[TenTec] Omni VI & SignaLink USB

[Jim Brown]

On 7/10/2014 6:41 AM, Bob McGraw - K4TAX wrote:
> Tigertronics i.e. SignaLink has transformers on the input and output 
> signals for isolation.

A transformer does NOT provide isolation to differential mode signals. 
It only blocks common mode signals. For example, audio voltage on both 
windings of a 1:1 transformer are equal, and no impedance is added to 
the differential circuit. In a 4:1 transformer, the voltages are 
proportional to the turns ratio, and the impedance looking into the 
transformer is equal to terminating Z multiplied by the square of the 
turns ratio.

Remember that "600 ohms" has not been used in pro audio for at least 40 
years. Output stages are low-Z sources, typically 100 ohms for pro 
products, 300 ohms or more for consumer stuff, and input stages are high 
impedance, typically 10K for pro products, 50K for consumer products. 
The impedance of a microphone is defined as 1/5 of it's recommended 
minimum load impedance, and a low-Z mic's output Z is typically on the 
order of 100 ohms. All commonly used ham mics are low-Z, wired unbalanced.

As you have described the schematic, with individual 10K in series with 
both the mic and line inputs, I agree there should be good isolation. 
Perhaps the schematic is not accurate?

73, Jim K9YC