as a Bi-Amp Crossover

  The first stage gave the signal a fair boost with a gain of 37 dB.  It also included the 2122 Hz low pass pole so that the signal leaving this stage was flat from 500 Hz on. Here was the top end of my crossover. All that was missing was to add a high pass filter to the output via the coupling capacitor working into the high frequency amplifier of my system. This filter is needed to continue the 500 Hz transition point down to DC.   
  The lower half requires more gain, as the signal level below 50 Hz is 20 dB down compared to the mid and high frequencies. The exact amount of boosting required does not require any precision as the speakers will always differ in sensitivity as the amplifiers used often do. I found that the +33 dB provided by the 12AT7 based stage worked adequately.
  But gain alone will not suffice. The 500 Hz crossover point must be converted from a high pass to a low pass one. Adding a 50 Hz low pass filter to the signal path both equalizes the low frequencies to a flat response and establishes the 500 Hz low pass point.
  If you were looking for the volume control, it is inside the preamp. A 100 cathode potentiometer was used to adjust the output signal simultaneously for both high and low outputs. What about FM, CDs, tapes? I lived on the wrong side of a mountain and I could not receive FM and the rest hadn't mattered to me.

   The equalization curve impressed on the phonoghaph helps improve the signal to noise ratio at high frequencies and increase the possible playback time. The job the preamp has is to provide a reverse equalization curve to restore the signal to flat. It must both boost the low frequencies by 20 dB and decrease the high frequencies by 20 dB relative to the mid-band frequencies. The frequency transition points are at 50 Hz, 500 Hz, and 2122 Hz.
    Now what happens if the flat output signal is sent to an electronic crossover for a bi-amped system? The signal is split into two paths and each receives its own frequency contouring. It is almost as if the signal must be built up and taken apart again. Or least that was my reasoning, 15 years ago, when I owned a bi-amped system that consisted of Strathern ribbons on the top and Audex 6.5 inches on the bottom with a crossover point of 500 Hz.
   What if the much of the signal on the record could be preserved and delivered unprocessed? Why flatten only to bend later? Why build an amplifier with 60 dB of gain and throw away 20 dB of it at the mid frequencies? These questions compelled me to design a bi-amped phono preamplifier.


pg. 10


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