REPORT ON FINAL 10/100K:1 AMPLIFIER PERFORMANCE.
I got the revised PCB in recently from JLCPCB. I built up the board using new components except for the Electrolytic capacitor and the largest film caps (2.2uF and 22uF) which I salvaged from one of my prototypes. The board worked with no issues. Performance is slightly better than my best previous prototype. The difference is probably due to superior board layout, especially since I re-used the same Electrolytic capacitor, so capacitor leakage should be the same as the prototype.
Noise floor tests. Tests were done using a 50 ohm resistor across the input and also with a NiMH battery based low-noise source.
To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case.
The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.
Andreas
I bought 8x NiMH AA (EverReady) cells from Digikey and charged them with a new T4S Tenergy charger bought for this purpose. I found the output voltage surprising noisy as shown in an attached scope waveform of the output on my 10K:1 amp. I added some filtering components to clean up the output. The battery was housed in a cast AL enclosure with BNC connector output. Schematic for battery box is also attached.
Noise floor of the new amplifier with 50 ohms across the input (10K:1 mode with 1K input resistance) was typically <120nV pk-pk.
Nose floor of new amplifier using the low-noise NiMH source box was typically <150nV pk-pk.
I also measured a couple of LTZ-1000 based 10V references I have built recently. One is housed in a battery backed Vref chassis and the other is just a PCB in a plastic bag on the bench. Value of <1.5 uV were measured with the Vref chassis being a bit quieter as expected.
Some photos and board As-built schematic are attached below. This project has been a great learning experience and just another adventure traveling the rabbit hole of volt-nut pursuits.
Insatman