careful not to damage the input of the sound card with excessively high voltage (e.g. by setting the volume too high). Also, if the amplifier is AC powered and grounded (headphone-only amplifiers are not in most cases), there may be problems due to ground loops.

Hardware setup: connect the male jacks of the splitter to the line input of the sound card, and the output of whatever is to be measured; if it is an amplifier, set the volume to zero for now, and connect the sound card line output to its input. Configure the sound card as necessary for loopback recording, setting the input and output levels in the mixer.

In the example test, I used a Xonar D1 sound card, DT770 Pro 250 Ohm headphones, and this program. When first run, it asks for the audio input and output device, select them as needed. Then, in the menu at the top, click "Settings" to open a configuration window. Not all the following changes may be necessary or useful for this simple measurement, but I set the FFT size to 16384, sampling frequency to 96000 (or use whatever works best with your hardware), clicked "Confirm", then set the smoothing window to "Blackman opt." and the bit depth to 24 (if supported), and checked "Extensible". There is nothing else to configure here that is relevant, so the window can be closed by clicking OK. At the right of the scope (top) display, "DCremoval" and "Values" should be checked. Now open the waveform generator by clicking "Wave" in the top menu. In this window, check both "Interlock" settings, set "Levels (% full scale)" to zero for now, and the frequency to 1500 Hz. Why 1500 Hz ? Because with the headphones used, the reactance and distortion are low at that frequency. With other headphones, a different frequency may work better, but 1500-2000 Hz is good for most full size dynamic ones. On the "Setup" tab, set the waveform generator sample rate and bit depth similarly to the input. With everything set up, the test can be started.

Enable the audio input and output by clicking "On" in both the top menu of the main window, and in the waveform generator window. If there are no errors, increase the volume (with "Levels (% full scale)" and/or the amplifier volume knob) until there is some signal on the scope display that is high enough to be clean, but low enough not to be distorted (or make the test headphone too loud).
The signal level is displayed at "TRms (%fs)". If you check "Infinite avg", this can be averaged over a longer period of time for more accurate results. Do so without the headphone load first, and wait until the displayed value stabilizes enough and stops changing consistently in one direction. Write down the RMS level (I got 2.7654145, for example). Uncheck the averaging to clear the result, and then repeat the measurement with the headphone load connected. In the example test, the RMS was 1.9992840. The measurement is complete, so the program can be closed.

Calculating the output impedance from the measured levels is simple, but you have to know the impedance of the load (that is why resistors are preferred for an accurate result). In my example, I assumed it is 260 Ohm. The formula is:
  Z_out = Z_load * ((V_unloaded / V_loaded) - 1)
With the example values:
  Z_out = 260 * ((2.7654145 / 1.9992840) - 1) = 99.63 Ohm
This is quite close to the correct result of 100 Ohm.
I did the test again with the rear channel output, and got:
  Z_out = 260 * ((2.9916870 / 2.9480940) - 1) = 3.84 Ohm

Of course, any other tools and software may be used, you just need to find out the ratio of the unloaded and loaded output voltage with a known load impedance, and then apply the above formula.
With a sound card, there is also some inaccuracy due to the low input impedance of the card itself (as opposed to a DMM), which affects the result as if it was connected in parallel with the output impedance of the source. Sticking with the above example, correcting the result for an input impedance of 3800 Ohm gives:
  Z_out = 1 / (1/99.63 - 1/3800) = 102.31 Ohm
 

For comparison, the same test with a 220 Ohm resistor load gave 23.199% unloaded and 16.037% loaded level. That means (see formulas above) 98.25 Ohm impedance, or 100.86 Ohm with input impedance correction.

 

Finally, with a simple DMM (measures AC voltage reasonably accurately from ~30 to ~2000 Hz) and 400 Hz frequency, 1.362 Vrms and 0.936 Vrms were measured using the 220 Ohm dummy load. This translates to 100.13 Ohm output impedance.

 


Edited by stv014 - 3/31/12 at 7:07am
post #7 of 25
Thread Starter 

Thanks guys for all your replies!

 

I'm in dialog with a guy that can do several measurements in a "proffessional" enviroment. I will come back to you whn we have some of the results! :)

post #8 of 25
Quote:
Originally Posted by Labbetuss View Post

Thanks guys for all your replies!

 

I'm in dialog with a guy that can do several measurements in a "proffessional" enviroment. I will come back to you whn we have some of the results! :)



Good to hear! If it's as high as some folks think it is, I'll probably go for an O2 or maybe an m-stage for my low impedance cans. The mkV sounds great with the hd580 which is 300 ohms.  

post #9 of 25
Thread Starter 

I just got an email from David at Little Dot. He said the output impedance om Mk V is 32ohms. Thanks to David for replying!


Edited by Labbetuss - 4/3/12 at 8:36am
post #10 of 25
Quote:
Originally Posted by Labbetuss View Post

I just got an email from David at Little Dot. He said the output impedance om Mk V is 32ohms. Thanks to David for replying!

Good to know....thanks. So it shouldn't be a problem with the k70X I don't think since they're double that.... but possibly with phones under 32 ohms? In my own experience with some 25 ohm headphones there's no issues, but perhaps there's some frequency response deviations that I'm not aware of.
 

 

post #11 of 25
Thread Starter 

Ideally the headphone-impedance should be 8x the output impedance. If the output impedance is lower than this it will significantly affect the system response in different ways. So, ideally headphones with no lower impedance than 250ohms should be used on this amplifier... No wonder people that combines this amp with K701 find the sound anemic and strange!

post #12 of 25

But Denons are even lower in impedance than the 70X, and lot's of folks loved them with the mkV....so I don't think the frequency response deviations are as extreme as you make them out to be. The 70X still sound very flat(frequency wise) with the mkV, other than a treble peak, but people report that with other amps as well....some with lower output impedance than the mkV. my k240DF has a treble peak with the mkV and that's a 600ohm headphone. I don't imagine that 'headphones should have 8X the output impedance of the amp' is written in stone. It's probably more complicated than that.

post #13 of 25
Thread Starter 

I only refer to all the different posts I've read about what I wrote in my previous post regarding peoples reactions to the high impedant Mk V together with relatively low impedant headphones. This is also readable on the little dot forum. When people have used headphones that had lower impedance than Mk V's output impedance, many get audible artefacts in the system. Buzzing, hizzing and noise for example. Don't worry lejaz, you can buy an impedance adapter which increases the load that the amp "senses", and the problem is gone. :)

 

It all depends if you like system as it is or not. There is no final answer here regarding sound and different peoples subjective preferences.

 

Lot of good technical reading in the article i linked to though! To me it looks dramatic enough. I've just ordered an Mk V myself, so it'll be fun testing it! ;)

 

Edit:

 

Regarding your 240dk. Offcourse the soundsignature of the unit itself will play a signifikant role....


Edited by Labbetuss - 4/3/12 at 3:59pm
post #14 of 25

Hmmm, the DF needs a bit of gain. I could give it 10 dB of gain, a bit of volume and it pretty much woke up. The Mark V is okay but given the sound signature of the DF expect a lot of volume corrections in between tracks and albums. 

 

Even so, if the Mark V would have a gain switch I would have kept using it...

post #15 of 25

Yes, I read through a lot of nwavguy's information on output impedance. He seems to be one of the most knowledgeable people on that particular subject. I was thinking of trying the O2 as a little experiment, since it has a very low output impedance. I'm kind of curious if I could tell the difference between the O2 and the mkV with the k702. If I find one second hand for a good price, I may grab it. I'm curious how the O2 would do with 25 ohm headphones. BTW, what phones are you going to be using with the mkV? I was told that the impedance adapter will effectively increase the output impedance of the amp.....which is the opposite of what I want. I'm no electronics expert.....that's only what I was told here on the forum. 

Quote:
Originally Posted by Labbetuss View Post

I only refer to all the different posts I've read about what I wrote in my previous post regarding peoples reactions to the high impedant Mk V together with relatively low impedant headphones. This is also readable on the little dot forum. When people have used headphones that had lower impedance than Mk V's output impedance, many get audible artefacts in the system. Buzzing, hizzing and noise for example. Don't worry lejaz, you can buy an impedance adapter which increases the load that the amp "senses", and the problem is gone. :)

 

It all depends if you like system as it is or not. There is no final answer here regarding sound and different peoples subjective preferences.

 

Lot of good technical reading in the article i linked to though! To me it looks dramatic enough. I've just ordered an Mk V myself, so it'll be fun testing it! ;)

 

Edit:

 

Regarding your 240dk. Offcourse the soundsignature of the unit itself will play a signifikant role....



 


Edited by lejaz - 4/3/12 at 4:40pm
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