Inrad offers an inexpensive mod kit for the Omni VI/VI+ to improve ssb receive audio.
I've installed the mod along with some careful measurements. The mod accomplishes two improvements:
The mod involves removing the IF/AF Board and replacing three components. I've only done the mod with leaded (not SMT) components. It is an easy job for those with a bit of solder rework experience. The board removes easily - 5 screws along with numerous well marked connectors. Start to finish, it takes about 30 minutes.
HOWEVER (and a big however), you should bear in mind that the receive audio always goes through the Omni 6 dsp circuit.
The dsp circuit has two big effects on the ssb audio passband:
However, if you simply bypass the dsp, the rx audio will have an unacceptably high level of "hiss" without the dsp 3400hz low pass filter. And even if you both bypass the dsp and add a low pass audio filter to take out the hiss, you are left with the difficult task of finding a workable solution for smooth cw T/R switching and sidetone injection which is accomplished in the Omni 6 dsp. The solution I went with was to bypass dsp with added 3KHz LPF on ssb and use the dsp audio for CW, although I added 1KHz LPF on cw as well to clean up objectionable artifacts.
Realize that Ten-Tec did in fact have a clear vision in mind when they carefully designed the stock ssb audio to behave the way it does. With the stock 2400hz filter fitted and the BFO aligned per spec, the audio passband is 250-2650hz. So the stock notch filter behavior and stock dsp high pass filtering makes sense in light of that plus the fact that Ten-Tec engineers were doing their best to cut out as much noise as they could from the passband. They were battling with a level of internal rx noise that had been slowly increasing since the Omni A days and had tried other audio noise filtering tricks previously with the Corsair II and Omni V.
But for many of us, a ssb rx passband lower cutoff of 250Hz is just too high. Read on.
Although the Omni 6 DSP was a laudable effort in bringing new signal processing technology to an amateur hf xcvr, it removes all audio below 260hz. Some operators won't be bothered by this but many of us are. It's not just an issue of diminished fidelity. For example, when running narrow ssb receive bandwidths, I find it desirable to shift the receive passband toward the low end of the spectrum (i.e., 150-200Hz). There is only one way to get more low frequency ssb rx audio from the Omni 6: bypass the audio dsp loop. TMP mini-coax cables 54 and 56 send the audio between the IF/AF board and the DSP CODEC on the Logic Board. Bypassing this loop will restore audio below 260hz. Because of the difficulty in achieving smooth T/R audio transitions and sidetone injection with cw, I will limit this discussion to dsp bypassing for ssb rx audio only while continuing to use dsp rx audio (with its smooth T/R transitions and sidetone) for cw. In designing a ssb rx audio dsp bypass solution, the main requirement is to implement low pass filtering (~ 3KHz) to remove high frequency hiss.
Basically, this is what you want to do in your bypass circuit, added to IF/AF board:
There was an unforseen benefit realized by the dsp bypass. The stock Omni 6 has a high amount of low-level, clearly audible, digital "grundge" in the rx audio - various tones/buzzes/encoder tuning ("zipper") noise, etc - and these were totally removed.
Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters (and carefully aligning BFO oscillators and carrier null) will transform the Omni 6 ssb rx audio and remove all the frustrating grundge from the Omni 6 cw rx audio.
The Corsair II is actually a much easier radio to modify for exceptional audio over the Omni 6. The need to selectively bypass the dsp in the Omni 6 makes it an unusually difficult job. But the CAT port in the Omni 6 is an undeniably huge advantage.
Just to reiterate, careful carrier null alignment is a necessity - even with careful adjustment, the LSB/USB carrier suppression is barely acceptable with the 2.8Khz filter installed in the 9MHz i.f.