In the realm of modern radio broadcasting, particularly with FM technology, the quality of transmission has reached remarkable levels. As a result, the fidelity of the receiver is largely influenced by the design and performance of its audio components. This has led to significant focus on improving low-frequency amplifiers, speakers, and the overall receiver structure. Among these, the speaker is often considered the weakest link in the system, making any enhancement in its performance highly valuable.
High-quality amplifiers typically use large speakers, which feature larger diaphragms. However, the complex movement of these diaphragms can introduce high-order harmonics, leading to nonlinear distortion. Additionally, during short sound bursts, the cone may continue vibrating for an extended period, resulting in a unique type of distortion known as transient distortion.
To address this issue, Soviet researcher A.A. Kurikovsky introduced a specialized negative feedback circuit in his book *Amateur Radio New Technology*. This circuit effectively suppresses speaker distortion. Although not widely adopted by hobbyists, it works by detecting when the speaker cone fails to respond correctly to the input voltage. In such cases, a new voltage is generated across the voice coil. By inverting this voltage and feeding it back into the amplifier’s input, the distortion can be significantly reduced.
Figure 1 (a and b) illustrates two variations of this circuit. In the first version, a bridge circuit is used to extract only the distortion voltage from the speaker’s voice coil, avoiding interference from the useful output signal. A potentiometer adjusts the bridge balance, while a capacitor (ranging from 500pF to 5000pF) helps manage the speaker’s low resonance frequency. The 6C5C tube amplifies the feedback voltage, ensuring that only the distortion component is fed back, preserving the integrity of the original signal.
In the second circuit (Figure 1b), the cathode current of the 6π3C tube generates a voltage across resistor R2. Another voltage is also present on R2 through resistor R3, which comes from the speaker’s voice coil. When the speaker is undistorted, these two voltages cancel each other out. However, if distortion occurs, a distortion-related voltage appears on the voice coil but not in the screen flow of the tube. This imbalance results in a distorted voltage being applied between the gate and cathode of the first tube, creating a negative feedback loop. This feedback is then amplified and inverted before being sent back to the voice coil, helping to dampen the cone’s unwanted vibrations and reduce distortion.
This method ensures that only the distortion is corrected, without affecting the amplification of the desired signal. It represents an elegant solution to one of the most challenging issues in audio reproduction, especially in high-fidelity systems.
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