A particularly good aspect of this amplifier is the current limiting circuit. The sensing circuit is a standard
rail load line limiting circuit but it is the drive circuit that is important. As transistors V12 & V6 (BC640)
turn on transistor V11 (BC639), it pulls bias current away from the amp through diodes D1 & D4 (BAV21)
and pulls drive away from the op amp IC1 (LM1458) through the diode/resistor pair D2/R12 (BAV21/3k3)
& D3/R23. Individually each topology acts to limit the current in the amp but it is the combination of the
two and the fine tuning of there interaction that produces the characteristic soft limiting without the harsh
crossover distortion. It is not until the amp is in hard clip does the amp produce the high freq crossover
distortion. This makes for nice sounding current limit that allows for soft distorted peaks to get through but
limits continuous excessive current while maintaining thermal stability.
Speaker Output Terminal Strip
(240V version)
The screw terminals on the left hand side of the strip allow access to the direct speaker outputs of the
amplifier. Reading from left to right the terminals are:
COM Common or “-” for low impedance speaker loads (4 ohms)
4
Positive “+” for 4 ohm speaker loads (use with common)
COM Common or “-” for 70v or100v speaker loads (maximum load of 40 ohms at 100v)
70
Positive “+” for 70v line speaker loads (use with common)
100
Positive “+” for 100v line speaker loads (use with common)
Please ensure that the correct “Common” is used. Low impedance and 70/100v loads can be used
simultaneously but please pay careful attention to the overall speaker load.
Note: The minimum impedance (or maximum load) at 100 volt line should be no less than 40 Ohms.
(115V version)
The screw terminals on the left hand side of the strip allow access to the direct speaker outputs of the
amplifier. Reading from left to right the terminals are:
COM Common or “-” for low impedance speaker loads (4 ohms).
4
Positive “+” for 4 ohm speaker loads (use with common)
COM Common or “-” for 25v or 70v speaker loads (maximum load of 20 ohms at 70v)
25
Positive “+” for 25v line speaker loads (use with common)
70
Positive “+” for 70v line speaker loads (use with common)
Please ensure that the correct “Common” is used. Low impedance and 25/70v loads can be used
simultaneously but please pay careful attention to the overall speaker load.
Note: The minimum impedance (or maximum load) at 70 volt line should be no less than 20.
Terminal Strip
The remaining terminals read as:
Spare
Tone Generator Common (use with one of the 4 tones listed below)
Pre-Announce
Chime
Alert
Tone
Bell
Chime
Evacuation
Tone
[600 Ohm Telephone Input] (115V version)
