Signal Tracer (Old school)

Taking in your suggestions, here is revision 2 of the circuit.

@John Wiltrout I was thinking the highlighted cap (C1) would provide the higher impedance needed. Am I thinking wrong?

I love the idea of the probe, that will be the way to go.  Thank you for the  schematic, I am afraid I am not fluent with the tubes, however.

I am focusing on solid state, for now, I would like to move into the vacuum tubes later, I love the vintage tube amplifiers.

@shabaz  

I pulled the transistors off of the circuit, and plan to see if that can, in fact, drive a speaker/headphones. I have a feeling I will need something to drive them, so I am thinking of adding some fets like you mentioned in place of the PNP/NPN's.

I like the input you have on this, do you have any posts or schematics of you circuits posted? or that you can share? Maby we can work together.? 

Just a thought.

By the way, the led bar is added to the circuit.

Hi Lupe,

My circuit uses a surface-mount-only part, and isn't ready for prime-time, but as an alternative I drew up the circuit below - uses through-hole parts.

It would be the high impedance input stage, so you could attach the right side of it to your LM358, or a LM386 circuit.

Any popular N-ch JFET could be tried any of these are common  2N38192N3819 2N5486 J310

It has a very high input impedance (of the order of Mohms). It will distort and clip perhaps above a few volts input, but that's ok

(doesn't matter for your application). It will need a screened box.

I have not built this circuit but it should function(played around with the  2N38192N3819 a lot in the past : )

For the speaker stage, I personally really like the LM386, it is a fine audio amp for such test applications, but if you want to use the LM358, then here is a known working circuit

that I built up a while ago as a quick test amp for some circuit (it is not hi-fi). It uses the dual op amp version of the '358 (LM2904)  for stereo but you only need one channel of

course so could be replaced with the '358 (the pin numbers will be different).

You don't need the input capacitor drawn below since the JFET circuit above already has that.

This circuit was tested with headphones (and it was loud), not a speaker, but it should be ok-ish with a 64-ohm speaker - that needs testing though!

Very good ideas. I have incorporated it and updated the schematic.

Hoping to have something on a breadboard, and prototype by the end of the month.

Using a 358, I have on hand to see how it goes.

I added C6, that will be in the probe, and I may need to add a diode, but if I do it will be switchable.

Are you going to have the FET right next to the probe tip?

It seems to me, the original has two main difficiencies with regard to the input.

Firsty, the large ratio between the circuit-to-probe-tip capacitance (a few pf) and the screened cable capacitance to ground (several hundred pf) means that the signal that the amplifier input sees is so low that it needs a lot of gain to get it back to a usable level. The original designer didn't have much choice - it would have been too difficult to put a valve at the probe tip - but you have the advantage that, with small surface-mount components, you could. By driving the screened cable with the FET, that capacitance is no longer a factor, and the signal reduction at the input becomes more modest - it will still be there, because of the capacitance from the probe back to the probe case, but should be much less. [If you wanted to be really clever, you might actively drive a shield behind the probe tip and reduce the capacitance even further, though that's probably going too far (and might not even work - I'm just throwing in a random idea there).]

Secondly, the high-pass filter formed by the few pf at the input and the input impedance at the amplifier really rolls off the signal at low frequencies. You're already dealing with that by using the FET, but you might be able to push it up further if it's close in to the tip [where you want pick-up anyway because that's the whole point].

I think you've simplified your circuit too much.

The artificial ground (potential divider and cap) and the dc blocking capacitor on the gain resistors were necessary.

The input potentiometer doesn't do anything very useful the way you've connected it.

Are you using a simulator that tells you that the circuit you've drawn works ok?

I see what you mean for the pot,I  was using it as a over voltage trim, but I can simply place a zener diode in place instead.

On a sim it works fine, but, we will never know until I get something in a breadboard.

And I am waiting on a new scope, and camera to come in before I can start testing it up.

I see what you mean for the pot,I  was using it as a over voltage trim, but I can simply place a zener diode in place instead.

On a sim it works fine, but, we will never know until I get something in a breadboard.

And I am waiting on a new scope, and camera to come in before I can start testing it up.