Realizing Hi-Fi Audio with 3 Ten-Tec Rigs

My listening is done with either a pair of Jamo CS5 amplified speakers or a pair of
Sennheiser HD-580 headphones connected to a Timewave DSP-599ZX box.  Either way, they only 
need to be driven with a line level signal source.  Without exception, every amateur rig
I've ever listened to had inferior audio output amplification between the product detector
and audio output.  I've found it easy to tap the audio from various rigs at or soon after the 
product detector, resulting in incredible improvements in fidelity of received ssb audio.
These improvements include more extended passband response, smoother and more listenable audio
and also much quieter audio.  If the tapped signal is very low level, I use my Behringer
MX-802A mixer to amplify it to line level using its "AUX SEND" bus.

Another alternative to external amplification is to replace the rig's af ouput stages with 
something better.  For the enterprising homebrewer, check out audiphile opamp info or some
sample circuits at Headwize.


Triton IV Corsair Omni VI

Triton IV

It is VERY easy to add a line level audio output to the Triton IV. If you look at the IF-AGC board (80279) schematic, the terminal pin labelled "Audio" on the schematic (off C21) is what we want. What I did was run a shielded cable from that pin to an RCA jack on the rear panel. I decided to use the "sidetone" jack, but you can use anyone you want. Just remove the wire currently connected to it, and tape/label the wire so you can reconnect, if desired, in the future. With the 544 bottom cover removed, and with 544 resting on its top with front panel toward you: if-agc board is to the left of the pto, with 2 if cans on it. There is a row of 7 stake pins at the rear of the board (in back of blue S meter pot) You will attach a shielded cable as follows: center conductor to third pin from the right, shield to last pin on right. Route the cable to the RCA jack you decided to use. Solder the center conductor of the cable to the center pin of the RCA jack and the shield to the shield lug. This mod takes the audio off before the AF Gain control. I control the volume with the level control of my speakers. It would be easy to take the audio off after the gain control, by consulting the schematic. Future additions to this mod will include replacing some of the coupling capacitors that are still in the audio path with more audio-friendly capacitors and perhaps higher values to enable slightly more bass extension. It may also be necessary to adjust C3 and tweak T1 on the SSB generator board (page 3-26 of the manual) to properly center the ssb passband and pull in some bass extension. If you adjust C3, you should also check that the CW offset hasn't been affected. You may need to adjust C2 to bring the CW offset back to 750hz. Note - these adjustments should only be undertaken by those experienced with such alignment. A final note is that Ten-Tec may have used different xtal filters during the span of the Triton IV/544 so your rig may be equipped with either a 2.5Khz or 2.7Khz xtal filter. It probably would be fairly easy to wire in an Inrad 2.8Khz xtal filter for ultimate fidelity. If you replace the filter, be prepared to align the SSB generator board, but this is not very difficult for those with a little experience, a dummy load, an oscilloscope and a general coverage receiver or frequency counter.

Corsair

I have followed the same methodology to add a line out to my Corsair I. After experimentation, I've taken the audio off after the first op-amp (notch filter) following the product detector. Listening tests as well as passband spectrum measurements indicate a vast improvement in audio clarity as well as bass extension. But the improvements in the receive audio are not just a matter or flatter/wider - it takes on a more natural, uncolored, listenable quality. It is my intention to eventually feed the product detector output directly into an FET buffer followed by an audio-friendly OPA132/4 op amp, and connect the OPA134 output as the line out signal. The op-amps in the Corsair have very mediocre audio performance and are very noisy. The 9Mhz crystal filter (2.4Khz) was replaced with an INRAD 2.8Khz filter and the first crystal filter position of the 6.3Mhz if was bypassed with a .01uF capacitor to run "wide open." Also, the ssb-n carrier offset was set at -200hz by aligning C3 on the ssb generator board for 9000.200Khz The carrier offset was determined experimentally by measuring the receive audio passband spectrum at several offset values. A passband of 30 - 2750hz was ultimately achieved. The SB-N to SB-R symmetry is perfect and on receive, the Corsair I has the most identical sounding USB vs LSB response that I've ever heard from a radio. Transmit audio passband has also been measured and it matches the receive audio passband. INRAD includes a note with this filter only, that says "you may need to adjust the carrier balance for optimum performance upon installation". I found that with my Corsair, I was indeed experiencing a bit of carrier in my transmit audio after installation. Making the adjustment is very easy to do if you own an oscilloscope. You put the scope probe on the output connector (74) of the DSB board. You tweak T1 (output xfmr) for maximum signal (rcv mode). Hit the PTT switch to go into xmit mode and adjust R1 and R2 for minimum signal. A significant improvement in the smoothness of your transmit audio can be had by driving the balanced modulator directly from your external mic preamp. Connection to the balanced modulator (U1 on DSB board) can be found quickly - there is a connector on the DSB board bringing in TX AF from the mic VOX board. You can tap in right there, and purists might want to bypass/replace the 1uF electrolytic blocking capacitor immediately following. What I did was remove the DSB board, which is surprisingly easy. On the solder side of the board, I soldered a 1uF nonpolar poly capacitor at the junction of L4 and C20 and then ran a shielded cable from the other side of the cap out to the rear panel "Audio In" jack, which normally is wired just like the mic connector but can now be a balanced modulator input. There is a noticeable "grunge" in the stock Corsair's tx audio from its onboard mic preamp, and this will completely eliminate the grunge. Purists will want to unplug the TX AF cable, but for a slight penalty you can keep it connected if you also need to run a mic directly into the front panel mic jack. Finally, I recommend using a high quality audio isolation transformer for the connection between your external mic preamp and the balanced modulator. W2IHY currently sells them for $10.

Omni VI

The stock Omni VI(+) suffers from a very constricted passband in ssb. ARRL measured about 1600hz in their review. After experimenting a bit, I found a way to completely open the passband up to the full width of the xtal filters. For those seeking hi-fi ssb rx audio from the Omni VI, and who are willing to amplify the audio externally, a night and day improvement can be had from tapping the product detector audio off U3-6 on the if/af board through a 1uF capacitor tack soldered to the pin, and a shielded cable brought out for amplification. It is a requirement that you then adjust C10/C12 on the TX Audio board to optimize the receive passband. The Inrad 2.8Khz filters are really a necessity to complete the "mod" for the Omni VI. It goes without saying that the notch filter, dsp, sidetone, fm, etc. are bypassed with this audio output. Good Luck and happy listening!

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