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6. Input/Limiter PCB Assembly
Note:
Refer to the Input/Limiter PCB Schematic Diagram, sheet 1 of 1 for the following.
The information inside the brackets [ ] is the schematic grid location on the sheet.
The Input/Limiter PCB assembly provides:
•
Input buffering for line level signals
•
Input short circuit and ESD protection
•
Power regulation for audio
•
Automatic gain change for balanced or unbalanced input signals
•
Limiting
•
Hard clipping for very large transient signals
•
LED control
•
Cooling fan control
•
Under-voltage detection and ENABLE control for the power amplifier.
Gain Structure of the Input/Limiter PCB
The overall gain of the preamplifier pcb, when not limiting, is unity for unbalanced signals, and
-6 dB for balanced signals. It is designed to keep the steady-state signal output below approxi-
mately 0 dBV, and hard clip transients to about 5 V peak
Input Buffer
J1 [D1] and J2 [C1] are balanced TRS connectors to provide an input jack and a “pass-through”
parallel connection. D301 through D304 [D1 to D3] provide input short circuit protection and
ESD protection by clamping to the power supply rails. C307, C308, C316 and C330 [C/D1]
provide RF decoupling as well as additional ESD protection to the input. C364 through C367
[D1 to D3] are not installed on production units.
U301B [D3] is a Bessel filter with a relatively high input impedance of 40k Ohms. This input
impedance was selected to avoid low frequency roll-off with the PS1 power stand bass line
output used as an input. The PS1 has a relatively high impedance with a capacitance of 2.2 uF
in series with 560 Ohms.
THEORY OF OPERATION
Input: 2Vrms
(balanced cable)
Gain: 0 dB
Balanced to
Single-Ended
Conversion
1.5 Vdc nominal
from PS1 with
balanced cable
Detector threshold 0.7 Vdc
Unbalanced input detector
Gain adjustment for
balanced or unbalanced
signal input
Gain:
+5dB (balanced)
+12dB (unbalanced)
Pad gain -5dB
1 Vrms (balanced)
0.5 Vrms (unbalanced)
2 Vrms (balanced input)
2 Vrms (unbalanced input)
Gain: 0 dB
2 Vrms
Gain: -14 dB
Limiter
Limiter Threshold: 0.6V peak
0.4 Vrms
(0.564 V peak
Gain
Adjustment
1 Vrms
Gain: -5dB
