I was thinking along the lines of a LM386 input but all this was to look at the basics of learning transistor AC calcs not building an amp per se. I have the DC stuff down pretty well but just learning the AC side of transistors. I didn't think about doing the RC calc and would have been just for the single resonant f not a broad band but a good heads up anyway so thx again. I'm still learning to use the scope and this was a bit of a jam for me. Time to put it down as a lesson learned and move on.

 

An LM386 is a power amplifier. When it is set to have a gain of 200 times its output has some hiss and with an input of only 10mV then its output is 2000mV which is only 2V. 2V RMS into an 8 ohm speaker is only 0.5W like a cheap clock radio.

 

Signal input across 100nF coupling cap and scope probe on base. mA meter across base to xstr. Emitter w/ 100nF cap.

With a circuit built like that, and out in the open on a workbench, pickup of all manner of interference is unavoidable.

You need shielding.

For breadboarding circuits involving low-level signals and/or high impedances, I built a "quiet box" out of a 6"x8" cookie tin. The metal box is connected to circuit ground, and external connections are brought in via a barrier strip on one end of the box. With the lid on, I've got good electrostatic shielding.

 

A quick way of eliminating common mode signal problems
is to do differential measurement. The new and old way -

Regards, Dana.

 

@OBW0549 "quiet box" out of a 6"x8" cookie tin WOW! I am impressed, that is literally thinking inside the box! Note to self: File this away for a future project. Thx! A little more detail on the box penetration/connections plz.

 

I would begin by characterising your oscilloscope, probe, and power supply.
If I had the time I would grab a Siglent and show you what I mean. I am away from my home base and wouldn't be able to get to it for awhile.

1) Start with the oscilloscope with no probe connected. Measure the noise at x1 and x10 input mode. In future, you want to use the oscilloscope and probe in the x10 attenuation mode.

2) Connect the probe, set to x10 attenuation, and ground the probe tip to the grounding clip. Measure.

3) Remove the ground clip from the tip. Measure.

4) Connect ground clip to GND or COMMON of your power supply. Connect probe tip to same. Measure.

5) Connect ground clip to GND or COMMON of your power supply. Connect probe tip to +V on power supply. Measure both DC and AC voltages.

6) If noise level is unsatisfactory, try a linear DC power supply. Try a PP3 9V battery. If your circuit needs higher voltage, try two 9V batteries in series.

 

With a circuit built like that, and out in the open on a workbench, pickup of all manner of interference is unavoidable.

You need shielding.

For breadboarding circuits involving low-level signals and/or high impedances, I built a "quiet box" out of a 6"x8" cookie tin. The metal box is connected to circuit ground, and external connections are brought in via a barrier strip on one end of the box. With the lid on, I've got good electrostatic shielding.

That's quite elegant. (Hmmm, where can I find a BIG cookie in a RECTANGULAR tin?)

 

Thx! A little more detail on the box penetration/connections plz

The first picture gives you a view of the barrier strip from outside the box. I put a 1" or so length of #18 bus wire on each terminal so as to have a handy place to clip on DVM, scope, etc.

The second pic is a view inside the box showing the breadboard and its connections to the barrier strip interior terminals. The red, black and blued wires are for power and ground, and the three white wires are for I/O. The wires are soldered to the barrier strip terminals and are all long enough to reach anywhere on the breadboard. Note the black wire connecting the barrier strip power black wire & terminal to the solder lug on the barrier mounting screw; this grounds the enclosure to the breadboard circuit.





You don't need to use a barrier strip for the connections; just about anything would do, such as a panel-mount DB9 connector or a round DIN connector.

 

That's quite elegant. (Hmmm, where can I find a BIG cookie in a RECTANGULAR tin?)

No need for it to be rectangular; a large-diameter round container, such as those used for Christmas cookies or other sugar cookies, will do fine. Just use the inside surface of the can lid to mount the breadboard and barrier strip or other I/O connector(s).

 

@OBW0549 K thx, Yeah I was already thinking some BNC ports for the sig gen and scope. Thx Again!

Sam

 

@OBW0549 K thx, Yeah I was already thinking some BNC ports for the sig gen and scope. Thx Again!

Yup. Some BNC jacks would make a good addition. Perhaps also some RCA jacks and/or 1/8" stereo phono jacks. Maybe I'll do that on version 2.0...