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Here’s a photo of a fully assembled FAB3110 Output Transformer Module.
You can use any API-type output transformer on this module. Shown here is an EA transformer, but you can also use transformers from Cinemag, Crimson Audio, etc…
There is a Logic control on the FAB module, allowing you to switch your transformer ratio from 1:1 to 1:2 (+6dB gain). Just apply any +3.3V, +5V or V+ voltage on this logic pin.
The FAB4312 is a standard Non-Inverting Amplifier using OpAmps, accommodating both the PDIP8 IC format, and the API 2520-footprint Discrete OpAmp format.
If you’ll be installing a Discrete OpAmp, MillMax sockets are required to be installed on the board. An external connection, RGAIN, is required to control the gain of this amplifier. One typically uses a 10K or 22K Reverse Log Potentiometer for RGAIN in this circuit configuration.
Feel free to change the values of RF, RGAIN and R3 to suit your purpose. The formula for Voltage Gain for the amplifier is conveniently printed on the FAB Module.
For CF, we’re using a 100pf C0G NPO ceramic capacitor. You may have to adjust this capacitor value to suit the OpAmp you’ll be using in case you encounter instability or oscillations.
In addition, a trimmer resistor (R2) is provided to let you adjust DC Offset Voltage as close as possible to 0 Volts. By adjusting the DC offset to as little as possible, you reduce clicking noise when changing gain settings. This may not be an issue if you’ll be using a potentiometer for RGAIN, but if you’ll be using a selector switch to vary the value of RGAIN, then you’d want your DC offset as small as possible.
The FAB3110 – Output Transformer Module is designed for PCB mounting of typical output transformers like the Cinemag CMOQ-2S and compatibles. These type of transformers are commonly used in “API-type/style” preamps.
You typically connect the FAB3110 Output Transformer Module after your amplifying module (FAB4000 series modules). That way, your processed/amplified signal chain will go first through the output transformer.
A FAB3010 XLR Ouput Module could then be connected after the FAB3110 Output Transformer Module.