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Re: Driving headphones at high peak levels
On 9 Dec 2014 at 12:56, Steve Beet wrote:
>
> Bob, that's a very interesting and thorough analysis,
> but the weak point of the sound card you recommend seems
> to be the low-frequency response.
The output low-frequency cutoff is 3 Hz. Line In is 41 Hz,
which is why the loopback plots look bad.
>As the 2 ohm output
> you refer to is labelled "line out" in your document,
> doesn't that suggest that it is designed for driving
> high imedance loads?
The device has no separate headphone output. There is no
problem with using a low-impedance output to drive high-
impedance loads... that's the ideal situation. It's
harder (more expensive) to build a high performance output
stage with low output impedance, so it's common to have
high-Z Line outputs and low-Z (and maybe reduced
performance) Headphone outputs. But since this is a 6-
channel output device they probably wanted to eliminate the
need for a separate Headphone jack on the compact case.
> If so, there is probably a
> decoupling capacitor on the output so that low
> frequencies are attenuated and driving a low load
> impedance (e.g. headphones) will decrease the time
> constant and make this defect worse.
There is definitely a decoupling capacitor on the output,
as on pretty much all sound cards. (They need to produce
ground-referenced AC signals, and they have a single +5 VDC
power source, so they use AC coupling on the inputs and
outputs, and reference the internal signals at typically
about 2 VDC.)
But you have mentioned a valid point, which I will check
out shortly. If I find big differences for loaded low-end
responses, I'll add another line to the test report.
Thanks for pointing this out!
Note, however, the OP wants to give pulsatile stimuli, not
steady sub-sonics. <g>
Best regards,
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!