Motherboard output impedance

[TwoEyedJack]

Hello, I while reading some Realtek DAC datasheets (used in computer motherboards such as the alc887 or alc 1200/1220) i noticed that many of them have different have a high output impedance on their rear line out usually around 150-200ohm and a lower one on the amplified output that are normally used for the front panel headphone connection. I suspect the frequency response and power handling of the front and rear Connections should also differ, even though I have read that some motherboards omit to connect the headphone output (P-OUT) anywhere(i guess they just swap the output between front and rear I/O depending on whether there is something connected on the front, there are however some motherboards/drivers that allow you to have audio outputted both from the front and rear at the same time, which probably use the headphone output.)

My question is, how can people get away with plugging in their 32ohm headphones into the 200ohm rear line out output, wouldn't that be similar to shorting the output stage ?

 

[fpitas]

Often outputs feature a series resistor to damp oscillations that can occur from feeding cable capacity. If that's the case, you just get a wacky response at your headphones that follows the impedance curve, like an amp with a low damping factor feeding a speaker.

 

[NTK]

That's what the 200 ohm output resistance is for. If you short the output, the output stage will still see a 200 ohm load, therefore limiting the short-circuit output current.

 

[fpitas]

That's what the 200 ohm output resistance is for. If you short the output, the output stage will still see a 200 ohm load, therefore limiting the short-circuit output current.

That too

 

[TwoEyedJack]

So the headphone out and the lineout don't have different output impedances ? Would that make the front I/O and the rear I/O ports the same ?

This is taken from the spec sheet of the alc887. Notice the different outputs. I thought one was wired in the back and the other on the front.
I must have a wrong understanding of impedance matching, i though inserting a lower impedance load on a high impedance source was a big no-no,

 

[NTK]

So the headphone out and the lineout don't have different output impedances ? Would that make the front I/O and the rear I/O ports the same ?

This is taken from the spec sheet of the alc887. Notice the different outputs. I thought one was wired in the back and the other on the front.
I must have a wrong understanding of impedance matching, i though inserting a lower impedance load on a high impedance source was a big no-no,

View attachment 239665

The spec says amplified output has 2 ohm resistance, whereas the non-amplified out is 200 ohm. So the headphone out and the line out should probably be different.

You just have to look at the voltage divider equation. The voltage across the load is:

V_load = V_source * R_load / (R_load + R_source)​

If R_source >> R_load, then V_load will be a tiny fraction of V_source. Also, if R_load changes with frequency (and R_source is comparable to R_load or larger), then the V_load to V_source ratio will change a lot with frequency. Both are undesirable.

However, the line out is intended to feed a load impedance of typically 10 kohm. Therefore, a R_source of 200 ohm is still low (relative to the load of 10 kohm).

The rule of thumb is R_source should be ideally <1/20 of R_load (minimum of R_load if R_load is frequency dependent).

 

[TwoEyedJack]

The spec says amplified output has 2 ohm resistance, whereas the non-amplified out is 200 ohm. So the headphone out and the line out should probably be different.

You just have to look at the voltage divider equation. The voltage across the load is:

V_load = V_source * R_load / (R_load + R_source)​

If R_source >> R_load, then V_load will be a tiny fraction of V_source. Also, if R_load changes with frequency (and R_source is comparable to R_load or larger), then the V_load to V_source ratio will change a lot with frequency. Both are undesirable.

However, the line out is intended to feed a load impedance of typically 10 kohm. Therefore, a R_source of 200 ohm is still low (relative to the load of 10 kohm).

The rule of thumb is R_source should be ideally <1/20 of R_load (minimum of R_load if R_load is frequency dependent).

Thank you for this explanation . This furthers my argument though, how can some people get away with plugging in their 16 ohm headphones on the 200ohm lineout without completely destroying audio and power. This is something that is commonly recommended in tech sites as to minimize the effect of EMI from the rest of the pc components acting on the cable that runs from the motherboard to the front panel.

 

[NTK]

It won't hurt the hardware, as the high line out resistance is current (and therefore power) limiting.

They would just get a much reduced maximum output, plus frequency response modulation due to frequency dependent impedance of the headphones. If their headphones are sensitive enough, they may not notice the limited max output. And they can get used to the frequency response modulation with time as our hearing adapts to it.