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
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