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Audio Tracer Probe
 The audio tracer probe described here is an 'add on' for the Mr Carlson's Super Probe project. His project is a very
sensitive
'non-contact'
probe that feeds into a control unit which is basically just a self
contained audio amplifier and loudspeaker. The control unit has a 5 pin
DIN socket on the front which not only carries the audio input but also
9 volts from the battery, making it very convenient to connect other
accessories. So I thought it would be good to build another more
conventional 'contact' probe that can be used with a low cost
oscilloscope probe.
It's basically a single FET (Field Effect Transistor) that converts the
relatively low input impedance of the amplifier in the control unit (in
this case an LM386) to a much higher impedance, enabling it to 'probe'
a faulty audio circuit in order to listen for the prescence of a
signal, without placing any load on the circuit.
 Schematic
Field Effect Transistors have an extremely high gate impedance making them ideal for use as
a buffer between the circuit under test and the amplifier being used to
listen with. Its input impedance isn't infinate though, due to the gate bias
resistors, but it's still very high with the two 2.2MΩ resistors
used here giving an input impedance of 1.1MΩ (these values can be
increased if required). It has a gain figure of just under 1 so it isn't a
preamp, it's not meant to be, it's role is simply to appear 'invisible' when
probing around a circuit without influencing it in any way. The input has a 4.7KΩ limiting resistor feeding a 5.1V zener clamping diode which gives basic protection against voltages above +5.1V and below -0.6V. Crude but effective. There is of course no reason why this circuit can't be used in other
applications, maybe as a buffer to match a guitar pickup to the input of
a low impedance amplifier, in
which case the
protection components can be omitted.
 Veroboard Layout
The
stripboard is a standard size of 9 x 25 holes but in this case it had
to be cut down to 7 x 21 holes so it would fit the limited space inside the enclosure.
Connection to the board is via a 3 way PCB screw terminal block, where
a short length of balanced microphone cable connects. The other end of
the cable goes to a 5 pin
DIN plug that matches the 5 pin DIN socket on the control unit,
providing battery power, audio signal and ground.
 With Scope Probe
The major flaw with FETs
is that they are not manufactured equal. If you were to purchase ten of the same type
from the same manufacturer, they could each have a different
specification... not good from a designers point of view. Imagine
trying to build a completely matched stereo preamp! This is probably
why they're used a lot in things like guitar pedals, where their high
input impedance is required and because a bit of distortion may even
add to the desired effect. So I'm not trying for perfection here as the
purpose of this probe is essentially just to hear if an audio signal is
present or not. I know people will rip this design to shreds, but it is
what it is and it works fine for this purpose. If you want an FET
circuit that meets Hi-Fi expectations, then there is a lot of
information out there on the web, but be prepared for conflicting opinions,
pages of calculations and much experimentation. At the end of the
day, I won't be sitting down with a glass of wine listening to my
favourite piece of music through this device!
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