Bose Personalized Amplification System, Troubleshooting Manual

Page: 13 / 50

13
THEORY OF OPERATION
Table 2. Nominal and Peak Signal Levels
The minimum input level that would allow the system to reach full output (with all gain controls
all the way up) is -72 dBV. This is the output level of a Shure SM58 being exposed to about 78
dB SPL and should be sufficient for almost any application.
This gain scaling puts the main responsibility for the high-signal performance of the system on
the digital limiters. The limiters need to make sure that the output never exceeds 0dBV. The extra
6 dB headroom in the DSP can be used to compensate for gain variance in the power amp and
to allow the DSP to overdrive the power amps for very short periods of time. It may turn out,
that, for certain instruments, this will be required to achieve maximum SPL at acceptable sound
quality.
Point Nominal Peak Comment
Line In, Trim
@ Max -30dBV un-
bal
- 12 dBV Ch1 / Ch2 Vol. + Master @ 12:00. Maximum input
limited by input INA163 output clipping.
Line In, Trim
@ 12:00 -1dBV dBV
un-bal
+17 dBV Ch1 / Ch2 Vol. + Master @ 12:00. Maximum input
limited by input INA163 output clipping.
Line In, Trim
@ min +12dBV un-
bal
+18 dBV Ch1 / Ch2 Vol. + Master @ 12:00. Maximum input
limited by input Buffer.
Mic., Trim @
max -50 dBV
balanced
-32 dBV
balanced
This nominal = the output level of a Shure SM58
microphone that is exposed to 104 dBSPL.
Mic., Trim @
12:00 -21 dBV
balanced
-3 dBV
Balanced
Mic., Trim @
min -8 dBV
balanced
+10dBV
balanced
This nominal = the output level of an AKG C4000
microphone that is exposed to 120 dBSPL.
Ch1, Ch2
Line out XLR +6 dBV
balanced
+24 dBV
balanced
Dig. Vol.
Control, input 0 dBV +18dBV
Dig. Vol.
Control,
output -10 dBV +18dBV The nominal gain setting here is -10dB for the
volume c20 max out assumes Volume @
Max. (+10 max)
Insert Send -10 dBV +8dBV,
+18dBV
Peak @ 12:00 / Max Volume. Nominal is
conservative but should also work well for cheap
stomp boxes
Insert Return -10 dBV +6dBV Assumes FX device in loop is set for unity gain
Peak limited by A/D input overload.
A/D input -10 dBV +4.5dBV Assumes max voltage at the A/D = +6dBV=0dBFS,
D/A output filter gain = -1.5dB
D/A output 0 dBV +4.5dBV
(debug)
0dBV
(normal)
The total excess gain in the DSP will be 16 dB, with
a nominal setting of +10dB. Again we assume
0dBFS = +6dBV. It is the responsibility of the digital
limiter to ensure that power amp does not clip, i.e.
that the output will not exceed 0 dBV
Bass Out 0 dBV 0/+4.5dBV
+6/10.5bal Limiter engaged (normal operation) / Debug
Bass Out can be used Balanced / Un-balanced
Amp In 0 dBV 0 dBV Nominal levels throughout the system are
referenced to full output power, so nominal & peak
are the same
Amp Out 30 dBV 30 dBV This is maximum output at clip level ~250W
@ 4 ohms