Audio Signal Leveler (Compressor) and VOX/Squelch Controller
In all my Ham and SWL activities, I always wanted a device that would work like a microphone VOX circuit, but for the audio output. This would be particularly nice
to reduce 'listener fatigue' for long-term listening to weak stations...down in the mud...and maybe during noisy atmospheric conditions. Additionally, the different
volume levels of weak stations and loud, local ones required constant adjustment of the audio gain control (volume) on the receiver. And, while I was wishing for
all these conveniences, why not a good equalizer that would make listening to music or voice more enjoyable. At a minimum, that would have to be a 3-band
circuit for the bass, treble and the voice-only bands. I really don't like empty dreams, but prefer the reality of those dreams, so the only option was to start
experimenting. After many build-and-test excersizes, here is the final device. For now, I can stop dreaming and start "using."
The VOX circuit. This is a little kit I found at Jameco
Electronics. Having their PC Bd. is much better than
hand wiring a circuit on perfboard.

This is designed for mic-level audio. If you are coming
in with line-level audio, you will need to reduce the
gain of U1A by reducing the values of R12 and R2.
Also "H11" should become a 5K pot with its wiper
feeding pin 2.

The output from Q1 can switch a low current signal
type relay. I chose to use an opto-relay since they
have no mechanical parts...and the clicking might
become annoying.

A good opto-relay would be a form-c configuration
(SPDT) like the TCC-110. The output of the relay can
then control the audio line by switching it on and
off...silently.

Below are some applications for the opto-relay device.
This is the compressor, which I prefer to call
an "Audio Leveler." You drive the input to
more than 3 volts of audio and the output
will always be (about) 0.77 V RMS.

The labels in the schematic explain what the
circuit is doing.

NOTE:
The VOX and Compressor circuits are
fed through double-throw toggles so
they can be enabled or bypassed...see
front panel controls.
These are some various indicator circuits. Note that they have different functions. The original function was to indicate if a carrier had data (voice) on it of was
just an empty carrier or maybe you are tuned to a frequency where there is no signal. The capacitor that hangs off the signal rectifier which feeds the switching
transistor is critical to the mode and function of the circuits. In the case of the "carrier/Data" function, I found that 1 uF is ideal. However, if you use the circuit as
a "Silence Alarm" then that cap value must be increased. 470uF gives about 3 seconds of delay before the circuit activates the relays. In the case of the VOX
(above), that value is selectable to adjust the amount of time before the VOX kicks in (see front panel switch).
This is the layout and wiring. It's a prototype that has been changed many times, so it's a bit messy.
Another nice little kit that supplies a finished PC bd is the Future Kit Mono Tone Board (kit). I used one for the low-pass (bass) and high-pass (treble) controlls
and a second one for the "voice-only" (presence) control. That was accomplished by wiring the bass and treble to a dual gang pot. So that when you turn it
CCW, both bass and treble get attenuated which provides a nice voice-only response. Conversely, you can turn it CW when you have a good music signal and
enhance the playback quality. Obviously, both the functions could be done with the bass and treble, only, but that would be less convenient due to more knob
twisting. Either way, it works nicely. It's you call how you want to do it.
See it in action here on youtube video

Play around with the different modules
and arrange things to your liking.

Have fun with it


Thanks to my friend Rick Cutter whose enthusiasm for these kinds of projects inspired me to update
this page. Look for his SWL projects on
youtube.
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