Crown CTS1200 Reference Manual page 8

2/Pt (L/10)
Z = (70.7) = 5000/(Ps * 10 )
where
Z = primary impedance in ohms
Pt = total power in watts
Ps = power to speaker in watts
L = insertion loss in dB.
Other transformer manufacturers do not compensate for insertion loss. In this case, the primary impedance
matches its rating. However, the power delivered to the speaker is less than the power applied, due to the
insertion loss.
Ps = Ptr/L
where
Ps = power to speaker in watts
Ptr = power drawn by transformer in watts
L = insertion loss (as a ratio)
2/Z
To determine whether a transformer is compensated, measure the power (E ) delivered to the speaker
when connected to 70.7 volts. If it is less than the rated power, the transformer is not compensated for
insertion loss.
When making speaker SPL calculations based on sensitivity ratings, subtract the insertion loss in dB from
the loudspeaker sensitivity rating (if the transformer is not compensated for insertion loss).
In transformers that compensate for insertion loss, the speaker receives the power indicated. Consequently,
each transformer draws a little more power from the line than is indicated. The final impedance will be too
low if you add power equal to the amplifier power.
With non-compensated transformers, the labeled power is not the power received, so the speaker SPL will
be lower than calculated. The impedance will read correctly, but the acoustic output will be lower than
expected.
APPENDIX: Line loss
On the Crown website is a calculator that determines the line loss of 2-conductor cable
on a constant voltage line. See the Line Loss Calculator at
http://www.crownaudio.com/apps_htm/designtools/line-loss.htm
REFERENCES
Daniels, Drew. Notes on 70-Volt and Distributed System Presentation, db, March/April 1988.
Davis, Don. Sound System Engineering, 2nd Ed., Indianapolis, Howard W. Sams Co., 1987, pp. 85-87, 402-
405.
138905-1 10-05
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