On Tue, 2006-07-04 at 20:56 +0000, Fraser Robertson wrote:
> Hi all, is possible to transmit good quality SSB from this rig? I get
> reports of muffled audio. Listening on another RX I know what they mean.
> I've re-aligned the driver & pa bias, carrier oscillators etc. I've
> isolated the mic ground as by design it shares the PTT ground and digital
> noise was getting into the mic amp.
> With a two tone test the signal looks ok on a scope and spectrum analyzer
> (products 26dB and 38dB down).
> I've tried different power supplies, different mics and audio sources.
> It sounds reasonable on receive, to me anyway, and works well on CW.
> The rig has the stock filters fitted. A friend has the Inrad SSB filter and
> says he gets muffled audio reports too.
> I've ruled out RF feedback and am running out of ideas. Any advice would be
> appreciated. The audio is readable but not crisp clear quality. (The rig
> is an upgraded Omni6).
> Thanks / 73 Fraser G4BJM
Well, there may be room for some more detailed troubleshooting.
What are you using for microphone? What is its impedance? What is the
radio input impedance, and how capacitance is across the microphone
wiring, in the microphone and in the radio?
Is there a sock over the microphone, either cotton, wool, or foam for
How far from the microphone are you talking?
By muffled, do you mean lacking in high frequencies, or is there
distortion on the higher frequencies, or is there distortion on the
lower audio frequencies?
On a scale of 1 to 10, where is the microphone gain set on the radio?
How much average power does the output show on voice? How much peak
power? Is there a fast/slow ALC setting? Likely not, but I ask anyway.
When you aligned the radio did you set the transmit carrier oscillator
frequency to be just 20 dB down the slope of the filter and was that
about 300 Hz from the filter passband corner frequency? Or did you shoot
for more bass and set the carrier closer to the filter which then takes
off some highs?
Any of these or a combination of them can lead to loss of higher audio
frequencies. Driving a low impedance radio with a high impedance
microphone is really effective at killing high. Too much RFI filter
shunt capacitance on the microphone wire can be really good at killing
high frequencies from a high impedance microphone. Talking really close
to a microphone can enhance the lows, (which is why so may TV singers
swallow the microphone to get it to pick up their puny sounds and make
them sound greater) at the cost of highs. A wind filter costs highs.
Let us know these details and maybe we can offer suggestions for change.
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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