Watt music amplifier: This is a robust, no-frills medium power amplifier that is particularly suited to use in ‘combo type, portable amplifiers used by guitar players and jazz musicians. The amplifier is a straightforward combination of an integrated audio driver IC, the LM391-80, and push-pull power output stage designed with bipolar transistors.
A few peculiarities of the design will be dis-cussed. The NTC, which is in thermal contact with the power output transistors, enables the LM391 to switch off the power stage when this gets too hot. The onset point of this thermal protection lies at an NTC current of about 200 uA. The electrolytic capacitor that shunts the NTC serves to provide a ‘soft start’, which prevents a loud click or other disconcerting noises when the amplifier is switched on. It may happen that the protection is too sensitive, in which case some experimenting with the value of R4, or that of the NTC is in order.
It is possible to implement feedback in -the( amplifier by connecting R23 to series network C5-R7. The latter pal R10, determine the Frequency res the amplifier, which may need adjusting to meet individual requirements. The component values given here will, however, be all right for most applications.
The effect of different values of C5 and R7 is simple to measure (or hear) by shorting out R23 temporarily. For 4(ohm) loudspeakers, R23 is lowered to 0.18 (ohm). Unfortunately, the LM391-80 is prone to oscillation, which is suppressed by components Rx, C6, C8 and C9 (in most cases, C6 may be omitted). Resistor Rx, in particular, reduces the open-loop gain. If Rx is used, Ry must be fitted to compensate the resulting off-set voltage. Components R22 and C12 form a Boucherot network that serves to stabilize the amplifier at high frequencies.
The input of the amplifier should be driven by a low-impedance source capable of supplying ‘line’ level audio signals (0 dB). Network RI-C1 attenuates signals above 50 kHz or so.
The quiescent current of the amplifier is set by preset P1. Set this control to 0(ohm) initially, and adjust it until a quiescent current of 50 mA flows. This may be increased to 400 mA► if low distortion is desired.
The power transistors are all located at the same side of the PCB so that they can be bolted on to a common heatsink, together with the NTC. The heat sink should be fairly large and have a thermal resistance of I K/W or smaller.
Note that L1 consists of 20 turns of 0.8 mm dia. enameled copper wire wound around R21. C9 is a ceramic capacitor.
Finally, some measured data (supply voltage: ±35 V; R23 short-circuited):
• 3-dB bandwidth (8 f2): approx. 11 Hz to 20 kHz
• THD (third harmonic distortion) at 1 kHz:
1 W into 8(ohm): 0.006% (Iq = 400mA)
1 W into 8 (ohm): 0.02% (Uin = 50 mA)
65 W into 8 (ohm): 0.02% (Uin = 873 mV)
80 W into 4 (ohm): 0.2% (Uin = 700 mV; onset level of current limit)
Parts list
Resistors: RI:R3:R6:R15:R16 = 1 (ohm)
R2:R10:R13;R14 = 100 k(ohm)
R4= 82 k(ohm)
R5 = 3.3 k(ohm)
R7= 150 k(ohm)
R8 = 47 k(ohm)
R9 = 4.7 k(ohm)
R11:R12 = 270 (ohm)
R17;R18 = 100 (ohm)
R19;R20 = 0.15 (ohm)/5 W
R21 = 1 (ohm)/1 W
R22 = 10 (ohm)/ I W
R23 = 0.39 (ohm)/5 W
Rx = I M(ohm) (see text)
Ry = 909 k(ohm) (see text)
P1 = 10 k(ohm) preset H
Capacitors:
Cl = 2.2 uF, 63V
C2 = 3.3 nF
C3 = 10 uF, 40V
C4 = 4.7 uF, 16V
C5 = 220 nF
C6 = not fitted (see text)
C7.C12;Ci4;Ci6 = 100 nF
C8 = 39 pF
C9 = 47 nF, ceramic
C10;C11= 1 nF
C13;C15 = 10uF 63 V
Semiconductors:
D1;D2 = 1N4006
T1 = BD237
T2 = 13D238
T3 = I3D250C
T4 = BD249C
