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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
Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!