On Wed, Jul 27, 2011 at 01:41:51AM +0300, Sampo Syreeni wrote: > On 2011-07-26, Fons Adriaensen wrote: > >> I certainly don't want you to waste your money on fancy speaker >> cables. > > Never thought otherwise. That's obviously never been what we do here. ;) > >> But resistance does matter, so a good cross section such as 2.5 mm^2 >> puts you on the safe side. > > What I was trying to ask is, what's the real problem with resistance, > especially with regard to a passive speaker and a modern, A/B class > solid state end stage? I mean, I don't really see cable resistance > shifting their operating point much, even with feedback, within the > audible range. > > What is it that I'm missing?
When the voice coil of speaker moves in the magnetic field it is surrounded by it generates a voltage proportional to its velocity. Ideally that voltage should be equal to the one produced by the amplifier: in that case the amplifier has complete control over the movement. You can easily test this: disconnect the speaker and gently push the cone of the woofer. You will see it moves quite easily. Now connect the speaker and switch on the amplifier, OR just short-circuit the speaker terminals. In both cases the speaker sees a very low impedance, and it will resist movement. In practice there is a problem: any resistance in series with the 'ideal' voice coil means that those two voltages are not equal and the amplifier is not fully in control. The resistance that appears in series is the the sum of the DC resistance of the voice coil itself, cable resistance and the output impedance of the amplifier. This sum should be as small as possible, and cable resistance can be a significant part of it. One advantage of integrated amps/speakers is that the amplifier can be designed to compensate for this resistance by giving it a negative impedance. This has to be controlled very carefully - overdoing it makes the whole thing unstable and ready to auto-destruct. Which is why it can't be done with separate amps and speakers. Ciao, -- FA _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
