Subject: Re: USB sound cards From: Bob Masta <audio@xxxxxxxx> Date: Thu, 11 Dec 2014 12:03:13 -0500 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>In general, I think most sound cards can drive an 8 ohm speaker without problems, though maybe not at very high levels. 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