dbx 120A - Manual (Page 11)

Operating Notes
3
120A
120A User Manual
Section 1
Avoiding Transients
Any sharp noises from your sound system can be hard on your system, and since the 120A
amplifies and augments all low-frequency information, extra caution is called for at all times.
To be safe, turn the power amp on last, and off first. and always keep the main volume control
low or off when turning the system or any of its components on.
To further reduce the chance of dangerous transients, make sure your system is operating at its
optimum performance. We recommend you have all switches, button and knobs on your equip-
ment cleaned by a qualified person (especially if any rasp or crackle is present). If you are
using a turntable, check to make sure that the cueing is gentle.
Speaker Placement
For systems with multiple speakers, we recommend placing each speaker so that the distances
from the center of its woofer to the three nearest boundaries are as different as possible. These
distances are measured along the speaker cabinet sides, not in a straight line. When these
distances are the same, the bass response of most speakers is roughest, with peaks and dips
more than 10dB apart over just a few notes. Putting a conventional box-shaped speaker
woofer down in a corner will make it sound very bassy, but for the evenness of bass, it’s the
worst spot available.
The formula for calculating maximum different distances a, b, and c and least bumpy
speaker placement is: a/b= b/c, or b2 = ac, where a, b, and c are never equal.
The same acoustic laws apply to your listening position: the smoothest bass will result when
the distances from your head to the three nearest boundaries are as different as possible. In
the same manner, the boomiest rooms have all three dimensions the same and the smoothest
have them maximally different.
If you do reposition your speakers, you should also reset any output level controls on the back
of the speaker cabinet. Many speakers offer a slight HF or MF boost with these controls all the
way up, and the new increase in bass might be better balanced by such a maximum setting.
The 120A is not intended only as a device to create wall crumbling low-end. Judicious use with
a pair of full range speakers can create the sense that a pair of speakers with 15” woofers is
being used. An additional benefit of improving the sound of smaller speakers is that a pair of
smaller speakers and a 120A is more portable than a pair of two larger speakers.
Turntable Feedback and Low Frequency Rumble
If you are using the 120A with a turntable, the increased levels of bass may make your turn-
table system more susceptible to feedback, wherein the turntable base, platter, cartridge/stylus,
and/or the disc itself actually pickup and replay bass from the speakers. One symptom is an
increasing, flapping rumble as you turn the volume up (severely, the system begins to howl)
that disappears or is markedly reduced when the tonearm is lifted off the record. The cure is
to isolate the turntable, making it immune to low-frequency vibrations. Any LF rumble in the
system from turntable, air conditioning, through mics, etc., will be boosted by the 120A.
To reduce turntable feedback, begin by locating the turntable and the speakers as far apart as
practical. Next, short of replacing the turntable, since some turntables are much more prone
to feedback than others, shock-absorber feet designed specifically for this problem. A massive
base or table under the turntable may decouple it from the cases.
Note for CD Players: High levels of Bass can cause CD players to skip. It is a good idea to
locate CD players away from low-end drivers and loudspeakers, and shock mount them.