Ramsey Electronics MX-5 Assembly And Instruction Manual

Ramsey Electronics Model No. MX-5

Now here’s the simple easy to use mixer for all of you home
brew DJs out there! This mixer has two line inputs and one
microphone so that you can hook up all of your audio
sources to any transmitter or audio amplifier. Perfect for
mixing those car tapes before the big trip!

• 2 Line inputs and one mic input, all most people will ever

need!

• Overload protection on the microphone input to “soften”

distortion

• Requires regulated 9 to 15VDC supply or battery.

• Line output for driving amplifiers or transmitters such as the

FM-10a

• Long slider pots for accurate mixes every time!

• Nice small extruded case for portability and space saving

MX-5 Page 1

RAMSEY TRANSMITTER KITS

• FM-10a,25 FM Stereo Transmitters

• FM-1,2,4,5,6 FM Wireless Microphones

• PB-1 Telephone Transmitter

RAMSEY RECEIVER KITS

• FR-1 FM Broadcast Receiver

• AR-1 Aircraft Band Receiver

• SR-1 Short-wave Receiver

• AA-7 Active Antenna

• SC-1 Short-wave Converter

RAMSEY HOBBY KITS

• ASA-602 2x60 Watt RMS Car Amp

• ASA-604 4x60 Watt RMS Car Amp

• SF-2 Super Crossover Filter

• SP-1 Speakerphone

• MX-5,10 Stereo Mixers

• PH-14,15,16 Peak hold Meters, great
for VU meters!

• LC-1 Inductance-Capacitance Meter

RAMSEY AMATEUR RADIO KITS

• FX Series VHF and UHF Transceivers

• HR Series HF All Mode Receivers

• QRP Series HF CW Transmitters

• CW-700 Micro Memory CW Keyer

• PA Series VHF and UHF Power Amplifiers

• Packet Computer Interfaces

• QRP Power Amplifiers

RAMSEY MINI-KITS
Many other kits are available for hobby, school, Scouts and just plain FUN.
New kits are always under development. Write or call for our free Ramsey
catalog.

MX-5 STEREO MIXER INSTRUCTION MANUAL

Ramsey Electronics publication No. MMX-5 Revision 1.0

First printing: May. 1996 MRW

COPYRIGHT 1996 by Ramsey Electronics, Inc. 793 Canning Parkway, 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.

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Ramsey Publication No. MMX-5

Price $5.00

KIT ASSEMBLY

AND INSTRUCTION MANUAL FOR

MX-5 STEREO THREE

INPUT MIXER

TABLE OF CONTENTS

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

How Does It Work? ........................ 5

Learn As You Build ........................ 7

Parts List ........................................ 9

Construction ................................. 10

Schematic Diagram ...................... 12

Setup And Testing ....................... 16

Troubleshooting ........................... 17

Parts Value Diagram .................... 18

Parts Layout Diagram .................. 19

Using the MX-5 ............................ 21

RAMSEY ELECTRONICS, INC.

793 Canning Parkway

Victor, New York 14564

Phone [phone]

Fax [phone]

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INTRODUCTION TO THE MX-5

Upon designing a more complex version of this kit, the MX-10, we determined
that not everyone needed all of the features in the world, so we came up with
this simple design. This is a high quality low-noise mixer capable of handling
two stereo line inputs and one mono microphone input. This gives most users
all the inputs that they will ever need for making car tapes, DJing for a wedding,
using with our stereo transmitter kits etc.

Mixers are used in all sorts of audio applications such as rock concerts, DJ
booths in bars, radio stations, recording studios and so on. If you've ever
looked at a mixer board for a concert, you would notice that they have upwards
of fifty or so channels, and seven or more controls for each channel. Each
channel has reverb, tone controls, delay, and more. Our MX-5 has only two
inputs for the line level which may include a CD player and a tape deck. The
MX-5 does not have any tonal controls for each channel, but almost all users of
this project will want a flat response anyhow. If they do not, they will run the
output of the mixer directly into an equalizer to custom-tailor their sound. For
example when a person makes a car tape, they may want to boost the bass to
get over road noise.

The MX-5 was designed for simplicity, low noise, and low cost. Its small size
makes it easy to carry around and store. Its long travel on the controls make it
easy to mix audio signals smoothly and accurately. We hope you enjoy building
and using the MX-5, we sure did at Ramsey!

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MX-5 CIRCUIT DESCRIPTION

We will use the schematic diagram to step through the circuit and find out what
makes it “tick”. As you can see, there is only one IC in the entire circuit, but
there are four individual operational amplifiers in the one IC. U1:A and U1:B are
the main components of the mixer. They are set up in what is called a summing
amplifier.

If we were to look at the left channel only,
we can see what one of the summing
amplifiers looks like. The three inputs to this
amplifier are seen as R1, R9, and R10.
These resistors determine how much gain
each of the inputs has, in this case they are
all equal. To find the gain of each branch,
there is a simple equation:

RF = R4 and Rin is any one of the three branch resistors. In

R

f

A =

branch

R3 and R13 determine how much line level signal is actually sent to each
branch of the summing amplifier by using slider control potentiometers. So for
the circuit up to this point to have a gain of one, the control will be set at 1/3 of
its full scale setting.

this case the gain is 33K/10K = 3.3. To find the output level
with a given input level such as 1V P/P, multiply the input

R

in

voltage by the gain so 1*3.3 = 3.3V P/P out.

The microphone amplifier is slightly different from what you may have
expected. Not only does it have gain, but it also has two diodes in the feedback
of one of the amplifiers. First we will talk about the gain of the circuit. A typical
microphone when talked into will have an output of about 100mV, while line
level audio has a level about 1V. To get the microphone signal up to line level,
we will need an amplifier with a gain of 10. In our case we have chosen an non­inverting amplifier for the job. A non-inverting amplifier has the nice feature of a
very high input impedance. This prevents loading on some high impedance
microphones which causes poor sensitivity. To find the gain of a non-inverting
amplifier, the equation is as follows:

In our case we chose R23 at 220K ohms, so for a gain

R

23

A

mic

The next stage of the microphone amplifier has two diodes in the feedback
loop. What are these for? you may ask. Well they are called clipping diodes.
Diodes have a property of needing about .7 volts across them before they turn
on. On signals under .7V P/P, the gain of the second stage is determined by Ri
of 10K (R18) and Rf of 10K (R16). This gives us a gain of 1. But if our signal

1

+=

of 10, R21 needed to be about 1/10th of that value or

24.4K. Since that is not a standard value we used a

R

21

22K ohm resistor which is close enough. (A gain of 11)

MX-5 Page 5

becomes greater than .7V P/P, then the diodes D1 and D2 begin to turn on.
This brings R14 into the gain equation as well. Now you have an Rf = R14 in
parallel with R16. This brings the overall gain down to less than .1, now the
amplifier is working as an attenuator. The best part of the diodes is that they
don’t just “snap” on, they have some variance before they are on fully, so this
creates what is called “soft clipping”. This soft clipping is a close relative of
distortion, but much more tolerable. This soft clipping circuit prevents a person
from overloading amplifiers or speakers by preventing high volume levels from
exiting the mixer.

The output from the microphone amplifier is then mixed in with the line level
audio as well. Before exiting the MX-5 though, the output of U1:A and U1:B is
controlled by the master level control, R6.

The power supply for the MX-5 is simple enough, it consists of a 9-18VDC
input or battery, and a voltage divider to provide for a split supply. The spilt
supply is necessary to operate op-amps as we have done. C9 is used to filter
out most of the noises that may be present in your power supply since they
can infiltrate the audio.

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RAMSEY “LEARN-AS-YOU-BUILD” ASSEMBLY STRATEGY

Be sure to read through all of the steps, and check the boxes as you go to be
sure you didn't miss any important steps. Although you may be in a hurry to see
results, before you switch on the power check all wiring and capacitors for
proper orientation. Also check the board for any possible solder shorts, and/or
cold solder joints. All of these mistakes could have detrimental effects on your
kit - not to mention your ego!

Kit building tips:

Use a good soldering technique - let your soldering iron tip gently heat the
traces to which you are soldering, heating both wires and pads simultaneously.
Apply the solder on the iron and the pad when the pad is hot enough to melt the
solder. The finished joint should look like a drop of water on paper, somewhat
soaked in.

The boards for the MX series of kit have components on both sides of the
board, but there is a top side that we put most of the components on. This is
the side that has little or no traces on it, but is covered with mostly copper.
When parts are installed, the part is placed flat to the board, and the leads are
bent on the backside of the board to prevent the part from falling out before
soldering (1). The part is then soldered securely to the board (2-4), and the
remaining lead length is then clipped off (5). Notice how the solder joint looks
on close up, clean and smooth with no holes or sharp points (6).

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