SETTING UP THE SYSTEM
This can be tricky.
I used a TEF analyzer and a $40,000 1/24th-octave Brüel & Kjær real-time
analyzer and intensity probe set of phase-matched microphones (another
$14,000). First, I did energy-time measurements to set the delays in the
amplifier. This proved to be difficult, since I had to take distance
ranging measurements of each driver separately to make sure I was looking at arrival times from the intended measurement object. After I
got the delays set, I checked frequency response the best I could in the
space available, then resorted to the real-time analyzer. Be aware that
the frequency responses you get with these two methods are very
different because TEF windows its measurements to try to exclude
reflections and examine only direct sound from the source, while
real-time analysis includes all returning room energy information along
with that from the speaker. I like a balance of both measurement
methods, because one lets you fine tune the energy output of the
speaker, and the other lets you adjust large trends like the general
"too-bright" high end you will likely notice if you try to obtain flat
output to 20 kHz from the direct-sound readings of a truly power-flat or
"constant-directivity" horn. Such horns produce more high frequency
energy in the first place because they distribute the energy over a
larger angle, that is to say, if one equalizes such a horn for flat
response, then more high frequency energy will be pumped into the
listening space than would be the case for other types of horns. Most
people are not used to listening to power-flat top-end, and will find it
too brassy. Only minimal-miked big-band recordings will sound right with
the system adjusted this way. For your playback system head end, I
recommend that you use an extremely low-noise preamp with simple
Baxendall type "tone controls." The bass and treble turnover frequencies
are a matter of taste, but 100 Hz and 10 kHz or 200 Hz and 5 kHz seems
to work well to adjust this system to music on recordings. To my ear,
parametric equalization is less pleasing and is certainly prone to
putting more phase aberrations in more audible frequency ranges than are
simple tone controls. One-third octave "graphic" equalizers are
completely useless for high fidelity use and one-octave units are even
worse-avoid these. They introduce horrendous phase shift problems of
their own, and if not properly analyzed with the system they are
driving, will virtually always degrade performance.
Begin adjusting with the amp master gains set low,
and turn the preamp all the way up. Slowly advance the amp gains until
you can achieve proper balance and slightly louder than necessary
output. This will ensure that you will have the least possible amplifier
hiss from the speakers. Amplifier hiss is a phenomenon that rarely
troubles owners of low-efficiency speakers, but these monsters are
efficient enough to make the transistor junction noise of poorly
designed amplifiers quite audible.
If any of you reading this decide to build this
system, it will cost around $10,000 including amps and all. If you get
that serious, if you are rich and adventurous and don't care what stereo
salesmen think, you are the type of person who would really enjoy this
system.
© 1997 Drew
Daniels