I did not realize that differences in driver resistance would mess up the crossover. I measured the DC resistance of the old PSB tweeter and it is 4.6 ohms. According to the specs for the Vifa, it measures 2.9 ohms. So would I use a 1.7 ohm resistor. Or should I be going with "nominal resistance"? PSB lists the Image 4T as 6 ohms nominal. The Vifa tweeter is 4 ohms.
A 1.7 ohm resistor would be best. Your main objective with this resistor is to compensate for the difference in impedances between the tweeters, and the DC resistance is a better indication of this difference because nominal impedances are, to some extent, approximations that allow manufacturers to group speakers into approximate categories (4 ohm, 6 ohm, 8 ohm, etc speakers). In this case the difference wouldn't be big (1.7 vs 2 ohms) and the difference in sound quality might not even be noticeable, but still, 1.7 would be better. In reality you'll actually have to use a 1.8 ohm resistor because 1.7 ohm resistors aren't a standard value.So, looks like you'll need an L-pad after all, since a 1.8 ohm resistor won't provide much attenuation. When doing the L-pad calculations, use the nominal impedance of the tweeter, as I mentioned before. This may sound contradictory to what I just said above about using the DC resistance, but the situations are slightly different. Above, the value depended on the *difference* between the tweeters, whereas here the values depend on the actual impedance of the tweeter at the frequencies of interest (which would be frequencies at and above the crossover point). A speaker driver's impedance varies drastically with frequency (check the datasheet of almost any driver and you'll see what I mean; for this Vifa tweeter, unfortunately, there is no graph of the impedance vs frequency), and generally with tweeters the actual impedance at the crossover frequency and above is closer to the nominal impedance than it is to the DC resistance. So, when calculating an L-pad, if you don't have a graph of the impedance vs frequency then it's best to use the nominal impedance.All of the above is, really, just approximations for various reasons:- We don't have a graph of the impedance vs frequency of either of the tweeters- We don't know how the original crossover was designed. Some manufacturers design a crossover by using the driver's nominal impedance (which is bad), whereas others use the actual impedance at the crossover frequency (good). So, if the crossover in your PSB speakers was designed using the nominal impedance then the issues that it would cause would probably overshadow most of the design concerns I've addressed here (although, still, the differences won't be too noticeable).- The current crossover might have other elements that affect the response, such as a Zobel network, tweeter resonance compensation, etcBUT, in spite of these approximations you're still going to end up with a really awesome system! These approximations won't be noticeable by very many people.Finally: power ratings of resistors. If you're not going to be running these speakers at concert level SPLs on a regular basis, then 5W resistors should be fine. However, if you do regularly play them loudly for more than a few minutes then 10W resistors would be a safer way to go.
Date published: 2012-02-09