Abstract
The object of this paper is to describe a practical method of amplifying heart sounds that may be used to teach students abnormalities of these sounds, to distinguish murmurs and to elucidate cardiac arrhythmias. This procedure is particularly useful in teaching auscultation to students.
Many amplifiers† have been described both in the United States and abroad. These usually have been bulky and not portable and many require the use of batteries. We have designed a high-gain 3-stage amplifier that is portable, inexpensive, operates on alternating current and does not require the use of batteries. The essential features consist of a type 57 tube of high amplification value, which is resistance coupled to another 57 tube in the second stage; and this in turn is resistance coupled in a 2A5 tube in the third stage. The power supply is the customary circuit and is heavily filtered using 30 henry chokes and 28 microfarads of condenser. A 2-ampere fuse was installed to prevent burning out the transformer in the event the amplifier is plugged into direct current by mistake. A 12-inch permanent magnet speaker was selected because of a better tone response than the magnetic speaker, and because it is lighter in weight than the dynamic speaker. An output transformer, while not required, was found to improve the tone response. It is possible with certain modifications to use this amplifier with direct current.
The choice of a suitable microphone presented the greatest problem in our work. The aim of a portable and inexpensive outfit ruled out the condenser microphone as it is bulky and requires a separate power supply. The same is true of the ribbon and dynamic types of microphones.
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