DL7MAJ, March 30th 2010
New Anti-Vox Output for the R-4C
When replacing the original AF-Amplifier in the R-4C with a „new“ IC-based amplifier, the
Anti-Vox output won´t operate because the audiotransformer T13 can´t be used. This
transformer has a secondary winding especially for the anti-vox and supplies at normal audiooutput (speaker or headset) app. 20Vpp for the anti-vox in the T-4X. This is app. 20 to 40
times the voltage supplied to the audio-output. The T-4X requires such a high voltage for the
A small audio-amplifier solves this problem – see picture 1.
Picture 1 Anti-Vox Amplifier
This amplifier is supplied with 150VDC to get the required output-voltage, this voltage is
taken from C164.
• Use a bipolar npn-transistor with Ucbo > 250V; the BD159 has 375V.
• The collector-resistor (33 k Ω) has a thermal dissipation of app. 150mW .
• The emitter-resistor (68 Ω) increases the input-impedance of the circuit and helps to avoid
audible distortion in the speaker at high output voltages. Otherwise, the base-emitter-diode
would become conductive.
• The input-resistor (100 Ω) defines the DC-potential to ground, if there is no speaker or
headset connected.
• The 2M7 sets the bias and should be selected for a collector-voltage of app. 80VDC.
• The input is connected parallel to the audio-amplifiers output and thus parallel to the
speaker (LS) or headphones.
• C104, C105 and R87 are the original components and remain unchanged.
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DL7MAJ, March 30th 2010
This circuit is soldered onto an experimental board and located at the place of the removed
T13 – see picture 2. I used a small tinned metalstrip, bended it, drilled two holes into it and
soldered the board onto it. The holes are the same as for T13.
Picture 2 New Board, T13 removed
If you want to contact the author:
Stefan Steger, DL7MAJ, eMail: dl7maj@darc.de
Homepage: www.dl7maj.de
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