Quest Engineering QTA1120P User Manual - page 10

QTA-

1120P/1240P

User Manual

User Manual

Q-Tech Commercial Series

QTA-1120P/1240P

8

Output

Impedance

Output

Impedance

Output

Impedance

0.1W

100K

Ω

10W

1K

Ω

40W

250

Ω

0.5W

20K

Ω

15W

667

Ω

50W

200

Ω

1W

10K

Ω

20W

500

Ω

75W

133

Ω

2W

5K

Ω

25W

400

Ω

100W

100

Ω

3W

3.33K

Ω

30W

333

Ω

150W

66.7

Ω

5W

2K

Ω

35W

286

Ω

200W

50

Ω

(100V plan) P = E x E / Z = 100x100 / Z = 10,000 / Z

P: Power(W), Z : Impedance (

Ω

), E : 100V(Designed voltage of a general-use amplifier)

High impedance of speakers and output calculation table for 100Volt systems

For the benefit of those who want to calculate 

the power of the system with a formula. See 

below:

Determining power by calculating 

total impedance

The alternative to counting the speaker taping 

watts is to calculate the total impedance of 

the line, which will also indicate the wattage 

necessary to drive the system correctly. 

The computational formula is voltage squared 

divided by impedance:  

Power = Voltage2 / Impedance .

When the speaker line reaches its top voltage 

of 70 Volts, the formula is:  

Power = 5000 / Impedance

 (because 70 

squared equals 5000). 

To measure impedance a dedicated meter is 

required. These use a 1k frequency to send an 

alternating current through the transformer(s). 

The resulting figure indicated by the meter’s 

dial can be cross referenced with the meter’s 

impedance chart to see what value should be 

expected and the power that will be required.