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  An Audio Frequency Response Modification for the Hamtronics™ TA451 UHF Transmitter
By Brad Andrews KB9BPF
18 July 2007
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Since 1999 the Macoupin County Radio Club 146.865 repeater has been using a remote receiver link consisting of a Hamtronics VHF receiver and UHF transmitter, and I have always thought it sounded "tinny." When upgrading the repeater to full duplex utilizing the local GE MASTR Pro receiver with the remote receiver link and a simple voter, the difference in audio quality between the two receivers was striking.

I began working to smooth out the audio response of the remote link system and found that the tinny audio quality was being introduced by the Hamtronics TA451 UHF link transmitter, specifically the microphone preamplifier.

One of the frequency response determining components is C2 (2.2uF), the DC-blocking capacitor in series with R2 (100K) feeding the base Q1, but the capacitive reactance of C2 is swamped out by the much larger value of R2 and does not affect frequency response much except at very low frequencies, below 10 Hz

Bias for the base of the first transistor Q1 is derived from the emitter biasing resistor R7 (1.2K) of the second transistor Q2, which is bypassed by C4 (10uF).

This also provides a path for negative feedback, but the amount of feedback is not well defined, relying mostly on the capacitive reactance of the emitter bypass cap C4 to create the negative feedback signal. Since capacitive reactance changes inversely with frequency, AC voltage across C4 would increase significantly at low frequencies, creating a larger feedback signal, decreasing amp gain at lower frequencies.

In order to create a more stable and well-defined feedback signal, I added an unbypassed resistor Rf in the emitter portion of the Q2. The result was a significantly flatter frequency response and the ability to set the gain to a reasonable figure.

Gain of the stage is defined as: G=V(out) ÷ V(in)
Ignoring the effects of the capacitors, Gain can be further defined by: G=((R6+Rf) ÷ Rf) × (R3÷R2)
To include the effects of the capacitors, add the capacitive reactance of C4 in parallel with R7 to Rf and the capacitive reactance of C2 to R2.

With Rf=150ohms and C4=100uF, gain is within 0.1dB of 19.2dB at 50 Hz and up.

Click on the image below for a full size image:

Click here for a rotated version of the above oriented for printing. It's a large file, you may want to select the "Fit to page" option in your printer properties.


Brad Andrews KB9BPF can be contacted at (callsign) //at// arrl //dot// net
(the email address has been disguised in an attempt to thwart the spambots).

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