Loudspeaker Designs and Articles for the DIY Enthusiast
|A Design utilizing both
More on Crossovers:
The QTP woofer topology: The woofer acoustic transfer function most closely
emulates 2nd order behavior with a Q of about 0.4, compared to a Traditional LR
has a Q around 0.5, and has an fc of 1700 Hz. The woofer is connected with
normal polarity, but is in phase quadrature with the mid. This is also the simpler
and less expensive topology in this instance.
The NTP woofer topology, on the other hand has a woofer transfer function that resembles a 2nd order LR acoustic function with an fc of 350 Hz and
requires the woofer to be connected with reversed polarity.
In both designs the midrange and tweeter transfer functions are unchanged. This both simplified the design and also minimized the number of switch poles
required to switch between the two designs. The midrange high pass filter closely emulates a 2nd order acoustic function, again with a Q of about 0.4. and
an fc of 325 Hz. Those familiar with John’s TP articles know that there is a Q boost required in these transfer functions, but my functions do not strictly
comply with his calculations. –I merely chose fc’s and transfer functions that resulted in a reasonably flat on axis response. (Another reason to call this
design ‘Quasi’.) I used a 3rd order electrical low pass filter on the midrange, which summed with the mid’s actual response, provided a 5th order BW
acoustic response at 10 kHz.
The tweeter high pass filter most closely resembles a 4th order acoustic transfer function at 12 kHz. Why did I cross so high? One reason was I wanted to
keep the tweeter phase issues as far away from the midrange and woofer as possible, minimizing its effect on the QTP aspects of the design. The other
reason was that the TB mid response ‘fell off of a cliff’ around this frequency, but below 10 K exhibited excellent off axis response. The ND16 really only
provides ‘air’ in this design, but is audibly missing if not present. Comb filtering was a concern, but turned out to be nearly inaudible in practice.