JBL 7124B
¥ 240,500 (around 1975)
Commentary
An AGC amplifier that automatically adjusts the volume of the PA system in response to ambient noise and noise.
Suitable for use in airports and street situations where ambient noise levels fluctuate constantly.
In the 7124B, ambient noise is sampled by a microphone and the output level is changed up to 20 dB based on this.
Since the control circuit continuously changes the program level, there is no unnatural step effect.
Use a high-impedance dynamic type or ceramic type microphone, or add a 5901 microphone transformer when using a low-impedance microphone.
It also has a 50k Ω high-level unbalanced input circuit, which can be converted a 15k Ω balanced bridge or a 600 Ω matching input by using a 5195 matching / bridge transformer.
The 7124B is installed between the preamplifier and the power amplifier and has a maximum output level of + 10 dbm.
600 Ω balanced output can also be obtained by using 5195.
Model Rating
| Type | AGC Amplifier |
| Gain | 0dB with full sensing signal -20dB with no sensing signal |
| Maximum output level | +10dB |
| Playback frequency band | 20 Hz to 20 kHz ± 1 dB |
| Attack time | 1 second for 10dB increase 3 seconds for 20dB increase |
| Delay time | 1 second for 10dB decrease 3 seconds for 20dB decrease |
| External dimensions | Width 483x Height 89x Depth 152 mm |
| Weight | 3.6kg |