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R2XL1 • 4
INTRODUCTION
Welcome to another quality kit brought to you by Ramsey Electronics, Inc.
This kit was designed to allow you to connect consumer audio devices to pro-
fessional equipment with as little hassle as possible. You would think that a
product like this was readily available, but it isn’t. Now you can easily go be-
tween a standard CD player and a studio style mixer, or even run a profes-
sional stereo transmitter directly from your computer!
It’s true that with the correct wiring that you can easily connect consumer au-
dio devices to professional equipment. The problem however is that the result-
ing levels are way too low! The R2XL1 provides plenty of gain so that you can
bring consumer levels of 0.7V RMS up to the required 1.228V RMS used by
studio style devices. This required a gain of 1.228 / .7 or 1.75 to achieve. We
expanded that by over 10 times giving you an adjustable gain range of 0.2 to
20.2! This gives you plenty of control to get those levels just right (especially
when coming from a computer or portable CD player).
A very simple circuit in principle, the R2XL1 uses two quad FET input
opamps to get the needed gain as well as giving you the required +- balanced
outputs to drive low impedance XLR inputs.
Another nice feature we added was the Stereo/Mono switch. It takes the au-
dio presented on the right input and sends it to both left and right XLR amplifi-
ers when pressed in. This allows you to drive a professional stereo audio de-
vice from a monaural audio source with a press of a button.
CIRCUIT DESCRIPTION
The R2XL1 is an audio pre-amplifier that offers the user a wide range of ad-
justable output levels. To analyze this circuit we will first follow the signal path
from input to output and then take a close look at the power supply section.
The power supply performs a nifty trick in order to get an additional supply rail
from a single power source so we’ll take the time to discuss it.
Looking at J1, the dual RCA connector. Notice that the signal first goes
through a couple of 10 uF capacitors. These capacitors are called coupling
caps because they filter out any DC on the line while letting the AC signal (the
music) pass through. Removing any residual DC component that might be
present makes sure there will be no interference with the first amplification
stages. For now we will only look at the top half of the circuit which handles
the right channel audio path.
Following the first coupling capacitor we see R13, a 10K resistor to ground.
This serves the purpose of making sure the opamp stage side of the preamp
will always be at the ground potential. This is a common practice that helps to
reduce the likelihood of power pops and switching transients when you plug
and unplug your audio cables.