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PA2V2 output impedance?

post #1 of 18
Thread Starter 

Does anyone know what the output impedance of the PA2V2 is?  I've searched in vain and came up empty.  I am looking at a low-impedance set of IEMs for knocking around, but need to know if the PA2V2's output impedance is low enough (hopefully 1 ohm or so) to make the IEMs sing.  I tried dropping Gary at Electric Avenues a line, but he is probably busy cranking them out :-)

 

Thanks - Satya

post #2 of 18

i highly doubt any amps for headphones are 1 ohm rated

post #3 of 18

pa2v2 has adjustable gain(internal trims near the battery connector)

here's the data sheet on the opamp used

 

http://www.national.com/mpf/LM/LM4881.html#Overview

post #4 of 18
Thread Starter 

Hi All:

 

Thanks for responding.  I do know that there are at least a few that are 1 ohm or less: the Fiio E5 and E7 (both around 0.1 ohms), the 4556-based Cmoy (0.1 ohm),  and the Sansa Clip+ player (1 ohm) are that low (I got these values from the nwavguy blog).  I did finally hear from Gary at Electric Avenues, and he is not sure what the value is for the PA2V2.  I may try to rig up a test arrangement and see for meself.  

post #5 of 18

It's kind of sad that nobody knows, considering how popular this is.

 

You're right that it should be fairly easy to measure, though how accurately is going to depend on your gear.  With just a couple of 1% resistors of different values (just make sure they can dissipate that much power, though that shouldn't be a stretch at lower voltages) and a cheap multimeter, you should be able to get a reasonable estimate with a couple simple measurements and calculations.  Just set up the source into the load resistor for a simple voltage divider.

 

V_L = V_s * Z_L / (Z_L + Z_s)

 

V_L = voltage across load resistor, measured by multimeter

V_s = source voltage (playing back the same test tone, should be the same for multiple tests), probably unknown

Z_L = load impedance of the resistor you are using, can be measured too (and should be close to nominal value with a 1% tolerance resistor)

Z_s = source output impedance, unknown

 

With at least two resistor values and V_L measurements you should be good to go.  2 (or more) equations and 2 unknowns.

 

Make sure to report back what you find!

post #6 of 18
Thread Starter 

Thanks - that describes in good detail what I was thinking about, and I think my algebra matches yours.  Not sure what the tolerance on my resistors is (gotta dig through and find them first), although I will measure them on the DMM just to make sure, and use a couple matched ones for both channels.

 

In looking at my DMM, the lowest AC voltage range is 200 volts, a "little" above the PA2V2's range :-)  So, as soon as my buddy across town can get my his better DMM with a 0.2 and 2.0 volt range, I will see how it goes and report back.

post #7 of 18

You don't need to check both channels (well you can to make sure, but it should be similar), and definitely not simultaneously, though you can if you want.  Just connect one output channel to a resistor back to ground and leave the other output channel floating.  Matching isn't necessary.

 

Yeah it should be sufficient to just measure the impedance values with the DMM, so they don't really need to be 1% tolerance.  I just meant that you could skip that step if your DMM can't do that for a rough estimate, by using the nominal values directly.

post #8 of 18

Publishing a technical paper on the PA2V2? Sweet. 

post #9 of 18
Thread Starter 

Thanks - I was thinking the same thing.  Checking the exact resistance will be a snap, and I probably will do both channels just to make sure they are the same.

post #10 of 18
Thread Starter 

Hi all:


 

Thanks for your ideas and patience. I finally got my buddy's Fluke auto-ranging DMM and got to work on measuring the PA2V2, which I purchased from Electric Avenues about 15 months ago.

 

To first check the set-up, I measured the output of a Behringer UCA202 DAC/amp, which is known to have a 50 ohm output impedance. Using a 100 hz WAV test tone on my laptop as the test signal, and with a half-watt 100 ohm resistor as the load (98.6 ohms, to be precise), the Behringer measured at 50.4 ohms. Sweet.

 

Next, I sent the Behringer DAC's output to the the PA2V2 (operating on its charger), and with the 100-ohm resistor load, the output impedance of the PA2V2 was 2.52 ohms. Scaling down to a  11-ohm resistor, which being closer to the PA2V2's value likely made it a little more accurate, I got 3.29 ohms. 

 

So, it looks like the PA2V2 has about 3.3 ohm output impedance.  Headphones with the PA2V2 should therefore probably be at least 25 ohms or so to get good bass damping and a nice flat frequency response. With 16 ohm cans, this may cause the bass to be a bit flabby at higher volumes, and may lead to some minor frequency deviations if the cans themselves have a wide impedance range.

post #11 of 18

Some people aren't believers in large damping factor, but yeah.  I guess it depends on the headphones / IEMs, whether or not that should make much of a difference.  Around 3 ohms is pretty benign except maybe in an extreme case, so this result is nothing particularly special.

 

Thanks for the report!

post #12 of 18

I wish this information were available for all amps.  Thanks for the info!

post #13 of 18

was the internal gain pots changed at all for the test?

post #14 of 18
Quote:
Originally Posted by Ai James View Post

i highly doubt any amps for headphones are 1 ohm rated


Don't doubt. I have a 1ohm DACPort and an Audio-GD with 2ohm out. Others exist, too. For any can under 50-60ohms having output impedance under 5ohms will give flatter bass. Impedances of 10 or up will markedly increase bass output by 1-3db, maybe more depending on the phone (except for orthos which don't have much dependence on output impedance for tone).
post #15 of 18
Thread Starter 

I didn't change the internal gain, although I did play around with the main volume pot, and the output impedance pretty much stayed the same, so the volume pot must be ahead of the final output stage.  I did get an email from the manufacturer, and he thought 3.3 ohms was about right, not because there are any resistors in the output, but because of the output transistors he uses.  I believe the output gain adjustment is also ahead of the final gain stage, so adjusting the gain pots shouldn't affect the output impedance either.

 

I recently purchased an Audioengine N22 amp, partly because it has a dedicated headphone section with a Burr/Brown opamp.  I measured its output impedance to be 4.7 ohms, which is also what I was quoted by the company's tech support.  So, the 3.3 ohms on the PA2V2 is certainly not the worst, and may be marginally better with low-impedance cans than the N22.  However, with my Grado SR60's, at 32 ohms, I really can't tell any difference.

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