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Re: USB sound cards

In general, I think most sound cards can drive an 8 ohm 
speaker without problems, though maybe not at very high 

However, there *is* a problem getting low output impedance 
as well.  The native design of modern amplifier stages has 
essentially zero output impedance due to negative feedback 
(milliohms or less). That means that if you connect such an 
amp to a low-impedance load, the current draw can be 
high... high enough to damage the output stages, or at 
least cause massive distortion as they go into protective 
current limiting.  Since these are for consumer use, where 
anyone can plug in most anything that fits the jack, 
manufacturer's typically add some output impedance.

As mentioned previously, the cheapie no-name 5.1-channel 
USB device has the lowest output impedance of the handful 
that I've tested (2 ohms... others ranged from 7 to 413).  
With a low output impedance, this card relies on current 
limiting for protection; with a 50 ohm load it can deliver 
only 1.14 Vrms, versus 1.5 with no load.  I didn't test 
with 8 ohms, but I suspect it will drive it just fine at a 
lower level.  (I may test at 8 ohms as well, just for 
future reference.)

And per Steve Beet's suggestion, the load definitely 
affects the low-frequency response:  The cutoff is less 
than 0.1 Hz open circuit, 1.6 Hz at 1000 ohms, and 16 Hz at 
100 ohms.  That doesn't bode well for the response at 8 
ohms.  I suspect this pattern will be common to most cards, 
but it will be a while before I retest them all.

You could consider adding a separate power amplifier, but I 
think that would be defeating your original goal of a small 
desktop setup. (The amp would present a high-impedance load 
to the sound card, so you'd get a good low-end response 
from the card.  However, you might need to increase the 
amp's AC input decoupling capacitor.  Yep, any audio-type 
amp will be AC-coupled, since DC would be a big problem if 
applied to speakers... overheating the coil, distortion due 
to asymmetrical motion of the voice coil, etc.)

The simplest approach, however, is probably to use the card
to drive the speaker directly, and measure the true
frequency response with a reference mic.  Then use that
calibration to correct future measurements.

By the way, all sound cards that I know of are AC coupled.  
Those "audiophile" cards that claim "DC coupled" use a 
servo-amp scheme that moves the capacitor out of the direct 
path (for audiophiles with capacitor phobias), but they 
still have a low-end cutoff just like ordinary cards.  

Best regards,

Bob Masta

On 10 Dec 2014 at 23:53, Henrik Møller (private) wrote:

> Dear list,
> Along with the present discussion of Bob Carlyon's question about sound 
> cards driving headphones with high voltage, I have another question 
> about sound cards.
> I want to calibrate the low-frequency response of a microphone in a 
> small closed coupler driven by a small loudspeaker. Although I can do 
> this in a laboratory setup, I would like to make a desktop routine check 
> - i.e. with the smallest setup as possible.
> A USB sound card that can drive an 8 ohm loudspeaker directly would be 
> perfect. Some old Sound Blaster PCI cards could, but I wonder if there 
> are newer cards that can? To be more precise, the output impedance 
> should be much lower than 8 ohm, e.g. 1 ohm or less.
> I need a decent low-frequency response, too. Preferably to DC, but at 
> least to a few Hz.
> Latency and high voltage are not issues. Low noise and low price would 
> be nice but not necessary.
> I wonder if someone has any experience and proposals?
> Best regards,
> Henrik
> -- 
> Henrik Møller
> Professor Emeritus
> Aalborg University
> Phone: +45 9940 8711
> Private mobile phone: +45 4280 3950