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Motional feed back amplifier schematic circuit diagram

This versatile amplifier circuit is designed and submitted by Mr Seetharaman Subramanian from Chennai. The full credit of this article goes to him and we are very proud to publish this fabulous circuit here.

Seetharaman’s description about the circuit.

This concept was first introduced in the UK-based magazine Practical Electronics a long time ago. I utilized this idea to design a circuit between 1981 and 1986, which underwent extensive field trials and modifications before its final design was established in 1986. I constructed numerous amplifiers based on this design, ranging in power levels, for both personal use and for my friends. These amplifiers are still in operation in many households today. It’s worth noting that this concept can also be applied to existing amplifiers, and I encourage you to experience the exceptional clarity in bass response, midrange, and high frequencies, along with excellent transient response and minimal distortion. I hope you all enjoy the performance of this amplifier.

In the realm of audio sound reproduction, it is a well-recognized fact that dynamic loudspeakers exhibit more nonlinearity and generate more distortion than all other components of a sound system combined. This phenomenon is particularly pronounced at lower frequencies, where the large cone movements result in increased stiffness of both the inner spider and the outer surround, leading to nonlinear suspension compliance and subsequently, high distortion.

For instance, in a typical high-fidelity sound system, at a frequency of approximately 35 Hz, the total harmonic distortion of the amplifier might be as low as 0.01%, while the loudspeaker’s distortion can range from approximately 3.0% to 50.0%, depending on the volume level. If the motion of the loudspeaker cone can be detected and fed back to the earlier stages of the amplifier, this distortion can be significantly reduced.

Motional Feedback (MFB) was a speaker system developed by Philips Holland in the early 1970s. It introduced a feedback system to the woofers of HiFi loudspeakers, enabling them to achieve extended low-frequency responses even in relatively compact enclosures. The primary advantages included a highly controlled bass response, with any distortion caused by the enclosure or the woofer itself being promptly corrected by the feedback mechanism. These hand-crafted speakers delivered exceptional sound quality but were relatively costly.

As an alternative approach, instead of measuring cone movement, the current flowing through the voice coil (proportional to cone movement) can be monitored and used as feedback for cone movement. This innovative idea is employed in the design of this amplifier (with no claim of originality, as it had been previously documented in Practical Electronics UK Magazine, possibly even patented).

The amplifier used in this setup is a standard Philips audio application circuit, boasting specifications of 40 Watts RMS with less than 0.06% Total Harmonic Distortion into an 8-ohm speaker, and a frequency response from 20 Hz to 100 kHz, modified to suit our needs.

The amplifier features a conventional class B, directly coupled quasi-complementary output stage, powered by a single 56-volt supply (eliminating the need for regulated or dual power supplies). The BC157 serves as the pre-driver and half-supply stabilizer, while the BD139 acts as the driver. A BD139 and a BD140 form a complementary pair for the output driver stage, with 2N3055 serving as the final output stage. The speaker voice coil current is monitored through a 0.47-ohm resistor connected from one end of the speaker to ground. This signal is provided as negative feedback to the preceding stage through a 470-ohm resistor. The half supply at the speaker coupling capacitor can be adjusted by varying the 39K resistor (if necessary, a 100K preset can be substituted for the 39K to adjust the half supply with no input signal at the junction of both 0.47-ohm output transistors and the speaker coupling capacitor). The quiescent current through the output transistors can be adjusted using a 22-ohm resistor in series with two bias diodes (1N4007). For a 56-volt supply, the value for a 50mA quiescent current will fall between 15 and 33 ohms. The amplifier can be safeguarded with a simple 1.5Amp fast-acting fuse in the positive power supply.

The amplifier can be constructed on a 40-watt Philips amplifier application board or any standard, straight-line board. All three driver transistors require cooling clips (standard TO220 casing cooling fins), and the output transistors need a high-quality extruded alloy heat sink with an insulating mounting kit and a layer of silicon conductive grease for efficient heat dissipation.

The recently featured passive Bass/Treble tone control circuit in “Circuits Today,” which operates with a 12-volt supply and is known as the “Baxendall tone control circuit,” is an excellent choice for use as a preamplifier and tone control with this amplifier.

Circuit diagram

Motional feed back amplifier schematic circuit diagram

Power supply circuit

Motional feed back amplifier power supply schematic circuit diagram


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