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Topic: Higher impedance headphone is louder than the lower impedance one (Read 9441 times) previous topic - next topic
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Higher impedance headphone is louder than the lower impedance one

I casually compared an old Philips SHP2700 to my Sennheiser HD555 by plugging them back and forth a few times into my receiver. The HD555 was a fair bit louder. But according to specs, the HD555 is 120 Ohm, while the SHP2700 is a mere 32 Ohm. Shouldn't the SHP be much louder?

Higher impedance headphone is louder than the lower impedance one

Reply #1 –
Receiver headphone outs usually have a high output impedance. The output impedance forms a voltage divider.

Let's say the output impedance is 470 ohm.

HD555, Vout = 120/(120+470) = 0.203 = about -14 dB
SHP2700, Vout = 32/(32+470) = 0.064 = about -24 dB
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #2 –
Impedance does not imply loudness.

You're thinking of sensitivity.

Higher impedance headphone is louder than the lower impedance one

Reply #3 –
Quote from: saratoga on
Impedance does not imply loudness.

You're thinking of sensitivity.

If one assumed equal efficiency then the lower impedance device should be louder. However, the devices are not equally efficient.

Higher impedance headphone is louder than the lower impedance one

Reply #4 –
Oh yes, add the numbers I calculated above to the respective sensitivity (must be in dB SPL @1Vrms, not 1mW!) to arrive at the actual SPL.

(I was assuming similar sensitivity.)

edit:
HD555 = 103 dB SPL/V
SHP2700 = 89.0 dB SPL/mW = 104 dB SPL/V

These are manufacturer specs (measured at 1 kHz I guess), so take them with a grain of salt. Still, with the numbers above, expect around 10 dB lower volume with the SHP.
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #6 –
> Impedance does not imply loudness.

Well that's handy to know.

Quote
HD555, Vout = 120/(120+470) = 0.203 = about -14 dB
SHP2700, Vout = 32/(32+470) = 0.064 = about -24 dB


Once more I find there's clearly a ton of really basic EE things that I don't know. Thanks for the examples, I'll go read up a little.

I don't suppose there'll be much difference between plugging the headphones into my receiver versus directly into the sound card output? (I'm going to try anyway)

Higher impedance headphone is louder than the lower impedance one

Reply #7 –
> Still, with the numbers above, expect around 10 dB lower volume with the SHP.

From playing around with foobar's replaygain settings over the years, ~10dB sort of matches what I heard.

Higher impedance headphone is louder than the lower impedance one

Reply #8 –
Quote from: dhromed on
I don't suppose there'll be much difference between plugging the headphones into my receiver versus directly into the sound card output? (I'm going to try anyway)

There can be.
The sound card output impedance could be roughly around 100 ohm (many line-outs, onboard ...) or just 10 ohm (Xonar STX for example).

In the first case the difference decreases to 5 dB, in the second to below 2 dB.
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #9 –
I'd expect about 47-75 ohms for onboard audio, maybe 100. That assumes headphone driving mode, which in some cases has to be enabled explicitly.

Output impedance will also affect frequency response. Headphone drivers' impedance may vary as much as shown for speaker drivers - a 50 ohm HD555 might range between about 55 and 230 ohms, for example. If it's one of the common "bass-light" HD555s, it should appreciate about 50 ohms of output impedance and would tend to sound thin below that. Typical simple receiver headphone outputs with 330..470 ohms would overdo it though, providing boomy bass. Bass quality never gets too great with these cans, either there's too little or it's uneven with a midbass hump. "Bass-heavy" HD555s would sound OK on a low-impedance output but quickly get boomy.

Higher impedance headphone is louder than the lower impedance one

Reply #10 –
Quote from: stephan_g on
I'd expect about 47-75 ohms for onboard audio, maybe 100. That assumes headphone driving mode, which in some cases has to be enabled explicitly.


I think its a lot better than that these days.  The last few generations of realtek parts have all been low output impedance (few ohms).

Higher impedance headphone is louder than the lower impedance one

Reply #11 –
The chips themselves, yes (5 or 2 ohms typical). Board manufacturers tend to implement series resistors though, mostly in order to get by with 100 µF coupling caps even in case of 16 ohm loads (which would show unacceptable bass response otherwise). In my work notebook I also found the curious combination of a chip that would allow for DC coupling used with regular AC coupling.

Higher impedance headphone is louder than the lower impedance one

Reply #12 –
Maybe this has changed with ALC1150? On a few boards with ALC898 I've measured up to 200 ohms in the back, up to 80 at the front panel. IIRC Realtek themselves even suggested a 75 ohm series resistor.

edit: nope, according to specs the ALC898 front panel output seems to be even better (if the mainboard manufacturer implements the 33 ohm instead of 75 ohm series resistor suggestion by Realtek) than ALC1150's.
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #13 –
Shouldn't it be the contrary ? If I use a lower impedance headphone with my O2+ODAC, I have to lower the volume as well.
 

Higher impedance headphone is louder than the lower impedance one

Reply #14 –
O2 has close to 0 output impedance, so output voltage will be fairly stable regardless of load. Now given the same sensitivity (dB SPL @1 Vrms) you are of course right.

But with high output impedance the output voltage is divided into two parts:
- one part drops across the output impedance
- one part drops across the load (the headphone driver)

The ratio of these voltages depends on the ratio of the impedances.
Zout = 100 and Zload = 100 would make an internal voltage of 1V would drop to 0.5V before the load. That's -6 dBV.
Zout = 100 and Zload = 33.3 would drop this voltage to 0.25V. That's -12 dBV.


You see high output impedance at headphone jacks for a few reasons:
- it's a cheap way to improve stability of the amp
- it's a cheap way to limit current (e.g. to prevent the amp from taking damage during a short)
- it's a simple way to make sure that low-impedance earphones won't receive too much power
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #15 –
Quote from: xnor on
O2 has close to 0 output impedance, so output voltage will be fairly stable regardless of load. Now given the same sensitivity (dB SPL @1 Vrms) you are of course right.

But with high output impedance the output voltage is divided into two parts:
- one part drops across the output impedance
- one part drops across the load (the headphone driver)

The ratio of these voltages depends on the ratio of the impedances.
Zout = 100 and Zload = 100 would make an internal voltage of 1V would drop to 0.5V before the load. That's -6 dBV.
Zout = 100 and Zload = 33.3 would drop this voltage to 0.25V. That's -12 dBV.


Makes sense. Thanks for the explanations xnor.

Higher impedance headphone is louder than the lower impedance one

Reply #16 –
Quote from: xnor on
Maybe this has changed with ALC1150? On a few boards with ALC898 I've measured up to 200 ohms in the back, up to 80 at the front panel. IIRC Realtek themselves even suggested a 75 ohm series resistor.

~200 ohms is expected if the output is in line-out (non-amplified) mode; it should be possible to switch to amplified mode somewhere in Realtek's Sound Manager. ~80 ohms sounds about right for 2 ohms + 75 ohms + inductor. The ALC898 DS does recommend 33 ohms, but I suppose board manufacturers often have better things to do than tweaking details like that...
(You'd think that they'd at least have learned how to avoid internal ground loops in the last decade or so, but nope, apparently stuff like that still happens.)

Higher impedance headphone is louder than the lower impedance one

Reply #17 –
ALC898 datasheet says something like "use 33 ohm for HP-OUT" (instead of 75), but I couldn't find this in the ALC1150 datasheet.

Anyway, even ~35 ohm could cause large deviations with certain IEMs.
"I hear it when I see it."

Higher impedance headphone is louder than the lower impedance one

Reply #18 –
I'm curious if anyone has measured the actual impedance on recent high end motherboards?

Higher impedance headphone is louder than the lower impedance one

Reply #19 –
The most recent I can offer is an ASRock Z77 Extreme4 (ALC898) with a bit below 80 ohm on the front-panel headphone out.
"I hear it when I see it."