Interfacing one box to another is always a chore! Some of the things that
should always be checked before just connecting two things together, are
1) issues of signal level, and content of the desired signal source also
having an undesired DC component that might hinder the operation of the box
you are adding.
2) Impedance matching. This is especially important in cases such as RF, but
in this case, where the source is a power audio signal, and at low impedance
since it also drives a speaker, some mismatch could be tolerated at the cost
of audio distortion if you were only going to another audio device.
3) But for a multi mode digital TNC, you probably do not want distortion from
either overdriving from too high a signal level, nor do you want to have too
low a signal level. Know what level the follow on device wants. Find out if
that is too low, or too high relative to normal speaker volume, and adjust as
4) To accomplish 1), 2) and 3) above, you have to read the manuals. Now, I
know there is this pervasive habit among hams, of "if all else fails, read the
book", but at least get the schematics out, and see if the audio output you
are tapping has a capacitor coupling to the speaker, and measure to see that
there is not an audio signal riding on a DC component such that the DC level
might upset the follow on box.
5) Ways around a DC offset include a small audio coupling transformer, such
as the speaker impedance to 500 ohm or 1000 ohm one that Radio Shack has
carried for years. Signal level can be measured and adjusted with the volume
control, and if necessary stepped up or down with a transformer, if the
preceeding circuit has the needed power to drive both the speaker and the
accessory circuit. Many times for audio where high fidelity, (music) is not
the goal, you can turn a transformer "backwards" to adjust a voltage. Even
small filament transformers from tube era junkboxes make adequate audio
transformers, of high power rating.
Aircraft surplus 400 cycle transformers once so available to hams in surplus
work fine for audio. Capacitor coupling can block a DC component, as long as
the cap does not charge to some DC level and remain at that DC voltage. A
bleeder resistor to common of the circuit could be provided to provide a load
into which the cap discharges in that case.
6) without transformers, one could still use a combination of capacitor and
resistive voltage division to adjust a signal level, and block DC. A pot
with one end in series with a cap, and with the signal taken from the rotary
tap and the other end would work as well.
7) There are simple rules of thumb for picking component values. The DC
blocking cap that must pass audio should have a reactance one tenth the
circuit impedance at audio frequencies to be passed. Usually, you use the
lowest frequency in determining this. A resistor string that is a bleeder,
can be made a total of 10 times the impedance of the circuit, without taking
significant power during the normal circuit audio excursions. A wide variety
of component values will work for audio; use what is already in your junk box
before spending effort on exact values.
The above was written to be used as general guidelines for any interfacing,
hope it is helpful to some.
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