Uh oh, it's possible to destroy a scope that easily? Wouldn't it have some kind of internal overvoltage protection? I have a cheap multimeter that has a removable fuse, surely a relatively expensive scope would have something similiar? In any case, i'll certainly be more careful in future!
The ground of the probe is connected to the metal outside of the BNC plug (which is most often used) at the scope end. These are typically connected directly onto the chassis which is earthed. There is no fuse here.
If you clip the ground lead to something that has a voltage relative to mains earth, current will flow through the shield of the probe, to the scope, then through your mains cable to ground. Either the current will be limited by something, or something will melt. It may be a fuse in the equipment under test, it may be wires, or it may be your scope lead. It could also trigger an earth leakage breaker (which would be a good thing).
If you have something with an isolated supply, this won't happen, but if you connect 2 earths (which are connected together at the scope) to 2 points with different potential, then current will flow through one probe lead to the scope, through the chassis, and back through the other probe lead. Again, something will happen. If the device is powered by small batteries, then you might short one out (it might get warm) and nothing much will happen. For a higher power supply, see above (except you won't blow an earth leakage breaker).
This is one of the hidden traps of using a scope.
I am getting a pretty strong source without the amp, but with the amp I can connect the circuit to a speaker and I get a high-volume sound out, which is useful.
You should probably check that you're not getting clipping of your output.
One note about low-noise vs regular opamps. My understanding (admittedly limited) is that the low-noise amp is designed to reduce the noise generated by the amplification process, and NOT the noise in the signal itself, is this not correct? So, if the intention is to produce clean white noise, (which I want to feed into a computer to make a true random number generator) using a low noise amp would prevent "regular" noise getting into the noise source, which could create a predictable pattern in the supposedly white noise. Is this assumption correct at all?
In general, noise is noise. The noise you want has a constant power from DC to daylight. As long as all sources of noise are similar, you're OK. It *might* be a problem if you were trying to generate pink noise, but even then, all other sources of noise will be tiny compared to what the transistor is creating.
I'm not too worried about battery duration, I'm more concerned with proof of concept, but I take your point. What would be done in a commercial application? I've never seen products which require 2 9v batteries, would they use some kind of boost converter and power splitter like I am using?
They might. Or they might employ a circuit to generate the -ve rail. 2 batteries is probably simpler, but probably not as commercially acceptable.
I did try the zener's, they didn't work, I didn't think about the fact that they would be designed for low noise operation, that makes sense... That's probably why the transistors worked much better. When you say "kills transistors" do you mean the transistors will eventually burn out?
That's a very good question.
The transistor will have steadily deteriorating gain the longer you use it in this application. (If you're interested, you can read this thread
https://www.electronicspoint.com/emitter-base-breakdown-t257654.html)
Will that affect the noise spectrum or the amplitude of the noise? I really don't know. However it would be really interesting to find out.
What you said about the antenna makes sense, it just seemed plausible to me that it could pick something up. Since I'm trying to build an RNG from this, I want the noise to be a "ideal" as possible, other wise I am going to get predictable patterns in the data.
Let us know how you get on with that.
Thanks a lot for your help, I appreciate it!
No problems, it's a pleasure.