[pumin]

I'm trying to measure headphone output from my computer soundcard (model: RME HDSPe AIO PCIe , $700) and later I'll buy headphone that match its output level. I use procedure below:

- I download test tone from Download Audio Tone Files , play by Winamp
- I use multimeter (set to V-- , 200m ) measure headphone jack between ground and either the left or right channel.


1kHz_44100Hz_16bit meter reads 0.5 , (Does it means 0.5mV?)

100Hz_44100Hz_16bit meter swings, up to around 3.3 , (Does it means 3.3mV?)

Rock music Track meter swings, up to around 35 , (Does it means 0-35mV?)

The sound card has 30 ohm fixed resistor which is included in the audio path for short circuit protection. No impedance specified from manufacture, but real resistance. I also measure impedance, meter swings around 30.

- Are my procedures correct?
- Are these information enough to help me choose a suitable headphone?
- Without external amp, will these numbers be enough to drive AKG K701 (600ohm), BT 880(600ohms,96dB SPL, max 100mW), HD650? Or should I go low impedance phones e.g. Shure SRH840 (40ohms,102dB/mW) ?

thanks a bunch!

 

[jnewman]

It sounds like you are measuring DC voltage. You're not going to get anything useful that way (An audio signal alternates + and - and so, in DC terms, averages to basically zero. It alternates fast enough your multimeter doesn't display the changes). If you have a good multimeter, you might get meaningful measurements using an AC voltage setting.

You would probably get better data from the sound card's specifications and more meaningful answers here if you post that. However, just as important as voltage swing are a bunch of things there's no way you can measure with a multimeter.

I don't have any experience with the sets of headphones you mentioned, so I can't really help you on that one.

 

[pumin]

oh. I follow instruction from this page, and it tells to measure DC. I must misunderstand their purpose. As you can tell I'm not very good at this.
Measuring DC offset

Now I switch multimeter to AC (V~, 200)

100Hz_44100Hz_16bit, meter reads 2.0
1kHz_44100Hz_16bit, meter reads 2.0
250Hz_44100Hz_16bit, meter reads 2.0 - 2.1
10kHz_44100Hz_16bit, meter reads 1.9

Are these numbers helpful? Thanks!!

Note: I tried tech support, and they don't know their specs.

RME User Forum / HDSPe AIO PCIe - is headphone impedance 30 or 75 ohm ?

 

[jnewman]

DC offset is a "Bad Thing" which can damage headphones, so it is important to measure if you are unsure of your equipment.

2V RMS may not get some of the high-impedance not-very-sensitive headphones loud enough or drive them quite right... somebody else will probably come along and give you a better answer, though.

 

[pumin]

Based on my meter reading above, may you tell which headphones from table below will work?

For example,
K240DF requires 3.1V (for 100dB SPL), and K1000 requires 6.93V. Only these 2 won't match because it needs more voltage than my meter reading of 2V ? The rest of headphones will work fine because it requires less than 2V ? Am I interpret this correctly?

 

[pumin]

Oh, i just found a high-gain switch, now headphone measures 4.3vrms

I think I can answer my own question by using some calculations

Let's look at AKG K501 impedance range: 120Ω, sensitivity of 94dB SPL at 1mW input and a maximum power of 200mW.

The current requirement for 1mW is:

I=√ (P/R)= √ (1mW/120Ω) = 2.89mA

And voltage necessary for 1mW is:

v = 2.89mA x 120 = 0.35V

For maximum power you need:

I=√ (200mW/120Ω) = 40.8mA

v = 40.8mA × 120Ω = 4.89v

Given soundcard can provide 4.3v, it can run most of these cans, I think I'm good now. Thanks for pointing me to right direction.

-pumin

 

[jnewman]

It's not like headphones won't produce sound if your source can't supply enough voltage to get them to 100dB, they just won't be 100dB loud... if the phones are hard enough to drive, your sound card may not make them as loud as you would like, but they will otherwise work.

There are a whole host of other reasons why your sound card may or may not be ideal for directly driving headphones, like an ability to supply adequate current for low-impedance headphones and slew rate (how fast the output can change). Voltage swing alone does not tell the whole story and you can't really measure the differences with a multimeter. The headphone amplifier part of your sound card (as compared to its line out) may even add noise or distortion that could be skipped by using a separate amp on the line out (certainly it adds some, every electronic component does - how your sound card does as compared to standalone amps, I don't know).

 

[pumin]

Ok J. I'll keep those in mind. My next move will be headphone amp.

 

[m1abrams]

Quote:

Originally Posted by pumin /img/forum/go_quote.gif Ok J. I'll keep those in mind. My next move will be headphone amp.

Wait if you are planning on buying a headphone amp then why did you just go through this whole testing deal against your soundcard?

 

[pumin]

oh, I think I'll pick a headphone and stick for this soundcard for a while. As it's a professional soundcard with 4.3v output, it should last me for a bit. Usually home stereo comes with 2v output. And when I save up for an amp, that will be my next move for finer sound.

 

[m1abrams]

Quote:

Originally Posted by pumin /img/forum/go_quote.gif oh, I think I'll pick a headphone and stick for this soundcard for a while. As it's a professional soundcard with 4.3v output, it should last me for a bit. Usually home stereo comes with 2v output. And when I save up for an amp, that will be my next move for finer sound.

FYI - Voltage swing has little indication of how well it will drive a given set of headphones. That 2V out on a stereo is usually the line level outputs which are not designed to drive headphone.

The output should have low output impedance and this needs to be lower for headphones woth lower impendance. Also current drive ability is important.

Not saying your soundcard can not handle it, I know little about it. Just keep in mind that driving headphones may not have been the center point around that cards design.