Ashly Electronic Amplifier none - Manual (Page 29)

Figure 31
The summed amplitude response of
the Ashly 18dB per octave
crossovers. For these models, flat
summing is achieved by setting the
dB control to 3.
Another nice feature of the state-variable filter is that once you set the
damping for a particular response, you don't need to re-adjust it for new
crossover frequencies. For example, if you own an Ashly 12dB per octave
crossover and have set the damping (dB) control to 6 for flat summing, you can
then set the crossover frequency anywhere you want and the outputs will always
sum flat. This is not true of other crossover designs.
At this point, you might ask why we don't simply tune the filter responses for
flat
summing and remove the rolloff (dB) control from the front panel
altogether. After all, who wants less than a perfectly flat response? To
answer that question, you have to put everything back into perspective and
remember that the crossover is not goi.ng to be connected to a resistive
summing network. In the real world, you'll be hooking the crossover up to
amplifiers which will in turn be connected to speakers which will in turn be
acoustically coupled to the air and to each other. Loudspeaker reponse is not
likely to be ruler flat at the crossover frequency; instead, there will likely
be a hot spot or a deficiency in response due to the imperfect nature of the
speakers. When that happens, all those careful calculations about flat
summing fall flat on their face. All of the sudden, you find that you need an
equalizer right at the crossover point, and with an Ashly crossover, you've
already got it. Take another look at those response curves of figures 30 S 31
above; in practive, being able to modify the damping of the filter in your
crossover is just like having an equalizer tuned precisely to your chosen
crossover frequency. The Ashly SC series crossovers give you the best of both
worlds: electrically flat summing to satisfy the lab technician, and a choice
of response shapes to help you approach acoustically flat summing where it
really counts—in a real-life loudspeaker system.
CROSSOVER EVALUATIOM
A popular and convenient method of evaluating a particular crossover is to
look at summed amplitude response, which can be accomplished by simply wiring
all the outputs of the crossover together in parallel through an appropriate
resistive network. The theory behind this sort of examination is that if you
add all the outputs together, the sum of the individual pass-bands should
equal the input signal. That is, the crossover should be capable of a flat
summed response. It is often said, incorrectly, that 12dB per octave
23