I am thinking about changing the circuitry for tuning the 3340. My inspiration comes from Rob Hordijk’s article on tuning the AS3340.
For example, the Tempco circuit depends on balancing the currents coming out of pin 1 and pin 2. Changing the trimmer-resistor-combination on pin 1 for a matched 5k6 resistor in series with a trimmer and resistor, makes this easy. Now we can measure the voltage drop on both 5k6 resistors, and with 0.000 V between those resistors, the current out of both pins must be equal.
Has someone experimented with using a current mirror on these pins? Or with measuring the voltage between those 5k6 resistors and using that as feedback for a digital transistor? What other stuff could we try?
Choose a thermally stable ceramic capacitor for the 1n0 timing cap on pin 11 of the 3340. I use C0G (same as NP0) types for the best stability.
E.g. from CPC: CA05507 Capacitor C0G 1nF 5% 100V are 13p each +VAT so it’s a no-brainer.
I have a PET film capacitor there. I think they are even better. Or is there a specific reason to use ceramics for this?
EDIT: I did some searching and see NP0 are better indeed. Thanks for this info.
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Actually I see Rob Hordijk’s articla that you link to mentions C0G and NP0 capacitors, and also recommends some other types. NP0 is merely the US military designation for C0G, they are the same. Not sure if PET is more thermally stable, have a look at the manufacturer’s spec. I don’t build with SMD components, my eyesight can’t cope with them.
I did not did the experiment yet, but just by doing the calculations from the datasheet reference values, it seems that pin 2 is 1 diode drop above the negative supply rail. As the current mirror also drops 1 diode voltage drop, there is voltage over the resistor, and no current will be flowing.
There is no way this idea will work.