Fig. 2. Extreme compression range of the

ACP 1 is illustrated by this graph.

This unit is an extremely low-noise device and, therefore, input noise from the "frontend" of the compressor is negligible. The 2N3819 also provides a very high input impedance for this first stage. RK a I megohm potentiometer, serves as the input level control and is located on the front panel of the compressor.

Transistors Q2 and Q3 are NPN silicon â– leviccs used as common emitter amplifiers to boost the audio signal coupled through the sourcc output of Ql. The emitter swamping resistor of Q2 is bypassed through C6 via Q5 in a circuit arrangement which provides the basic compressor action. The emitter swamping resistor of Q3 is not bypassed and the stage operates in a slightly degenerative mode.

The output signal is developed across R9, a 5K ohm potentiometer which also serves as an output level control mounted on the back panel of the ACP-1. Regardless of the setting of R9, however, part of the output signal is coupled through C9 to diode D1 and transistor Q4. DI rectifies the audio signal segment coupled through C9 and the resultant dc current is amplified by Q4, another NPN silicon transistor.

The amplified dc output of Q4 is used to control Q5, a PNP germanium device, Q5 is operated in its linear resistance region and acts as a current-sensitive variable resistor. This variable resistance is in series with C6, the emitter bypass capacitor for Q2, The gain of Q2 changes as the resistance of Q5 changes since the effect of C6 is reduced and the stage operates in an increasingly degenerative mode.

Circuit Features

One of the significant differences over many other circuits that one notes in the foregoing description is the use of the 2N3819 FET input stage- The use of this

FEI provides two significant features. One is very high input impedance so that a correct match is provided to the usual hi-z crystal or dynamic microphone. Low impedance microphones require the use of a matching transformer for proper operation. The other significant feature is that the extreme low-noise characteristic of the FET prevents the noise buildup or noise "rush" that is common between speech pauses when using various other compressor circuits. The lack of noise "rush" is also partially due to the well chosen release time of about two seconds,

Another significant feature is the use of Q4 as a dc amplifier stage. The use of this extra stage provides a greater compression range than is possible when only a diode or dual-diode voltage multiplier is used to control the variable resistor element in a compressor, R1 1 and CIO determine the compression release time while the voltage divider formed by R13 and R14 set the output level at which compression action starts.


All the components supplied were of high quality ami a complete set of plugs and battery connectors are included (the battery itself is not supplied). The components all mount, except for the panel controls, on a heavy duty glass epoxv printed circuit board? The enclosure has a gloss brown speckle finish with white lettering. Provisions are made for internal mounting of a 9 volt transistor-type battery.

The assembly, circuil and operating details are all described in a 3Vi page folded sheet. Following the instructions given, the assembly time required was somewhat less than two hours.

Operating Experience

The unit was tried with a SB-34 transceiver and crystal microphone (as well as a SBE microphone through a matching transformer for the input). 1 he SB-34 has no ale circuitry and particularly outstanding results were obtained. Stations worked consistently reported an apparent 6-8 db increase in signal strength when using the compressor-Audio reports were clean and no appreciable noisiness from use of the compressor was reported. St was interesting to note that the tubes used in a linear with the SB-34 started to redden slightly, a sure sign that greater average power was being generated. Transmitters having a usual 8-10 db range ale circuit should exhibit only a slightly less improvement when using the compressor.

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