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How to Build a Two-Stage Detector-Amplifier Unit
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signal before it reaches the detector tube. However, the detector tube must also be possessed of qualities which will make of it a sensitive rectifier of these signals.
Honesty of reproduction, or in other words, the property of the amplifier to repeat faithfully the sounds as transmitted, is a function governed by the selection of a suitable audio-frequency transformer, plus the intelligent use of A, B, and C batteries.
All detector-amplifier circuits are pretty much alike. Their differences are mainly in the design which affects the control of the various parts entering into the construction of a completed unit. Undoubtedly a unit may be produced in which everything possible is variable: C battery adjustment to the amplifiers, grid leak, grid condenser, tapped transformer primaries and secondaries, and B batteryvoltages. It is hardly necessary to state that the tubes would be individually controlled loy separate rheostats. However, a unit such as this would soon loose its value if it were to be used in a permanent installation where there would be no need for all these controls once a satisfactory adjustment has been obtained. Such a completely variable unit would rather be suitable for the laboratory.
The volume produced by an audio amplifier
depends upon the number of stages of amplification which may safely be used without overloading the amplifier tubes. Volume also depends upon the sensitivity of the detector and the ability of the amplifier to take whatever is produced in the detector and amplify it without altering the signal characteristics. Briefly explained, this means that some amplifying transformers have the tendency to favor some voice and music notes over others, depending upon the electrical and mechanical makeup of the transformer. The distributed capacity in transformer windings causes a favoring of the lower frequencies over the higher frequencies. Also, when little iron is used in the core construction, it becomes over-saturated by the forceful variations of electromagnetic flux and prevents the transformer from functioning successfully.
GOOD QUALITY AND SUFFICIENT VOLUME
THE detector and two-stage audiofrequency amplifier described here is the result of experimentation along the lines as explained above. It has been reduced to a practicable working unit producing a very high quality of signal with plenty of volume.
This detector-amplifier may be used with any tuner now available, but has been especi
FIRST STAGE
AUDIO AMPLIFIER
A.F.
TRANSFORMER
SECOND STAGE AUDIO AMPLIFIER
O A 0+ -O
45 90-120 B
FIG. 2
The circuit of the detector-amplifier. The numbered units refer to those panel controls as marked on the panel illustration Fig. 3. In wiring it is well to make frequent use of this circuit and the schematic wiring
diagram, Fig. 7