A very simple tone correction designed with operational amplifier can be built using electronic diagram below which is based on LM833 op amp. Such a correction allows us to change the tone frequency response without to have any influence on that audio equipment, and for this purpose using a compact assembly.
This Door ring circuit with memory can be used as "memory ring" can be designed using this electronic circuit diagram. This is necessary especially in case of unexpected absences from home, described circuit will warn you with an LED if ring button was pressed or not. Using diode D1 and capacitor C1, the circuit is powered by transformer’s ring. It provides a DC voltage sufficient for memory ring circuit. Under normal conditions the transistor T1 and T2 drive is locked, constituting a form of lock T1.