TOA Electronics TA 1400 user manual Protection Features, 8.2Powering up - Soft start, Clip Limiter

8.2Powering up – Soft start

When you power up the amplifier it takes a couple of seconds to check its circuits. This is known as the "soft-start" or "slow-start" sequence. The fans then blow at high speed before going into "idle" and the two bottom green LED’s illuminate to show the amplifier is operational.

8.3Input attenuators

The two input level attenuators on the front panel adjust the signal level for their respective amplifier channel in all modes. They are calibrated in dB to help setting up active loudspeaker systems or cutting down unwanted noise from the input signal.

In bridged mode, both controls must be in the same position, so that the speaker load will be shared equally between the channels.

8.4 Indicators

Front Indicators

The yellow LEDs at the top indicate if any protection circuits are activated. These are described on page 13.

The Clip/limit indicator tells when the amplifier output is clipping or limiting. It has two different indication statuses:

•If the clip limiter is engaged it has a short time constant and it illuminates briefly.

•If the clip limiter is not engaged it has an increased time constant and it illuminates for a longer period.

The ”-40 dB” LED’s illuminate if the output signal is greater than -40dB (with 0dB as reference to full output power). These LED’s also act as signal present indicators.

The two bottom green ”ON” LED’s indicate that the output circuits are receiving the correct rail voltage.

9 PROTECTION FEATURES

Each Tannoy amplifier has many advanced protection features, protecting both the amplifier and the speakers connected to it should a fault condition arise. Under normal use these features are inaudible. All protection circuits are independent.

9.1.1 Clip Limiter

The clip limiter is included to prevent dangerous clipped signals reaching the speaker and damaging it. When an amplifier is severely overdriven, its output waveform is clipped (its peaks are squared off) –reducing the crest factor. In extreme cases, the waveform can approach that of a square wave. An amplifier is normally capable of producing far more power under these conditions than its normal undistorted rated output power.

The limiter works by monitoring the output and comparing the distortion produced between the input and output of the amplifier. If the distortion exceeds 1%THD for any reason (voltage or current clipping), the limiter reduces the input signal proportionally. Note that, if the signal is distorted or clipped before it reaches the amplifier, the clip limiter cannot detect it, and will not be activated. Under normal operation the clip limiting is inaudible. The limiter can be turned On or Off by depressing or pressing the relevant clip limit switch.

Some manufacturers of loudspeaker controllers do not recommend the use of clip limiters in amplifiers, as they tend to upset the tracking of the controller’s limiters.

Apart from this one exception, Tannoy recommend leaving the clip limiters switched "on" (button depressed). (As a side-effect, once the amplifier comes out of a protect condition, the output level has a slow rise time -the effect is like turning the gain up slowly).

Tannoy Limited	13
TA 1400 User Manual	11-06-02