Why would my headphones be louder when buffered vs driven direct from motherboard HP output?

[Deleted member 3116]

I noticed when driving headphones DT770 32 ohm directly from motherboard output vs buffering them there is large loss in volume compared to my HD6XX...

Why would this be?

The ALC887 audio chip in the mobo has a 2 ohm output impedance according to the datasheet, so the volume difference with 32 or 300ohm load should be negligible, but here they are so much louder when buffered that I can start to hear the hiss of the noise floor, while the HD6XX volume stay the roughly the same when buffered or not, as expected.

My only guess was that there could there be some sort of dynamic gain adjustment going on in the alc887 based on load impedance, since generally lower impedance will need less gain than high impedance headphones.
I didnt see anything like that in the datasheet, the output level is given as 1.2Vrms, it doesnt mention the load.

 

[Lambda]

Maybe the mainbord can’t handle the current?
maybe its not 2Ohm output impedance.

 

[Deleted member 3116]

Maybe the mainbord can’t handle the current?
maybe its not 2Ohm output impedance.

If it was running out of current then the noise floor would still be audible with nothing playing.

the 'non-amplified' output is 200 ohms on the realtek chip.
it's possible the mobo could bypass the HP amp for the rear output for some reason, though it calls it a headphone output in the manual

 

[Deleted member 3116]

After seeing that a couple of mobos measured on here both had at very high (>70 ohm) output impedance that has to be it, not like there is any other explanation for it.

 

[3125b]

The mainboards I have measured were around 80 Ohms.
And my subjective listening test confirms the effects of that, decent with sensitive and/or high impedance headphones, not good with medium to low impedance and low sensitivity headphones.

 

[twsecrest]

I noticed when driving headphones DT770 32 ohm directly from motherboard output vs buffering them there is large loss in volume compared to my HD6XX... Why would this be?
The ALC887 audio chip in the mobo has a 2 ohm output impedance according to the datasheet, so the volume difference with 32 or 300ohm load should be negligible, but here they are so much louder when buffered that I can start to hear the hiss of the noise floor, while the HD6XX volume stay the roughly the same when buffered or not, as expected.
My only guess was that there could there be some sort of dynamic gain adjustment going on in the alc887 based on load impedance, since generally lower impedance will need less gain than high impedance headphones.
I didn't see anything like that in the datasheet, the output level is given as 1.2Vrms, it doesn't mention the load.

Have no idea what you mean by "buffering"?

 

[twsecrest]

I noticed when driving headphones DT770 32 ohm directly from motherboard output vs buffering them there is large loss in volume compared to my HD6XX... Why would this be?
The ALC887 audio chip in the mobo has a 2 ohm output impedance according to the datasheet, so the volume difference with 32 or 300ohm load should be negligible, but here they are so much louder when buffered that I can start to hear the hiss of the noise floor, while the HD6XX volume stay the roughly the same when buffered or not, as expected.
My only guess was that there could there be some sort of dynamic gain adjustment going on in the alc887 based on load impedance, since generally lower impedance will need less gain than high impedance headphones.
I didn't see anything like that in the datasheet, the output level is given as 1.2Vrms, it doesn't mention the load.

The 32-Ohm DT770 would need less voltage, to get as loud as the 300-Ohm HD6XX.
But, the 32-Ohm DT770 might need more current (then the HD6XX). So it could be the motherboard's audio jack does not provide a good balance of voltage/current, that the dt770 needs?
(but I'm just guessing)

 

[Lambda]

Have no idea what you mean by "buffering"?

https://lmgtfy.app/?q=headphone+buffer
Basically an amp with out voltage gain but current gain

 

[solderdude]

I noticed when driving headphones DT770 32 ohm directly from motherboard output vs buffering them there is large loss in volume compared to my HD6XX...

Why would this be?

There is a difference in voltage efficiency. If anything they should not be equally loud.
Funny thing is it should be the other way around when you really have the 32 Ohm DT770.
Beyer = 109dB/V where HD6XX is 103dB/V so at the same volume level the DT770 should be noticeably louder.

When the HD6XX is louder you either do not have the 32 Ohm DT770 or the output resistance of the headphone section is not 2 ohm or there is some smart sensing going on.

 

[3125b]

Good point about the unsmart sensing (oh how I hate that). The newer Realtek chips do that, if you have the audio driver of the mainboard installed however, you can select a power setting manually (at least with my Asus).

If I calculate the maximum SPL with the 6XX vs. 32R 770 based on my Asus board I get 107dB for the 6XX vs. 105dB for the 770.
So yes, you are hearing that correctly, though it's not a big difference.

 

[Deleted member 3116]

There is a difference in voltage efficiency. If anything they should not be equally loud.
Funny thing is it should be the other way around when you really have the 32 Ohm DT770.
Beyer = 109dB/V where HD6XX is 103dB/V so at the same volume level the DT770 should be noticeably louder.

When the HD6XX is louder you either do not have the 32 Ohm DT770 or the output resistance of the headphone section is not 2 ohm or there is some smart sensing going on.

I was talking about the difference in volume between buffered and unbuffered for the 2 headphones there. It should basically be zero for both headphones if output impedance was really 2 ohms, the output impedance is so high that the D770 ended up sounding less sensitive than the HD6XX.

 

[solderdude]

What if you parallel 33 Ohm to the HD6XX ?

 

[Deleted member 3116]

I dont have a resistor to test , but i think it safe assume output impedance and possibly adaptive gain is the cause.

In some ways it is kind of good, because otherwise you would have nasty audible hiss on sensitive headphones with no way to attenuate it.
This might be reason they designed it to be so high.