Most amplification is supplied by the power amplifier. These amplifiers are typed in a particular class. The most common are referred to as class A, class B, and class D. They are distinguished by their power consumption, gain, and output abilities.

Each amplifier can be modified to limit the maximum output of the hearing aid. For linear amplification, the amplifier may be limited by peak clipping. This occurs when the electrical signal exceeds the maximum output of some component of the hearing aid circuit. This type of limiting causes various forms of distortion that has been found to reduce the intelligibility and the subjective quality of speech.

A hearing aid that has some type of level-dependent signal processing is termed a nonlinear hearing aid. Most nonlinear hearing aids reduce gain as input or output levels increase.

Nonlinear hearing aids are designed to amplify a wide range of sounds so that they are audible to the hearing-impaired listener without becoming uncomfortably loud. These aids usually use some form of compression circuit that reduces the gain of the instrument when either the input to the device or the output of the device exceeds a predetermined level. This process results in a comfortable amplification for the wearer and prevents the hearing aid from saturating.

Compression hearing aids can provide amplification of the speech components that are essential for intelligibility and can reduce impulsive or high-level sounds that normally cause discomfort.

The hearing aid receiver is an output transducer and handles more power than a microphone. Receivers in hearing aids are very small because of cosmetic considerations.

In general, larger receivers can supply larger output signals. Therefore, the small receivers on hearing aids may be taxed to their output capabilities.

The receiver also must be chosen to match its amplifier. A mismatch in design produces limited output and increases distortion.

Because of the receiver's open position in the external ear canal, it is vulnerable to damage from debris in the ear canal and from the aid being dropped. Manufacturers state that approximately 40% of hearing aids returned for service have damage or blockage to the receiver.

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