Large motional feedback system

• Introduction, requirements and system overview
• Power supply
• Input circuit
• Active cross-over filter unit
• LF amplifier with motional feedback
• MF amplifier with special distortion cancellation
• HF MOSFET amplifier with soft clipping
• Protections

Power supply

Mainpower supply

The main power supply is based on standard 50 Hz non-switching transformers with sufficient power headroom.

Some details of importance:

• A slowstart circuit is used to limit the inrushcurrents by means of a series resistor. Timing is done with a standard timer IC
• Leakage resistors are connected over the high voltage electrolytic capacitors to discharge them when the power supply is tested without a load. 120 V is unsafe. It is good to be aware that not only tube amplifiers can kill people with heart rithm problems. In that sense it is strange that the loudspekear terminals of high power amplifiers are often not isolated.
• Snubber circuits (RC) are connected to the rectifiers to reduce RF interference by snap-off effects.
• A thermal sensor checks the overall temperature of the (black) mounting plate as the power supply is located inside the woofer enclosure which has not a very good heat exchange with the outside. In practice the temperature rise is hardly more than a few degrees.

Sub power supply 1

The spower supply for the low power signal amplifiers and filters consists of two stages. First a single transistor series regulator is applied to reduce the 35 V to approximately 20 V which is the rough regulated voltage to the real voltage regulators theat either are placed at the power amplifiers (in this schematic) or at the input and filter units.

The 15V regulated supply consists of a standard series regulator IC and is terminated by a parallel regulator that only comes in action when the standard series regulator fails (short circuit). This is done because of the high cost to repair all operational amplifiers in case of failure. The resulting high current is limited by a current limiter transistor over the first series regulator. When even that circuit fails the "non-flammable resistor" (NFR) of 2.2 Ohm will act as a fuse. The LED's will not light when such an error occurs.