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Broadband magnetic loop preamplifier gain reduction by voltage

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neazoi

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I wonder if I can vary the gain of this small magnetic loop broadband preamplifier, by just lowering down the voltage fed to it?

This will allow me to remotely set the gain of it, without having to use extra cables (phantom power and gain set through the same coaxial). I hope that reducing the VCC won't disturb the balanced nature of the preamplifier???

Thank you
 

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To put it simple, yes, you can. Reducing Vcc will reduce gain. But this will make the transistors work in a less linear region, thus increasing intermodulation and cross modulation.
Here's a hint: place a 9V regulator at the input, feed the circuit with a variable voltage from 12 to 20 or 24V. The amplifier won't be affected, but you can use the variable voltage to drive some electronic attenuator. This is my idea: connect an LDR to the loop, have a LED illuminates the LDR, the LED will have cathode to GND, anode to a 10V zener in series with an appropriate resistor. Send 12V and you'll have max gain. Increase voltage and the LED will become brighter, the LDR's R will get lower, thus attenuating the signal with no distortion.
 

Depending on signal level and amplifier, you might encounter clipping / distortion before you see gain reduction from reduced headroom. Especially true if constant-current bias, less true if resistor (supply proportional) bias.

Would a variable attenuator at the receiving end, be out of the question? Mux-tapped ladder, after the input termination resistor?
 

@dick_freebird "a variable attenuator at the receiving end" is not a good choice if the incoming signal is so strong it overloads the preamplifier. Attenuation should be placed as early as possible in the receiving chain.
 

@dick_freebird "a variable attenuator at the receiving end" is not a good choice if the incoming signal is so strong it overloads the preamplifier. Attenuation should be placed as early as possible in the receiving chain.
Maybe. Depends on where and how much noise / overrange is picked up along the lineup.
 

@dick_freebird You're right. Attenuating right at the input degrades noise figure. Unnoticeable with strong signals, but with medium intensity signals it's advisable to begin attenuating downstream.
 

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