ProSound xover wiring - wtf is going on??
I had the unfortunate job of opening up a ProSound PA speaker today, in order to replace a blown bass driver (with a better one) and couldn't help noticing the crossover wiring...it is made up from a 2nd order high pass filter and LPad for the compression horn with 2 transistors (TIP122 and TIP127) on a huge heatsink and 2 power diodes. The horn is wired + to + surely it should be reverse phase for a 2nd order network. The bass driver has just got an iron cored inductor in series with it, 1st order, but really really bad design by the look of it generally.
I'm just wondering if anyone else has seen any ProSound internals before, and could shed any light on the transistor and diode business and the incorrect phase for the horn?
To cut a long story short, I instinctively bypassed the low pass filter completely, and reversed the polarity of the horn driver. It sounds tons better!!! What were they playing at???!!!
Electrical crossover order is not consequential when it comes to polarity of connection to the driver- only the resulting final acoustic slope is important.
The second order electrical circuit most likely accelerates the natural 2nd order roll off of the compression driver resulting in final acoustic slope of 24dB/oct or 4th order. That crossover type is generally connected with normal (plus to plus) polarity. It could also result in final acoustic slope of 18dB/oct, where the summation in frequency domain is the same regardless of polarity- only phase response changes.
The woofer having just a simple 1st order electrical LP filter on it could end up with a second, third or even 4th order acoustic roll off depending on the value of the inductor or in other words frequencies at which it the electrical filter is effective. So, the correct polarity of connection is always dictated by the resulting final acoustic response.
Also, the physical offset of drivers will have influence on the "proper" polarity of connection.
Without measuring you will not know for sure what polarity is better.
Putting in a "better" bass driver should always be followed by redoing of crossover. The raw frequency response and impedance will never match what was there originally, not to mention the TS parameters responsible for low frequencies behavior of your woofer in your box. Your new woofer could result in either overly boomy or weak low end depending on how it interacts with the box. Again, measuring or at least modeling would give you some answers.
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