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SoftStart Circuit for Power Amps

SoftStart Circuit for Power Amps

When your monster (or not so monster) power amplifier is switched on, the initial current drawn from the mains is many times that even at full power. There are two main reasons for this, as follows …
Transformers will draw a very heavy current at switch on, until the magnetic flux has stabilised. The effect is worst when power is applied as the AC voltage passes through zero, and is minimised if power is applied at the peak of the AC waveform. This is exactly the opposite to what you might expect

At power on, the filter capacitors are completely discharged, and act as a short circuit for a brief (but possibly destructive) period

These phenomena are well known to manufacturers of very high power amps used in PA and industrial applications, but ‘soft start’ circuits are not commonly used in consumer equipment. Anyone who has a large power amp – especially one that uses a toroidal transformer – will have noticed a momentary dimming of the lights when the amp is powered up. The current drawn is so high that other equipment is affected.

This high inrush current (as it is known) is stressful on many components in your amp, especially …


Fuses – these must be slow-blow, or nuisance fuse blowing will be common

Transformer – the massive current stresses the windings mechanically and electrically. It is not uncommon to hear a diminishing mechanical buzz as the chassis and transformer react to the magnetic stress Bridge rectifier – this must handle an initial current way beyond the normal, because it is forced to charge empty filter capacitors – these look like a short circuit until a respectable voltage has been reached Capacitors – the inrush current is many times the ripple current rating of the caps, and stresses the internal electrical connections

It should come as no surprise to learn that the majority of amplifier failures occur at power on (unless the operator does something foolish). This is exactly the same problem that causes your lights at home to ‘blow’ as you turn on the light switch. You rarely see a light bulb fail while you are quietly sitting there reading, it almost always happens at the moment that power is applied. It is exactly the same with power amplifiers.

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