QSC ISA280 - Table 1: Bias and Current Limit Adjustments

20 QSC Audio Products, LLC
TD-000163-00 Rev. B
Current limits properly into short, but current is weak at 2
ΩΩ
ΩΩ
Ω
• If the op amp rails are low (< 14–15 volts) when driving a 2-
ohm load without clipping, check the resistors R157 and R158
(Channel 1, all models), R198 (Channel 1, ISA750 and ISA800T
only) and diodes D115 and D116 (Channel 1, all models), or
R257 and R258 (Channel 2, all models), R298 (Channel 2,
ISA750 and ISA800T only) and diodes D215 and D216 (Channel
2, all models).
• If the op amp rails are normal (14–15 volts) when driving a 2-ohm
load without clipping, usually the output section gain is too low:
weak driver transistors, open output transistors, or open emitter
resistors. Check the value of the driver transistors’ emitter
resistors, too: R146 and R147 (Channel 1) or R246 and R247
(Channel 2).
Rail voltages unequal
The balance between the positive and negative rail voltages is set
by a voltage divider comprising resistors R118 and R119 (Channel
1) and R218 and R219 (Channel 2). If the amplifier channel passes
a signal but clips unevenly due to unequal rail voltages, this
voltage divider is the likely culprit.
Calibrations Adjust
ISA280
ISA300T
ISA300Ti
ISA450
ISA500T
ISA500Ti
ISA750
ISA800T
ISA800Ti
ISA 1350
Channel 1 bias: DC voltage across R147
(ISA1350: across R199 of upper module)
R131 (ISA1350: R171 of
upper module)
0.14 V 0.12 V 0.07 V 0.09 V
Channel 2 bias: DC voltage across R247
(ISA1350: across R199 of lower module)
R231 (ISA1350: R171 of
lower module)
0.14 V 0.12 V 0.07 V 0.09 V
Output current into shorted load
Channel 1: R139 & R140*
Channel 2: R239 & R240*
4–4.5 A 4–5 A 8.5–9 A 4.6–5 A
AC current when driving shorted load*
*
Channel 1: R139 & R140*
Channel 2: R239 & R240
*
3.75–4.5 A 4.5–5.5 A 5.5–6.5 A 3–3.3 A
Clipping voltage into 2 ohms (RMS)
Channel 1: Adjust R139 for
symmetry*
Channel 2: Adjust R239 for
symmetry
*
26–29 V 33.5–37.5 V 44–49 V 57–63 V
Clipping voltage into 2 ohms (peak)
Channel 1: Adjust R139 for
symmetry*
Channel 2: Adjust R239 for
symmetry*
36.8–41 V 47.4–53 V 62.2–69.3 V 80.6–89 V
Idle AC demand*
* (at ambient room
temperature; higher when hot)
0.4 A,
±10% 0.4 A, ±10% 0.6 A, ±10% 0.6 A, ±10%
* NOTE: Except ISA1350
** Figures shown are for 120V amplifiers; multiply current by 0.5 for 230V or by 1.2 for 100V models.
Table 1: Bias and current limit adjustments