Omni VI/Omni VI Plus SSB RX Audio: Inrad Audio Mod & DSP Bypass Mod

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:

1. Without the mod, the minimum notch filter frequency setting is 240hz ("Notch" knob fully ccw). Because there's no way to completely shut off the notch filter, there is always a 240hz "suckout" in the audio in the stock Omni 6. The mod lowers this to 50hz, below the ssb rx passband, by replacing a notch filter IC resistor. With the mod, the highest notch frequency ("Notch" knob fully cw) is now 2500hz.
2. Without the mod, the ssb rx passband tilts down with 2700hz level approx -10dB relative to 500hz level. The mod flattens the passband by replacing two capacitors at the product detector output.

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:

1. passband is rolled off by -25dB above 3400hz (filters out high frequency noise, i.e. "hiss")
2. passband is steeply rolled off below 260hz; this factor makes the first part of the Inrad mod of questionable utility unless you plan on modifying your Omni 6 to bypass its dsp and extend the low frequency response. 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.

To get around the 260hz high pass filtering of the Omni 6 dsp and still provide the needed low pass hiss filtering, I have done the following with great success: tap the Omni 6 audio right at the product detector output (pin 6) through a 1uF cap and bring it via shielded cable (you could also tap the audio after the notch filter) into an appropriate low pass filter to remove hiss. This can be an external equalizer/dsp unit or a homebrew op-amp circuit placed adjacent to the IF/AF board. I've tried both these methods with excellent results.

A side benefit of this filtering technique is that all the digital "grunge", encoder noise, and other annoying artifacts are totally removed from the rx audio.

The existing audio chain can be left intact as well, in case you ever want to listen through the Omni 6 headphone jack or speaker. I considered reinjecting the low pass filtered audio back into the Omni 6 audio chain in the vicinity of DSP return connector 54/56 on the IF/AF board, but the Omni 6's af output amplifier U8 has very poor low frequency response.

Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters will transform the Omni 6 ssb rx audio from mediocre to sublime. Although rx audio is the topic at hand, I've also optimized my tx audio by feeding line level (i.e., externally pre-amped) tx audio into the balanced modulator U4-5 through a 1uF cap and a 1.5K resistor. This mod leaves the existing Omni 6 tx audio chain intact.

One last project for the future is to implement a ssb monitor by feeding some of the tx audio through the unused analog switch U7D in parallel with the cw sidetone audio.

- Barry N1EU