We found this circuit in a 1973 documentation. The null adjustment trimpot is 10k. The circuit can be powered by symmetrical sources from 9 V. The diodes on the bridge can be 1N4148.
This circuit was found in the December 1992 Radio Electronis magazine. R2 is an NTC, there is a...
This passive tuned circuit was found in a 90s Popular Electronics. The coils must be according to the...
This circuit is from 80's Linear Technology documentation. Equivalent operational amplifiers can...