What causes low frequency rolloff under load ?

[percy]

When I measure the frequency response on my PC sound card using RMAA in loopback mode (line in=>line out) it measures ruler flat from 20hz-20Khz. But as soon as I put a load on it - like a headphone(33ohm) - the low freqency response starts rolling off as early as 70-80hz.

What could be the potential causes for this ? Could it be the output current capability of the output opamp ? If the opamp is capable of the output current that what could be the next possible cause ?

The output opamp is a TLV2465C http://focus.ti.com/docs/prod/folder...t/tlv2465.html

[neutron7]

does it roll off at all volume levels? or does it get worse as you increase it

[sangram]

Originally posted by percy


What could be the potential causes for this ?

Usually it would be the size of the output capacitor causing the rolloff.

I can't think of anything else actually.

[tiltedhalo]

Depending on many factors, frequency impedance, power and induction of the head phones and all the other variables. You get factor loading effect short of like a engine on a dyno 300hp at 5200RPM, but load the dyno and poof change of HP and torque due to RPM and load, same type of physics applys to electronics.

[Ouroboros]

As your sound card will be using a single +3.3v or possibly +5V rail, it will use an output coupling capacitor to block the dc bias on the output from getting to the headphone output. With low-impedance 32R phones, you'll need at least 470uF to get good LF response. The output cap on most sound cards is much lower than this, resulting in a first-order roll-off of the bass.

[AndrewT]

Hi,
is the sound card intended for driving a 32ohm headphone load?
An output of 1Vrms (about31mW) is 44mA peak current.

[sreten]

Originally posted by percy
Could it be the output current capability of the output opamp ?
If the opamp is capable of the output current that what could
be the next possible cause ?

Hi,

No, that would cause massive amounts of distortion.

Originally posted by sangram
Usually it would be the size of the output capacitor
causing the rolloff. I can't think of anything else actually.

Seconded.

/sreten.

[percy]

Thank you all. Your responses deserve some more information from me. Here are some pictures of the frequency response and distortion also if that helps give a better idea.

The freq response drops equally across the entire range - with an added rolloff in the low frequency range.

The output caps are all 100uf.

I dont know the recommended output impedance of the soundcard's output but the manual does say I can connect heaphones (and there is a headphone option in the windows/soundcard driver's mixer also which I dont think is doing anything). Oh btw its a Turtle Beach Santa Cruz card.

Its almost like a classic 1st order rolloff dont you think ?


Left (white) channel is the one connected to the headphone. Right (green) is looped directly to give you can idea of both the base soundcard and under load together.

[percy]

Total THD -

[percy]

60hz imd