Audio probe... and "Inject audio" probe

[Fp-www.Tonepad.com]

Having used the audio probe many times with great results, I have come up with a new debugging tool:

The "Injection probe".

Which is nothing other than a buffer (FET, like Tillman's) who's output is connected to a probe.

You take the probe and inject (thus the name) signal to different parts of the circuit.

With the audio probe you debug from input to output, connecting an audio source to the circuit's input. But what if you wanted to test only the output stage?

With the "injection probe" you debug from output to input. Connecting the amplifier to the circuit's output and injecting sound to different stages.

It would probably be a good idea to include an amplification stage and level control to the injection probe.

Ever wanted to hear how the tone control of the BMP or TS sound like on their own? use an injection probe and an audio probe to audition that part of the circuit.

Hopefully this is a new idea and not something already invented (I know there are signal generators/etc, but if this is a smart idea you'll know the difference).

Fp

[wui223]

My thought is why don't we combine the signal source with audio probe and maybe a small wattage amp for output also. So we can have all in one unit, signal generator+probe+output . Anyway would you share the schem of your injection probe ?

[vanhansen]

I like this idea.  Any way to make it fit in to a ball-point pen?   

[troubledtom]

 
  i like this idea, good job!
         - tom

[petemoore]

  I used to hook my guitar through a cap, [also need ground BTW].
  Then I'd try to strum it hard and use the few seconds of sustain to inject it's output for debugging.
  Went as far as to connect a keyboard to Same ground and cap [alone] and tape one key down for a source of input to inject so I could have my hands free.
  For high voltage circuits, the system you've described is a very good one, [safety first ya know], and is what I'd use as I'm familiar with injecting to debug, as only method needed.
  Probing and injecting are like symmetrical opposites. They both find the same things, but like, from opposite angles.
  Both is probably useful and argueably the best, I've just gotten used to injecting when debbugging 9V circuits because my thumb and a screwdriver or other conductive shaft for use as probe are always right there.

[R.G.]

Here are some random thoughts:
- use a good, low impedance buffer on the output, and insert some resistance between that and the output. This is to protect things when you try to force a buffer output to do some signal it doesn't want to do. Any analog signal injector shares this problem with digital logic probes.

- injected amplitude will vary - a lot - depending on the impedance of the point you're injecting it to.

- signal injector probes were common back in the tube era when impedances were lower and more hand tracing went on.

For really interesting testing, use a signal generator oscillator where you can flip the gain between just above the oscillation criteria, where it oscillates all the time, and just below it where it rings at the oscillation frequency for a while, then dies out. This is easier to do with some oscillators than others. If you then use a timer like the CMOS 555 to periodically whack the oscillator when it's below self oscillation, you get a fair analog of a guitar string struck and decaying away.

[Fp-www.Tonepad.com]

Interesting. I may dig into that a little more.

And, of course it would fit in a pen!

Fp