Seven Band Graphic Audio Equalizer & Real Time Spectrum Display
The finished and working unit.
One part of the receiving chain that is often overlooked or just disregarded is how we handle the recovered audio. This is kind of ironic. After all the pains and efforts we
go through to build the very best receiver we can, for the purpose of receiving audio from far away places, we tend to disregard the quality of that audio after we receive
it. Makes you think...huh? I've said it so many times before, but I'll risk the nauseam to make the point for the sake of those few folks who never heard my mantra; "I
seldom, if ever, listen to a receiver through its internal loudspeaker." That speaker is there strictly for the purpose of monitoring ... for you to "sample" the received
audio. Of course, for mobile, portable, field and other non-base use, the internal speaker has its obvious purpose.
Some time ago I built an audio console, and there have been several designed and built since. These consoles offer varying degrees of audio signal conditioning. With
the use of various filter circuits we have provided audio spectrum control from as basic as Bass and Treble control to fully Parametric Band Compensation. Once audio
tayloring has been experienced and used for some time, it becomes a necessity ... there is simply no going back. Of course a decent loudspeaker system is a must, lest
all careful efforts are wasted.
Here's what I finally came up with, without getting too much into the "studio" realm or the home-entertainment theater arena. This unit is practical for almost all broadcast
purposes, save the most serious audiophile's requirements. This is a 7-band equalizer. I call it "parametric" since its center frequencies can be shifted, but that is an
internal adjustment, and I prefer to keep it that way - as set and forget - for the sake of operational simplicity. The audio band consists of 10 octaves (or 11, depending
on how you slice them). Here we are concerned with the middle seven bands. This is more than adequate for SWLing, AM DXing, utility monitoring and Ham Radio
For the sake of clarity, here's the decal
Closeups of the decal after being applied
to the front panel.
The Primary and Secondary controls for
the analyzer display select either the input
or output (raw or equalized) signals. You
can measure the RMS or peak levels and
there is a "Hold" function to freeze the
data. The "Line Feed" control adjusts the
gain of the "send" signal level for the
transmitter feed, recorder, studio monitor,
A couple of photos of the inside layout prior
to cable dressing.
The Power switch
indicators show if the
Mains Power is properly
configured and if the
Earth Ground from the
chassis or equipment
rack is within a couple
microamps of dead