Ramsey Electronics R2XL1 User Manual

Ramsey Electronics Model No. R2XL1

Have you ever found yourself with two RCA outputs on your
audio equipment and you somehow have to connect them to
XLR inputs. While wiring RCA levels directly to XLR can work,
the levels will be way too low! So what do you do?
Here is the answer to your conversion dilemma. The R2XL1 not
only converts your connection types from RCA to XLR, it also
gives you the added advantage of independent channel gain
control for the proper levels!

• Powered by a 12VAC wall power transformer for ground isolation.

• Stereo/Mono switch so you can drive a stereo device with a mono

source.

• Variable gain settings from 0.2 to 20.2 for full control with

independent left and right adjustments. Lets you get your levels
perfect!

• Low noise design fully regulated for the best sound possible.

• 10 Hz to 50 kHz audio pass-band for clear, crisp sound.

• Durable male XLR converters for heavy repeated use.

R2XL1 • 1

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R2XL1 KIT INSTRUCTION MANUAL

Ramsey Electronics publication No. MR2XL1 Rev 1.3

First printing: June 2001

COPYRIGHT 2001 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York

14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.

R2XL1 • 2

Ramsey Publication No. MR2XL1

Price $5.00

KIT ASSEMBLY

AND INSTRUCTION MANUAL FOR

R2XL1 RCA TO XLR LEVEL

CONVERTER KIT

TABLE OF CONTENTS

Introduction .......................................... 4

Circuit Description ................................. 4

Parts List ...............................................7

Schematic Diagram .............................. 8

Parts Layout Diagram ...........................9

Learn as You Build ............................. 10

Assembly ............................................12

Using the R2XL1 .................................15

Professional Audio Wiring ................... 16

Custom Case Assembly ..................... 17

Troubleshooting .................................. 18

Warranty .............................................19

R2XL1 • 3

RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone [phone]

Fax [phone]

www.ramseykits.com

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 pro­fessional 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 be­tween a standard CD player and a studio style mixer, or even run a profes­sional stereo transmitter directly from your computer!

It’s true that with the correct wiring that you can easily connect consumer au­dio devices to professional equipment. The problem however is that the result­ing 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 au­dio presented on the right input and sends it to both left and right XLR amplifi­ers when pressed in. This allows you to drive a professional stereo audio de­vice 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 ad­justable 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.

R2XL1 • 4

The next component in line is R4. This 1K resistor in combination with R3
and R5 sets the gain of the first inverting opamp stage (U1:C). An inverting
opamp will take the signal presented on its input and flip the polarity of the sig­nal on its output giving you the inverse

value of the original. In our case, the
resulting output signal from the opamp (pin 8) will be opposite in polarity to
that of the original input signal on the left hand side of R4. The amplitude of
the signal will also change depending on the gain of the circuit. The formula
for determining the gain in this circuit is given by Av = -Rf / Ri, where Ri is the
input resistance of R4 and Rf is the feedback resistance of R3 + R5 together.
If we want to find the maximum gain of this stage, we can take our 1,000 (1K)
ohm input resistor and divide its value by our 10,000 (10K) ohm pot and 100
ohm feedback resistors added together (10,100 or 10.1K ohms). This works
out to a maximum gain of -10.1 (the negative value just means it is inverted).
How does this affect our signal? Take for example a +1V input present at R4.
Multiply the input signal value by the calculated gain factor (+1V x -10.1) to
obtain -10.1 volts on the output (pin 8) of the amplifier. That wasn’t so hard
was it?

Let’s move on. The signal output from pin 8 (U1:C) then goes through two
more opamp stages (U1:A and U1:B) for processing before we’re done! One
of these stages has a familiar topology. Take a look at U1:B. It’s another in­verting opamp using resistors of R1 and R2 to set its gain. Use the formula
from before to figure out for yourself what the gain has been set for. We’ll con­firm in a moment if you are correct.

The other opamp (U1:A ) is configured as a non-inverting amplifier. A non­inverting opamp does just what it sounds like, it preserves the original signal
polarity while offering the ability to amplify the signal (increase or decrease its
amplitude). The formula for determining the gain of this configuration is given
by Av = 1+ Rf / Ri, where Ri is the input resistance (0 ohms), and Rf is the
feedback resistance (0 ohms). Since there are no input or feedback resistors,
the gain works out to be 1 for this stage. This type of circuit is commonly
called a Voltage Follower (for obvious reasons) and it acts as a buffer.

Both stages U1:A and U1:B have their gain set to 1 and –1 respectively (did
you get –1 for the gain of the inverting amplifier?). Because one opamp is in­verting and the other is not, their outputs are always opposite of one another.
The end results is a combined gain of 2 for these two opamps together. How
does that work? Let’s analyze this. If the output of our first stage (U1:C) is +1
volt (to make the math easy) and it is amplified by the inverting amplifier (U1:
B) with a gain of –1, its output will be –1 Volt. When the same 1V input is ap­plied to the non-inverting stage (U1:A) with a gain of 1, its output will be 1 Volt.
Looking across the outputs of both amps (pins 1 and 7) the difference be­tween the two works out to be 1 - (-1) = 2 Volts. Hey… that’s pretty neat!

Armed with the information that our first stage (U1:C) has a maximum gain
of 10.1 and the second stage (U1:A and U1:B) has a fixed gain of 2, we can

R2XL1 • 5

determine that the total gain of the circuit is 20.2 by multiplying the series gain
values together (10.1 x 2). This gives us plenty of control to get our low level
RCA signals up to the higher XLR requirements.

This may be more than you ever wanted to know about a preamplifier de­sign, but we aren’t done yet! The output of the two opamps in the last stage
are sent through another set of coupling capacitors that removes any DC com­ponent we may have accidentally added when we amplified the audio. It also
helps us to isolate our preamplifier from any DC present from a poorly de­signed audio component that you may plug this into. We use larger 100 uF
capacitors here because XLR specifications say we need to drive down to 560
ohm impedances. The larger capacitors help us to preserve our low frequency
information. (The left channel works exactly the same as the right so take a
look at the schematic and apply your new found knowledge.)

All that seems fine with a stereo audio source, but how do you handle one
that is Mono? The mode selection switch (S1) allows you to take the right
channel audio input and send it simultaneously through both the left and right
amplifier stages for dual outputs are the right levels!

Now on to the power supply! Since we are going to run this product from a
12VAC power transformer we will need to convert the AC supply to a DC volt­age in order for our opamps to work properly. To accomplish this, we use a
bridge rectifier consisting of D1-4 to take the AC input voltage and convert it to
positive DC pulses. This directly is too noisy for our application so C12 was
added to smooth things out. C12 acts like a battery and holds the voltage at a
more consistent level to get rid of the pulses (also called ripple). Since C12 is
a very small “battery” (it discharges almost as quickly as it charges) so some
of the pulses are still present. To eliminate these we use a voltage regulator to
completely smooth out the remaining pulses. The output of VR1 is our positive
supply and is completely smooth and ready to use.

So where do we get the negative supply from? Simple, we use a voltage
doubler scheme. This works by using a coupling capacitor C11 in a bit differ­ent fashion. Here we are isolating the DC component of the positive half of the
supply and leaving only the AC component. This AC component is then recti­fied by D5 and D6 into negative pulses. The negative pulses are then
smoothed by C13 into a negative supply source with a bit of ripple. To remove
the remaining pulses we use a negative voltage regulator (VR2) to completely
smooth it out. There you go, our negative supply!

At this point we have two regulators VR1 and VR2 with outputs of +12V
and –12V respectively. This means we have plenty of supply voltage for our
opamps to work with. Usually opamps can have maximum output of 1.4 volts
within their supply voltage so our opamps will go from +10.6V to –10.6V giving
you a 21.2 Volt total swing! More than enough to drive XLR inputs with plenty
of dynamic range. Ok, I think it is time to start building our kit!

R2XL1 • 6