Voltage-controlled oscillator (CB20831E)

This circuit is from American documentation from the 90s. The transistor is general-purpose and operational amplifiers can be modern equivalents.

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Delayed SCR trigger (CB19336E)
The circuit shown is from an old European publication. In it we have the way to activate a relay...
Audio Line Amplifier (CB4769E)
We found this circuit in a European publication from the 1990s. Transistors are still common types...
Active three-tone filter (CB20808E)
This circuit was found in an American documentation and can be added to common amplifiers of any...