The FAB4300 Module is a Unity Buffer Amplifier Module, utilizing either a PDIP8 chip, or a Discrete OpAmp.

It has a single-ended input, and a single-ended output. The output of the OpAmp is connected back to it’s inverting input (IN-), while the input signal is applied to the non-inverting input (IN+).  The output of the OpAmp basically follows whatever is the input signal applied to IN+, thus  it’s named a voltage follower.

While the FAB4300 does not offer any amplification to the input signal, it serves a very important function. The input impedance of the FAB4300 Module is very high, meaning that the FAB4300 does not load down the signal source. At the same token, the output impedance of the FAB4300 Module is very low and thus it can drive loads very efficiently without any apparent loss in signal. You can say the FAB4300 acts like an impedance bridging circuit.

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This is our FAB4312 Non-Inverting Amplifier Module.  You can use any single opamp, 8-pin PDIP package like an LME49710, NE5534 or equivalent. It also features Mill-max sockets if you want to mount a 2520-style discrete opamp, like our DOA-12, DOA-17, 990C, API 2520, or compatible discrete opamp.

A trimmer is also provided to let you adjust DC offset to zero volts (0V). A 2-pin header is provided for connection of your gain control potentiometer (or selector switch, or digital potentiometer, or switched resistor ladder network).  In the future, we’ll be providing FAB modules that will offer these different options for controlling the gain of your amplifier.

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The DOA-12 is a high-performance, discrete operational amplifier well suited for demanding audio applications due to it’s low noise, high slew rate, high gain capability, high output capability, and operation over a wide range of power supply voltages. It uses the standard discrete opamp 2520/990 footprint… a roughly 1”x1” device with (6) pins, a format commonly found and used in pro-audio gear.

Running at +/-24Volts supply voltage, the DOA-12 is capable of reaching +26dBu output (loaded with a 600:600 output transformer). Running at a slightly lower +/-18Volts supply voltage, the DOA-12 still achieves an impressive +24dBu maximum output.

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The DOA-17 is our modern remake of a vintage discrete op amp design, using modern low-noise components and improved layout. It uses the standard discrete opamp 2520/990 footprint… a roughly 1”x1” device with (6) pins, a format commonly found and used in pro-audio gear.

Running at +/-18Volts supply voltage, the DOA-12 is capable of reaching +23dBu output (loaded with a 600:600 output transformer). If one uses the DOA-17 to drive an output transformer wired for 1:2, it is possible to create high-performance preamplifiers capable of reaching +29dBu output!

Each DOA-17 is designed and tested to be stable, and oscillation-free even at maximum output levels and gain settings due to careful selection of components, efficient PCB layout, design, and construction.

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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.

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