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
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 external to the Omni 6 via shielded cable (you could also tap the audio after the notch filter) AND feed this audio line into an appropriate low pass filter to remove hiss. I've used all of the following with good results (2 & 3 are preferred due to smoother sound):

1. Timewave DSP-599ZX high Z input B set to 0.0dBV, filtering set for 100-2800hz   OR
2. Behringer PEQ 2200 analog parametric equalizer set for 85khz lo cut, 2.5khz hi cut, band5 eq -15dB 10khz 1.2 octaves   OR
3. 4Khz Low Pass filter circuit designed by Elecraft for use in the K3 [[http://www.elecraft.com/manual/K3_Schematics_Jun_2010.pdf|schematics]])

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

The existing audio chain is left intact as well, in case you ever want to listen through the Omni 6 headphone jack or speaker. Another option for the experimenter is to re-inject the externally filtered audio back into the Omni 6 which would be easily accomplished via the DSP return connector 54/56 on the IF/AF board.

Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters will transform the Omni 6 ssb rx audio from mediocre to superb. Although rx audio is the topic at hand, you may want to consider feeding the tx audio directly into the Omni 6 balanced modulator (U4-5 on TX Audio board) via a 1uF capacitor if you are already using an external mic preamp and can adjust its output to a fairly low level.

- Barry N1EU