Balanced Line Level Adjustment
Standard Pi/T pads break the noise rejection of balanced lines (XLR, Ethernet, Twisted Pair). To drop signal without adding hum, you need a symmetric "H-Pad" or "O-Pad".
Design a Balanced PadThe Problem: Common Mode Rejection
A balanced signal (like Pro Audio +4dBu) relies on two lines carrying equal but opposite signals. Noise affects both lines equally and is cancelled out by the receiver (CMRR).
Warning: If you use a standard Unbalanced attenuator (Pi or T) on a balanced line, you effectively short one side to ground (or unbalance the impedances). This destroys the noise rejection, introducing hum and buzz.
The Solution: H-Pad and O-Pad
To attenuate a balanced signal while preserving its symmetry, we "mirror" the resistor network across the center line.
1. H-Pad (Bridged-H)
An H-Pad is essentially a T-Pad where the series resistors are split between the Top and Bottom lines. The shunt resistor remains in the center (floating, not grounded).
- Perfect Symmetry
- Preserves CMRR
2. O-Pad (Result of Pi)
Similarly, an O-Pad is a Pi-Pad where the top series resistor is mirrored on the bottom. This is less common than the H-Pad but valid for certain impedance scenarios.
Applications
- Pro Audio (Studio): Reducing a "hot" line-level synth output before a mic preamp.
- Telephony/DSL: Attenuating signals on twisted pair copper.
- Differential RF: Reducing drive levels to a differential ADC or Amplifier.