On Tue, Nov 09, 2010 at 08:44:05PM +0100, Jörn Nettingsmeier wrote: > On 11/07/2010 11:30 PM, peiman khosravi wrote: > > Thanks jörn for your enlightening notes. > > > > Firstly, the higher frequencies are there as the byproduct of certain > > FFT processes (e.g. spectral stretching). I am transposing or stretching > > the spectrum and so some of the lower spectral materials move up the > > spectrum. I could filter them out I guess but then why filter them if > > they are not audible. > > i see. well, maybe do filter them :)
You really should filter them out. Even if you can't hear them, at high levels they will take power. Most amps dislike driving high HF levels into capacitive loads (such as a long speaker cable). Pro amps often have bandpass filters right at the input, but they won't usually affect anything below 30 kHz or so. > i've seen p.a. power amps die after starting to oscillate in the > ultrasonic range. only a month ago, a qsc amp died on me this way. it > had 3 12" monitors connected to it (and yes, it's being marketed as > 2ohm-safe), and all of a sudden, its clipping leds went full on. How was the wiring ? 1) Separate (long) cables to each speaker. 2) Daisy-chained, one long cable and few short ones. 3) Daisy-chained, all long cables. > funnily > enough, it even *created* a signal at its inputs - when connected to a > cheap small yamaha mixer, the mixer's master meter would max out (and > fall back to zero when the amp was disconnected, so i can be sure it > wasn't the mixer that generated the problem signal). That's really scary. Not only did the amp generate this signal, but it managed to create a level at its input that even when driving the low output impedance of the mixer was enough to max out the meters... > now my uneducated guess is, if an amp-plus-speakers circuit is > susceptible to such oscillation, maybe excessive hf content triggers it, > and drives the amp into saturation. (even if it's not a runaway > oscillation but just long ringing at loop gains < 1.) > a clipping amp might produce undertones as well as overtones, so that > might actually produce artefacts in the audible range. Certainly possible. Power amps can do weird things, such as a single period or a short bursts of some very high frequency superimposed at a fixed point of the cycle of a lower frequency signal. Without input all seems OK. Ciao, -- FA There are three of them, and Alleline. _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
