RCA Hobby Circuits Manual - Preservation · PDF fileamplifier that is stabilized by the use of...
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Transcript of RCA Hobby Circuits Manual - Preservation · PDF fileamplifier that is stabilized by the use of...
RCA s*Lr*.sTATr
Hobby CircuitsManual
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Gircuits
Fig. 7l - Completed circuit board Jor the automatic keyer.
CIRCUIT NO. 7 _ MICROPHO]UE PREAMPLIFIER
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The microphone preamplifier iscapable of boosting the output of ady'namic microphone to a 0.5- to l-voltl:r'el. This level is compatible with therni.xer, volume compressor, and lineamplifier (Circuit No. 8) and the audioaorver amplifier circuit (Circuit No.l -) described in this Manual.
Gircuit OperationThe schematic diagram and parts
list for the microphone preamplifierare shown inFig. 72. The circuit con-sists of a two-stage direct-coupledamplifier that is stabilized by the use ofdc feedback. The circuit works well*'ith dynamic microphones havingimpedances from 200 ohms to 30,000ohms.
When the circuit is in operation,base bias current for the input transis-tor Qr is obtained from the emitter ofoutput transistor Q z through Rs. Q z
obtains its base bias current throughthe collector iesistor of Q r, Rr. Thisunique bias circuit provides dc feed-back for stabilization of the operatingpoints of the transistors. For example,
if the operating current of Qr increases,the collector voltage of Qr decreasesand reduces the voltage of the base ofQz This lower voltage causes a reduc-tion in the operating current of Qr.When the operating current of Qzdecreases, the voltage at the emitter ofQz also decreases. This voltagedecrease is reflected back to the base ofQr, which undergoes a current reduc-tion that offsets the initial increase.
This preamplifier circuit is designedto operate from an l8- to 20-voltsource; voltage in this range can beobtained from batteries or from apower supply. The power circuit can becommon to the power amplifier. Thepreamplifier circuit can tolerate vol-tages greater than 20 volts if R8 isincreased about 400 ohms for everyvolt above 20 volts. The current drainof the preamplifier is approximately2.5 milliamperes; the voltage gain isabout 1700.
Special GonsiderationsWhen the preamplifier is used with
a low-impedance dynamic microphone
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Circuits
crRcurT ruo. 15- ELEcTRorurc FUzz BoXThe fuzz box is intended to be used
rith a guitar; however, it may be usedrith any instrument whose musicaloutput is electrically amplified. It canbc used with the audio power amplifiercirorit described in this Manual. Thefu:z box changes the character of theround produced by an instrument andmakes possible the generation of arariety of sounds of which the instru-mcnt alone is not capable.
Gircuit OperationThe schematic diagram and parts
list for the electronic ltzz box areshown in Fig.97. The output of transis-tor Q r, a basic emitter-follower thatgives the Iuzzbox a high-impedanceinput, is applied to the base of Qz. Qz isbiased at almost cutoff and, therefore,amplifies only half the input signal.Potentiometer Rs is used to adjust the
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Frrts List
Cr : 0.01 microfarad, 25 volts orgreater
CrCsCr = 5 microfarads, 15 volts,electrolytic
Q'Q, : transistor, RCA SK3020Q1 : transistor, RCA SK3005R.r Rz Rs R? : 100,000 ohms, l/2
watt, lj%oR. : 3300ohms, l/2watt, l0%
Rs : potentiometer, 5000 ohms, lin-ear taper
R6 : 15,000 ohms, l/Zwatt, lUVoRs Rro : 2700 ohms, I 12 watt, l\VoR, : 47,000 ohms, I l2watt, ll%oR1r : potentiometer, 10,000 ohms,
linear taperS1 : switch, double-pole, double-
throw
Fig. 97 - Schematic diagram and parts list Jor the electronic Juzz box.
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input signal level to Q, (to approxi-mately I volt) and the amount of"flzz." Transistor Qa receives theoutput from Qz through Rs; Qa isbiased in such a way that the top halfof the sigral input to it is clipped; thisclipping action tends to create a squarewave. Potentiometer Rrr is used tovary the output level of the circuit.
The input voltage of this circuitshould be approximately I volt. Thecurrent drain for this circuit is approxi-mately 5 milliamperes.
RCA Hobby Circuits Manual
Construction
In operation , the fuzz box is nor-mally cut in and out as music is beingplayed. When the fttzz box is cut outthe musical instrument is connecteddirectly to the amplifier.
The drilling template for the elec-tronic fuzz box is shown at the back ofthis Manual; a component placementdiagram and a photograph of a com-pleted circuit board are shown in Figs.98 and 99 respectively.
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65Fig. 98 - Component placement diagram Jor the electronic Juzz box.
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Fig. 99 - Cornpleted circuit board Jor the eleetronic Juzz box
