his 140 - amplifiers, dampers, and acoustic plumbing

AMPLIFIERS, DAMPERS, & ACOUSTIC PLUMBING

• Class A amplifier

• It is a biased amplifier meaning that, even when no sound is present, electrical current is continuously flowing to the output transducer through the amplifier.

• It is the most simple form of amplifier.


• Class A amplifiers are very inefficient with their electrical current use.

• Are most suitable for low power hearing instruments.


• Class B Amplifiers

Are more efficient because they separate the “work load” into two amplifiers. One amplifier is pushing the output transducer diaphragm while the other amplifier is pulling it.


Class B amplifiers are used most often where high output amplification is required. They are often referred to as push-pull amplifier circuits.


• Class D amplifiers

• Are smaller due to the amplifier being inside the

same can as the output transducer/receiver.

• Are more efficient with the electrical current, because they switch off and on at a rapid pulse rate.


• Class D amplifiers are called “switching” amplifiers due to the rapid pulsed switching of current.

• The are used in the output stage of today’s digital amplifiers.


• Class H amplifiers

Are class A amplifiers with a sliding bias for greater efficiency of current use. They are still less efficient than a class D amplifier.


• Due to the sliding bias, the class H circuit has more distortion.

• It is still less efficient with current use than a class D amplifier.


• Compression amplifier

Its basic operation is that when the sound input increases into the hearing instrument, the amount of amplification output decreases.


Compression amplifiers may also be known as automatic gain control (AGC) or automatic volume control (AVC).

Compression amplification is fundamental in the development of digital algorithms.


• Digital Circuits/amplifiers

Use many of the fundamental concepts of the analog hearing instruments; however, it must have an analog to digital converter on the input side of the digital amplifier.


The A/D converter changes the analog electrical voltage into a string of numbers. These numbers can then be manipulated into very discreet signal patterns.

A digital to analog D/A convertor is necessary to convert the signal into an output transducer.


• Acoustic Dampers

Are necessary to smooth the inherent electrical peaks of amplifiers and output transducers.


Electrical peaks create distortion and unstable frequencies. Unstable frequencies are what make hearing instruments “whistle”/feedback.


Acoustic dampers are most often made as fused mesh inside of a stainless steel ring.

They may also be made from lamb’s wool and plastic foam.


Dampers may be located/inserted into the microphone or receiver opening of the transducers; or somewhere within the “plumbing” i.e. earhook, tubing.


• Acoustic couplers/”plumbing”

Modify the amplified sound as it exits into the ear canal.


• With various horn shapes of the couplings and damping, much of the frequency amplification can be modified before it exits into the ear canal.


• Acoustic plumbing/coupling can be related to the various shapes of musical instruments in the creation of musical notes. This shaping of the acoustic coupling can modify the frequencies received at the eardrum.


• Acoustic couplers include:

1. Earhooks

2. Earmolds

3. Various tubing

Note: We will learn more of the specific applications in

future coursework.

his 140 - amplifiers, dampers, and acoustic plumbing

Business

acoustic plumbingdampers

acoustic coupling

amplifier circuits

biased amplifier

simple form of amplifier

amplification output

output transducers

electrical current use