Amplifier Circuit DiagramsOver voltage protector

Stereo Protector Against D.C. Schematic Circuit Diagram

Speaker Protection Circuit:

Stereo protector against d.c.: If a direct-coupled output amplifier experiences a malfunction during operation, the speakers, especially the bass units, face potential risks. The bass, in particular, is susceptible to not being decoupled for direct current (d.c.) due to capacitors in the cross-resonance network. For example, if the output transistor fails, the bass units might be exposed to the full supply voltage at their terminals.

Circuit for Speaker Protection:

A circuit designed to protect speakers in the event of such breakdowns while simultaneously preventing disruptive ‘plops’ is illustrated in the diagram. It operates from an unregulated, unsymmetrical power supply and can be directly connected to the power supply of the output amplifier under normal circumstances.

Signal Bypass and Window Comparators:

The alternating current (a.c.) components of the signals in the output stage are effectively bypassed by R1 and two anti-series connected capacitors, C2 and C3. This results in the d.c. component of the loudspeaker signal at the junction R1-R2. Subsequently, it undergoes processing through potential dividers R2-R3 and is fed into window comparators IC1 and IC2. With the supply voltage fixed at 10 V by R13-D7, the window height is maintained at 2 V by R5. To clarify, u2 equals 6 V, and u3 equals 4 V. In the absence of d.c. at the output of the power amplifier, UI remains at 5 V. In this scenario, the outputs of ‘OR gates’ D1 and D2 register as logic high. However, if the d.c. component at the output of the power amplifier exceeds ±2 V, u1 becomes greater or smaller than either u2 or u3.

Stereo protector against d.c. schematic diagram

Circuit Operation and Relay Control:

When the power amplifier is activated alongside the provided circuit and the voltage u1 remains within the defined window, C4 starts charging through R8. After approximately 1.5 seconds, the state of the ‘Schmitt trigger’ IC1d changes, transitioning its output to logic high. Consequently, the relay is energized, seamlessly connecting the loudspeaker to the power amplifier, eliminating any audible ‘plop’ noise. In the event of a malfunction or an increase in the direct voltage at the power amplifier’s output, C4 discharges swiftly through R7 within 50 milliseconds. The output of IC1d then goes low, causing the relay to de-energize and disconnecting the loudspeaker from the output amplifier.

Component Selection for Voltage and Relay Operation:

To ensure proper functionality, resistor R13 and the operating voltage of the relay must align with the supply voltage. For a supply voltage range of 20–40 V, suitable values for R13 are 4.7 kΩ and 1 W, while for 12–20 V, 1 kΩ and 1/4 W are appropriate. If the supply voltage is, for example, 36 V, the relay’s operating voltage should be 24 V. The remaining 12 V should be dropped across an appropriate resistor, such as R16 with a value and rating of 820 Ω, 1/4 W, assuming the relay draws 15 mA. If there’s a need for circuit deactivation, S1 can be included. Closing this switch energizes the relay.

Stereo Power Amplifier Adaptation:

For a stereo power amplifier application, components R1–R3, C2, C3, D1, D2, D5, D6, IC1a, and IC1b need duplication. The additional circuit is connected in parallel with S1. It’s crucial to note that in this setup, the relay should feature two working contacts, or two relays with their contacts in series can be employed.

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