Ramsey Electronics MX10 User guide

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Ramsey Electronics Model No. MX10
Now here’s the simple easy to use mixer for all of you home
brew DJs out there! This mixer has all of the features you will
ever need to mix audio as the professionals do - in a small
compact size! Perfect for DJing on the desktop transmitter,
making car tapes, mixing for a wedding or anywhere else a
mixer could be needed.
•2 Line inputs, 1 phono and 2 mic inputs, all most people will
ever need!
•Overload protection on the microphones input to “soften”
distortion
•LED peak hold meters for accurate audio levels
•Line output for driving amplifiers or transmitters such as the
FM-25
•Slider pots control Main, Line1, Line2, panning and phono.
•Small size with BIG audio quality!
•Runs from 12VAC wall adapter

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RAMSEY TRANSMITTER KITS
• FM10A, FM25B FM Stereo Transmitters
• AM1, AM25 AM Transmitters
• TV6 Television Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Short-wave Receiver
• AA7 Active Antenna
• SC1 Short-wave Converter
RAMSEY HOBBY KITS
• SP-1 Speakerphone
• AVS10 Automatic Sequential Video Switcher
• WCT20 Cable Wizard Cable Tracer
• LABC1 Lead Acid Battery Charger
• ECG1 Heart Monitor
• BS1 “Bullshooter” Digital Voice Storage Unit
• AVS10 Automatic Sequential Video Switcher
• WCT20 Cable Wizard Cable Tracer
• TFM3 Tri-Field Meter
RAMSEY AMATEUR RADIO KITS
• DDF1 Doppler Direction Finder
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• 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.
MX10 STEREO MIXER INSTRUCTION MANUAL
Ramsey Electronics publication No. MMX10 Revision 1.0
First printing: May. 1996 MRW
COPYRIGHT 1996 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.

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MX10 STEREO THREE
INPUT MIXER
Ramsey Publication No. MMX10
Price $5.00
Table of Contents
Introduction ....................................4
Circuit Description...........................5
MX10 Parts List ............................10
Assembly Procedure.....................11
Schematic Diagram ......................16
Initial Testing.................................23
Troubleshooting ............................24
Layout Diagrams.................26,27,28
Using the MX10 ............................29
Warranty .......................................31
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
RAMSEY ELECTRONICS, INC.
590 Fishers Station Drive
Victor, New York 14564
Phone [phone]
Fax [phone]

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INTRODUCTION TO THE MX10
Seeing the need for a high fidelity low noise mixer to be used with our
micropower transmitters, we came up with this easy to build compact mixer.
The small size lends to the small size of the transmitter for portability. You can
easily fit an entire radio station inside of a briefcase, just try that with electronics
of just a few decades ago! While designed to go with our micropower
transmitters, the MX10 will work as well, if not better than most commercial
mixers on the market today.
Mixers are used in all sorts of audio applications such as rock concerts, DJ
booths in night clubs, weddings, 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 MX10
has two inputs for line levels which may include a CD player and a tape deck, it
also has one stereo input for a phono with RIAA equalization (boosts bass
response of records). The MX10 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.
We also included more than one microphone input on the MX10 which allows
two people to talk at the same time. When one microphone is plugged in to the
left channel mic, it exits the mixer in both channels. When both mics are
plugged in, the left one comes out the left channel, while the right one comes
out the right channel. This leads to a nice effect on a transmitter for a listener to
visualize the DJs.
The MX10 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. Now you can give those
professionals a run for their money. We hope you enjoy building and using the
MX10, we sure did at Ramsey!

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There are three inputs to this amplifier. The 10K ohm resistors determine how
much gain each of the inputs has, and the 33K resistor sets the overall gain. To
find the gain of each branch, there is a simple equation:
RF = 33K and Rin is any one of the three branch resistors.
In 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 voltage by the gain so 1*3.3 = 3.3V P/P out.
R9, R22, R14, and R58 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.
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 50mV, 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 around 20. In our case we have chosen an
non-inverting 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
of 20, R21 needed to be about 1/20th of that value or
about 10K. (Notice the values around U5:A)
MX10 CIRCUIT DESCRIPTION
We will use the schematic diagram to step through the circuit and find out what
makes it “tick”. There are plenty of different devices performing different tasks
in the MX10. The most common active component is the opamps. These are
used in a number of ways in the MX10, one of which is mixing.
To perform mixing, an opamp is set up in what is called a summing amplifier.
This amplifier does exactly what it says. It takes all of its inputs and adds them
together on the output. Following is an example of a summing amplifier in a
mixer, similar to the ones in the MX10.
in
f
bracnch R
R
A=
21
23
1R
R
Amic +=

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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 if the forward
bias direction before they turn on. On signals under .7V P/P, the gain of the
second stage is determined by Ri of 10K (R28) and Rf of 10K (R27). This gives
us a gain of 1. But if our signal becomes greater than .7V P/P, then the diodes
D1 and D2 begin to turn on. This brings R30 into the gain equation as well.
Now you have an Rf = R27 in parallel with R30. 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.
U6:A and U6:B are both RIAA equalization circuits. This compensates for the
peculiar frequency response you get on records due to the way vinyl and the
phonograph needle interact. This circuit boosts the bass and reduces the treble
to give a level response throughout the hearing range. The nice part of this
circuit is that it is easy to modify into a line input instead of a record input. This
is described later in the manual.
U4:D and U4:C are the summing amplifiers. These take the audio from the line
level inputs, the phono inputs and the two microphones and adds them all
together. R26 then adjusts the summed outputs to the peak hold meters, the
earphones, and the output jack.
The earphone amplifiers consisting of U3 and U2 are set up as summing
amplifiers as well. Each branch of the summing amplifier is connected the same
as the ones in U4. When you switch between CUE and play, the signal is just
redirected from the earphone summing circuits to the line level summing
RIAA Equalization
-40
-30
-20
-10
0
10
10
15.84
9
25.11
9
39.81
1
63.09
6
100
158.4
9
251.1
9
398.1
1
630.9
6
100
0
1584.
9
2511.
9
3981.
1
6309.
6
1000
0
1584
9
Frequency (Hz)
Gain (dB)

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circuits. The earphone amplifiers LM386s have quite a bit of kick to them, which
should be more than enough to out power most DJ systems.
The peak hold meters serve the function of displaying peak amplitudes in your
audio so your eye can see them. Normally a short duration pulse such as a
drum beat or strum of a guitar has a very short time duration for the initial
higher power sound. If we didn’t use a peak hold circuit, your eye would be
hard pressed to see the LEDs blink on the bargraph, or in the case of an analog
meter, the mechanical weight holds back the needle from going to the actual
peak.
The peak hold circuit consisting U4:A and U4:B combined with D6, R43, C14,
D19, R60, and C17 takes an instantaneous peak and stores it in the two 10uF
capacitors. The resistors then discharge the capacitors at a slow rate. These
capacitors and resistors extend the peak long enough in duration so that your
eye can see it. The function of the diode is to DC rectify the incoming audio
signals since we only are interested in the positive going signals. If we did not
DC rectify the signal, we would constantly positively charge the capacitor on a
positive pulse, then negatively charge it on a negative pulse. The net result
would be a meter that just barely twinkled at you. Here is an example of what a
peak hold circuit does for you. The dark gray is the original signal, and the light
gray is the peak hold signal. As you can see the light gray stays at a higher
amplitude for a longer period of time.
The power supply for the MX10 is a little different than you may have expected.
12VAC is plugged into J8, the power jack. The 12VAC is then rectified to DC
using diodes D29, 31, 34, and 35. This is then “smoothed” out using C30, which
averages out most of the bumps and lumps out of the power supply noises.
VR1, a voltage regulator takes out the rest of the bumps and lumps, and gives
us a clean smooth +12 VDC power source with very little noise. U8:C is then
set up to give us a buffered, regulated split supply voltage. The output of this
opamp is now used to generate a ground, while the supply voltages are now +5
and -5 volts (closer to +6 and -6) in reference to this “ground”. This method is
an easy way to get a split supply from a single supply such as ours.
TIME
AMPLITUDE

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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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As with all Ramsey kits, we want to mount the parts AS LOW AS POSSIBLE to
the board. A 1/4” lead length on a resistor not mounted close to the board can
act as an inductor or an antenna, causing all sorts of problems in your circuit.
Be aware though that if there are stand up components in your circuit, they
don’t need to be squished to the board. Keep the portion of the resistor closest
to the board mounted right on the board.
For each part, our word "Install" always means these steps:
❒1. Pick the correct part value to start with.
❒2. Insert it into the correct PC board location, making sure the part is
mounted flush to the PC board unless otherwise noted.
❒3. Orient it correctly, follow the PC board drawing and the written directions
for all parts - especially when there's a right way and a wrong way to solder
it in. (Diode bands, electrolytic capacitor polarity, transistor shapes, dotted
or notched ends of IC's, and so forth.)
❒4. Solder all connections unless directed otherwise. Use enough heat and
solder flow for clean, shiny, completed connections.
Keeping this in mind, let’s begin by sorting out our components and cross-
checking them against the parts list to make sure we have received everything.
NOTE TO NEWCOMERS: If you are a first time kit builder you may find this
manual easier to understand than you may have expected. Each part in the kit
is checked off as you go, while a detailed description of each part is given. If
you follow each step in the manual in order, and practice good soldering and kit
building skills, the kit is next to fail-safe. If a problem does occur, the manual
will lead you through step by step in the troubleshooting guide until you find the
problem and are able to correct it.

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RAMSEY MX10 PARTS LIST
Semiconductors
❒3 LF347 Dual operational amplifiers (U4,5,6)
❒2 LM386 Audio power amplifiers (U2,3)
❒2 LM3915 LED bargraph drivers (U7,8)
❒4 1N4002 type diodes (black epoxy body with white stripe)(D29,31,34,35)
❒6 1N4148 type diodes (small glass body with black stripe)(D1,2,5,6,8,19)
❒10 Small yellow LEDs (D3,10,11,12,13,14,22,23,24,25)
❒6 Small green LEDs (D4,7,9,18,20,21)
❒6 Small red LEDs (D15,16,17,26,27,28)
❒1 7808 +8 volt regulator (VR1)
Resistors
❒30 10K ohm resistors (brown-black-orange) R1,2,3,4,7,8,16,17,20,21,
27,28,33,34,36,37,39,45,46,48,49,51,52, 54,61,63,76,77,82,84).
❒12 100K ohm resistors (brown-black-yellow) R29,38,43,50,55,60,62,65,
70,71,80,85)
❒2 220K ohm resistors (red-red-yellow) (R32,44).
❒10 1K ohm resistors (brown-black-red) (R30,35,41,42,47,59,81,83,92,93)
❒4 1M ohm resistors (brown-black-green) (R53,56,64,66)
❒2 2 ohm resistors (red-black-gold) (R68,69)
❒2 180 ohm resistors (brown-gray-brown) (R57,67)
❒2 68K ohm resistors (blue-gray-orange) (R88,89)
Capacitors
❒18 10uF electrolytic capacitors (C1,2,4,6,7,9,10,11,14,15,17,18,20,21,
24,25,26,36)
❒1 470uF electrolytic capacitor (C30)
❒3 220uF electrolytic capacitors (C3,31,33)
❒7 .1uF ceramic capacitors (marked .1 or 104) (C12,13,28,29,32,34,35)
❒2 .0033uF ceramic capacitors (marked .0033 or 332) (C16,22)
❒2 .001uF ceramic capacitors (marked .001 or 102) (C19,27)
❒3 .01uF ceramic capacitors (marked .01, 10n, or 103) (C5,8,23)
Controls
❒5 10K ohm slider potentiometers (R9,14,22,26,58)
❒4 10K ohm top mount potentiometers (R6,13,31,40)
❒6 DPDT top board mount slide switches (S1,2,3,4,5,6)
Miscellaneous
❒2 Microphone jacks (3/8”) (J5,6)
❒1 2.5mm Power jack (J8)
❒5 3.5mm stereo jacks (J1,2,3,4,7)
❒1 6 foot grounding wire