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Large motional feedback system

Input circuit:

The input circuit provides both the activation of the system and a balanced and unbalanced signal input.

The activation is done in three ways. The first is a simple switch for manual on/off. The second is an external logical level where "on" corresponds to a connection to ground by an open-collector circuit, an external switch or an optocoupler of which the latter two are preferred over the first to prevent groundloop currents. The third activation is by an internal automatic on\off and meant to be used when the system is connected to a simple unbalanced pre-amplifier without switched DC outputs or other means to activate the power amplifiers. The principle is based on sensing the signal at the unbalanced input and the detection circuit is a straightforward copy of the original circuit used in the Philips MFB loudspeakers.

The preferred mode of use is with external opto-coupled activation and a balanced input signal to limit common-mode interference and groundloop generated hum. The balanced input circuit is a standard instrumentation amplifier. The common mode rejection ratio of an instrumentation amplifier is maximum when the gain of the first differential stage with OPA 1 and 2 is maximum relative to the gain of the following differential-to-single-ended stage. In this case the input voltage is already quite high which means the gain is limited to a factor 3.4 for this stage. The differential-to-single-ended stage with OPA 3 is an attenuator that outputs the signal with a level of 2V peak, corresponding with a differential input voltage of 2.8V peak. This level of 2 V could be chosen higher in view of the +/- 15V power supply of the signal amplifiers used further in the system but originally this MFB loudspeaker system was designed with standard power amplifiers in mind with a gain of ≈30 (≈30dB) which corresponds to this signal level of 2V peak for a maximum output voltage at clipping of 60V.

The unbalanced (asymm) input is inserted at the positive input of OPA3 after attenuation with a potentiometer and buffering by OPA4. This buffer amplifier is neccessary to provide a virtual ground for the 1k8 resistor of the differential-to-single-ended attenuator. Otherwise the common-mode rejection ratio for the balanced input would be impaired.

input circuit

The power supply of the operational amplifiers is not drawn. It consists of standard +/-15 V series regulators that are supplied from the +/-20 V voltages from the Sub power supply-1 and is shared with the supply of the cross-over filter.

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