Posted by Lew on September 04, 2002 at 07:16:37:
In Reply to: M1 crossover frequency posted by Larry Klein on September 03, 2002 at 19:25:12:
Accumulated information from my own U2 and M1 spkrs and from others suggests that the x-over frequency is different for different spkrs based on the model and the vintage. In my U2s, the x-over frequency (really, the -3db point of the hi-pass filter) was about 1.2kHz. The component values were C=16uF, R=7.5 ohms. In my M1s, the "original" x-over frequency was 550Hz, with C = 48uF, R= 6 ohms. In both cases, the slope, as for any first-order RC network, is 6db per octave. If you have the "new" backplate for use with OTL or other tube amplifiers with a high-ish output impedance, the x-over point remains 550Hz, with C = 24uF, R = 12 ohms, and the added inductance in parallel with R (L=15mH). The addition of the inductor does not alter the x-over point (according to impedance v frequency measurements I have seen, but maybe not according to the math) but does create a second-order hi-pass filter, with slope = 12db/octave. A3s appear to use a different x-over point, i.e., with different values for R and C. Either that or the choice of x-over depends upon the age of the spkrs. In other words, new or late production A3s may be just like M1s, which I assume are just like A1s and U1s.