A Preamplifier For ‘Scope Signal Tracing
O’scope Preamplifier
Saturday, November 12, 2022
The FNIRSI digital oscilloscope in the previous post has limited vertical gain for audio signal tracing, so I built a preamplifier for it. The preamp has fixed gains of 10X (20dB) and 100X (40dB). It should also be useful for preamplifying low level signals with calibrated gain into the fixed line level inputs of my MOTU and TASCAM interfaces. It is AC coupled only, since small DC voltages are easily read with a voltmeter. Powered by two 9 volt batteries for portability and ground loop avoidance. I run the FNIRSI scope from a USB power bank for the same reasons.
The inputs are on BNC jacks for use with scope probes. The FNIRSI probes won’t adjust down enough for square wave response with this circuit because the input capacitance is too low. Standard probes for my Tek and B&K scopes work fine. The FNIRSI probes are apparently made specifically for that scope’s high input capacity. If you want to use the FNIRSI probes, add a 100pF cap from input to ground to match the scope.
Outputs are on RCA jacks because I have adapter cables from RCA to just about everything else.
The chassis is an aluminum stomp box with the jacks, a gain toggle switch, and the ON-OFF switch.
The circuit is built with an LF351 JFET op-amp, but other common op-amps will work, like a TL-071.
A pair of 1N914 or 1N4148 diodes protect the FET input from static and high voltages. Large signals are not expected at the inputs because of the gain and limited output of around 10V P/P. Input impedance is 1M ohm to match standard scope probes. The input capacitor is 1uF 250V mylar film and output cap is 1uF 50V ceramic. The power pins are bypassed with 100uF 16V electrolytics to ground.
That’s it. Simple box, simple circuit built with parts on hand. Photos:
