PA 225 / 250 (T)
PA 425 / 450 (T)
two/four channel power amplifier
IRP professional sound products L.P.
1 E. Beacon Light Lane
Chester, PA 19013, USA
phone 610-874-5663
Grounding
The chassis ground of the amplifier is connected with the ground of the AC power cord (the non-fused earthed
protective conductor).
As soon as several devices are connected with each other in a signal chain, a ground loop is created. Ground
loops are caused when current flows from the analog ground plane of one piece of equipment to the ground
plane of another. A compensating current travels between the different devices on the shields of the audio
cables. This is mostly causing humm-problems. On the back side of the amplifier you’ll find the GROUND LIFT
switch. This switch isolates the device chassis-ground from the pin 1 of the audio connectors and interrupts any
compensating current between different devices.
NEVER isolate the ground of the AC power cord (the non-fused earthed protective conductor) to eliminate
humm-problems. This is a violation of the law and dangerous !
Decibel Explanation
A decibel is a logarithmic scale commonly used to express differences in signal levels. It is useful in audio
because it can express a wide dynamic range with relatively small numbers (or a small movement on a meter),
and it more closely matches how we perceive sound.
The measurement quoted in dB describes the ratio between the quantity of two levels, the level being measured
and a reference. The absolute quantity of the signal is not relevant. This means that decibels are always
comparing one quantity to another. For example, when we measure gain in dB, we are comparing the output
level to the input level.
To describe an absolute value, the reference point must be known.
There are different reference points defined:
- dBV represents the level compared to 1 Volt RMS. 0dBV = 1V with no reference to impedance.
- dBu represents the level compared to 0,775 Volt RMS on an unloaded, open circuit.
- dBm represents the power level compared to 1 mWatt. This is a level compared to 0,775 Volt RMS across a
600 Ohm load impedance.
1dBV 2.2dBu, +4dBu equals 1.23 Volt RMS, the reference level of -10dBV is the equivalent to a level of
-7.8dBu.
Headroom is a measure (usually in dB) of how much higher the peaks of a signal can be compared to the
nominal level without clipping. That is, it compares the peak level (in volts RMS) to the nominal level (in volts
RMS). The difference between the two (in dB) is the headroom.
