Drake T-4XB Audio Improvement

By Chris Codella, W2PA

February 2009

In early 2007 I decided to try improving the audio section of my Drake T-4XB, part of a B-Line I had purchased back in 2000.  On-air reports consistently indicated a more restricted sound compared with my FT-1000D using the same microphone and no processing.

I began by modeling the two-stage microphone amplifier with the "5Spice" simulator, a freeware version of which is available for download.  The resulting frequency response curve for the as-designed circuit is pictured below.  The red curve (bottom) is measured at the input and the blue (top) at the output.  The cursor is positioned at a point 3dB down from the maximum output.

This shows that the 3dB rolloff point was up around 465 Hz, which explains the constrained audio sound.  I believe this was probably the intention of the designers - not to sound tinny, but to optimize for SSB communications.

Just to see what might be possible, I measured the response of the sideband filters - the resulting plot is shown below.

Evidently, the passband is about 2.5 kHz wide - just right for SSB and not too tight for better audio on the transmitted signal.

So I decided to try experimenting with different values of selected components in the microphone amplifier - V9A and V9B, a 12AX7A.  The relevant schematic section, scanned from the R-4B manual, is shown below.  This circuit is virtually the same in all versions of the T-4 and TR-4 (and, I suspect, the TR-3 too).

After some experimenting, I arrived at a configuration where the following values were added to existing capacitors, so as to make the modification easily removable. The new capacitors were installed in parallel with the existing ones.

C124 - add .0047uF  (mic. input)

C132 - add .01uF      (interstage coupling - out of V9a)

C133 - add .005uF    (interstage coupling - into V9b)

The values are not critical and I used parts I already had around. All were ceramic disk capacitors. I found that in each case the new capacitor could be added without disturbing the original parts.  The resultant characteristic is shown below:

The 3dB rolloff point is now down around 87Hz.  On-air tests confirm the improvement in audio characteristics.  I'm told there is now little or no difference, in how the audio sounds on the receiving end, between the T-4XB and the FT-1000D, using the same microphone and no additional processing.  I also performed this modification on my TR-4 with similarly good results.