Schematic Volt

This High Level Wideband RF Preamplifier can be made in two ways: (1) with the aid of a linear active element, or (2) with a non-linear element operating with negative feed-back. This circuit is of the second kind, using an RF power transistor as the active element. Feedback is also required to ensure correct termination (50 Q) of the aerial, since bipolar transistors normally exhibit a low input impedance. Also, the noise figure is not increased because virtually no signal is lost.

High Level Wideband RF Preamplifier Circuit Diagram:



The common-base amplifier is based on a UHF class A power transistor Type 2N5109 from Motorola. The feedback circuit is formed by RF transformer Th. The input and output impedance of the preamplifier is 50 4 for optimum perform-ance. Network R3-C5  may have to be added to  preclude oscillation outside the pass-band, which  ranges from about 100 kHz to 50 MHz. The gain is  approximately 9.5 dB, the noise figure is between 2  and 3 dB, and the third-order output intercept point  is at least 50 dBm.

The input/output transformer is wound on a Type FT37-75 ferrite core from Micrometals. The input winding is 1 turn, the output winding 5 turns with a tap at 3 turns.

Parts:
R1 = 47K
R2 = 100R
R2 = 6.8K
R4 = 68K
R5 = 2.7K-1/2W
R6 = 2.7K-1/2W
R7 = 2.2K
R8 = 39K
C1 = 100uF-25V
C2 = 100uF-25V
C3 = 100uF-25V
C4 = 47nF-63V
C5 = 47nF-63V
D1 = BZX79C18
D2 = BZX79C18
Q1 = BC337
Q2 = BC327
J1 = RCA Jack
IC1 = LM833, Opamp

Notes:

• R2, R3, R4, R7, R8, C4 & C5 should be low tolerance types.
• Schematic shows left channel and power supply.
• For stereo operation R1, R2, R3, R4, R7, R8; J1; C1, C4 & C5 must be doubled.
• Numbers in parentheses show IC1 right channe

Parts:

P1,P2___________10K Linear Potentiometers
 (twin concentric-spindle dual gang for stereo)

R1_____________100K 1/4W Resistor
R2,R6___________18K 1/4W Resistors
R3_______________3K3 1/4W Resistor (18K for shelving curves: see Notes)
R4,R5____________1K8 1/4W Resistors (10K for shelving curves: see Notes)
R7_____________560R 1/4W Resistor

C1_______________1µF 63V Polyester Capacitor
C2_______________4n7 63V Polyester Capacitor (3n3 for shelving curves: see Notes)
C3_______________1µ5 63V Polyester Capacitor (100nF for shelving curves: see Notes)
C4,C7__________100nF 63V Polyester Capacitors
C5,C8___________22µF 25V Electrolytic Capacitors
C6,C9_________2200µF 25V Electrolytic Capacitors

IC1___________TL072 Dual BIFET Op-Amp
IC2___________78L15 15V 100mA Positive Regulator IC
IC3___________79L15 15V 100mA Negative Regulator IC

D1,D2________1N4002 200V 1A Diodes

SW1____________DPST Toggle Switch

J1,J2__________RCA audio input sockets
J3_____________Mini DC Power Socket

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Comments:

Needing a Tone Control, this module can be inserted in the chain between the Control Center and the Power Amplifier.
The circuit is based on an original design of Reg Williamson, published on Electronics World + Wireless World, Circuit Ideas, January 1991.

This tone control circuit will take the form of either the traditional bass and treble adjustment or the shelving type, which is similar to a "tilt" control but with independent bass and treble.
Asymptotic slopes for both are about 4dB/octave, but begin to flatten out at 100Hz and 10kHz, reaching a maximum of ±15dB at the ends of the audio band.
Component values in parenthesis are those for the shelving type of control. In this case, the shelf begins at about 250Hz and 4kHz, flattening to a ±6dB maximum an octave above or below; the central point is 1kHz.
Reactive elements for the low end include a generalized impedance converter, which simulates an inductor in series with a resistor (gyrator).

When set to the electrical centers of their respective controls, the reactive elements of the equalizers are virtually out of circuit. With these settings, the circuit behaves as a unity gain amplifier with 100% negative feedback. Even so, reactive elements can be switched out altogether with a DPST on/off switch.

As with the other modules of this series, each electronic board can be fitted into a standard enclosure: Hammond extruded aluminum cases are well suited to host the boards of this preamp. In particular, the cases sized 16 x 10.3 x 5.3 cm or 22 x 10.3 x 5.3 cm have a very good look when stacked. See below an example of the possible arrangement of the front and rear panels of this module.

Notes:

• To build the shelving type of control, use the values shown in parenthesis for R3, R4, R5, C2 and C3
• The circuit diagram shows the Left channel only and the power supply.
• Some parts are in common to both channels and must not be doubled. These parts are: P1 and P2 (if twin concentric-spindle dual gang potentiometers are used), IC2, IC3, C4, C5, C6, C7, C8, C9, D1, D2 and J3.
• This module requires an external 15 - 18V ac (50mA minimum) Power Supply Adaptor.

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Technical data:

Input sensitivity:

1V RMS for 1V RMS output

Maximum output voltage:

9.5V RMS into 10K load

Frequency response:

flat from 20Hz to 23KHz

Total harmonic distortion @ 1.5V RMS output:

100Hz = 0.004% 1KHz = 0.004% 10KHz = 0.002%

Total harmonic distortion @ 6V RMS output:

100Hz = 0.003% 1KHz = 0.003% 10KHz = 0.03%

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