buffer amps and pre-amps

[SCPD]

This thread is to discuss the use of a buffer amplifier to provide isolation between the radio discriminator circuitry and the microphone or line input to a computer.

Here's a circuit I'm thinking about building. The novelty of this one is that it is a single transistor design - no op-amps. It's actually a guitar pre-amp circuit.

http://www.till.com/articles/GuitarPreamp/index.html

-rick

 

[SCPD]

Here are some others (I have not tried these).

These are low-Z input models but might work for some folks.

Dynamic Microphone Mini Amplifier (Fully Assembled)

Preamp - Dynamic Microphone Pre-Amp (Fully Assembled)

Dynamic Microphone Pre-Amp Kit Mic-1 looks like a low-cost kit.

VELLEMAN MICROPHONE PREAMP KIT

For a true high-input impedance circuit - this one mentioned by Tom "wa8pyr" will work.

As Tom mentions - a circuit that doesn't require two 9 volt batteries would be easier to mount inside a radio and draw a single supply voltage from the "host" radio.

FWIW, if your sound input has a "+20 db gain" pre-amp built-in, it might be one of
these.

 

[pmn929]

So Rick,
Did you build the first pre amp there? And if you did, what resistor values did you use? I would hardly think you would need 3M Ohm for a scanner.

 

[SCPD]

Did you build the first pre amp there?

I haven't had time.

And if you did, what resistor values did you use?

The 3.0 Meg-ohm makes sense to me. You want to sample the voltage with causing any additional load.
-rick

 

[pmn929]

Ok,
I built the first one that you listed, and here are the results:

Before:

After:

Not quite as rounded as I would have hoped.

 

[wa8pyr]

Here are some others (I have not tried these).

For a true high-input impedance circuit - this one mentioned by Tom "wa8pyr" will work.

As Tom mentions - a circuit that doesn't require two 9 volt batteries would be easier to mount inside a radio and draw a single supply voltage from the "host" radio.

Just breadboarded a buffer using the noted schematic and a 741 op amp, using one 9v battery for power (single rail supply). Works reasonably well although I think I'm going to have to tinker with resistor values a bit. There is still a little loading noticeable although very little, and the waveform appears smoother although still a wee bit jagged.

I'm also going to try the TL071 and see how that does.

Tom WA8PYR

 

[SCPD]

Not quite as rounded as I would have hoped.

Please set your oscilloscope program to a higher sample rate. The pixellated look is due to a relatively low sample rate.
-rick