Ramsey Electronics Model No. STC1
Now you can give your home stereo transmitter all of the features
found in a professional radio station! Control and prevent
overmodulation, interference from high frequency signals such as
TVs and older CD players, and “sweeten” the mix with Bass,
Presence, and Brilliance. A perfect companion for the FM10A and
the FM25 or any other FM radio transmitter!
• Gives your station that professional sound with real “Punch!”
• Ideal companion for FM10A and FM25
• Requires any DC voltage from 9 to 12VDC
• Uses line levels from CD players, tape decks, mixers etc.
• Bass, Presence, and Brilliance controls
• Dynamic range limiter to prevent overmodulation.
• 15kHz 8th order butterworth low pass filters on each output to
prevent high frequency interference with the stereo carrier.
• Clear, concise assembly instructions lead you to a finished product
that works FIRST time!
• Add our case and knob set for a finished ‘Pro’ look. Cases match all
Ramsey products.
STC1• 2
RAMSEY TRANSMITTER KITS
• FM10A, FM25B FM Stereo Transmitters
• TV6 Television Transmitter
• FM100B Super Pro FM Stereo 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
• SP1 Speakerphone
• MD3 Microwave Motion Detector
• PH14,15,16 Peak hold Meters, great for VU meters!
• LC1 Inductance-Capacitance Meter
• TFM3 Tri-Field Meter
• ECG1 Heart Monitor
• LABC1 Lead Acid Battery Charger
RAMSEY AMATEUR RADIO KITS
• DDF1 Doppler Direction Finder
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• VLF1 Low Bander Low Frequency SWL Converter
• 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.
STC1 STEREO TRANSMITTER COMPANION INSTRUCTION MANUAL
Ramsey Electronics publication No. MSTC1 Revision 1.3
First printing: Jan. 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.
STC1• 3
STC1 STEREO TRANSMITTER
COMPANION
Ramsey Publication No. STC1
Price $5.00
TABLE OF CONTENTS
Introduction .................................... 4
How Does It Work? ........................ 5
Learn As You Build ........................ 7
Parts List ........................................ 8
Construction ................................. 10
Schematic Diagram...................... 14
Setup And Testing........................ 21
Using The STC1........................... 22
Parts Layout Diagram .................. 24
Parts Value Diagram.................... 25
Troubleshooting ........................... 26
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
STC1• 4
INTRODUCTION TO THE STEREO TRANSMITTER COMPANION
Knowing how troublesome different audio sources can be when trying to
transmit them over the air, we have come up with a product that will eliminate
many of the problems. This kit allows the FM10A and the FM25 stereo
transmitters to have the same quality audio and transmission that the
professional radio stations have. The STC1 has many of the same features the
professional sound processors have, and will be sure to please your listening
audience when used with your home brew transmitter.
If you have experienced a steady whine while trying to transmit a television
signal or audio from an older CD player, this kit will help to reduce the problem.
The cause of this problem is the horizontal sweep frequency in a television, or
the 44kHz sampling frequency of the CD player mixing with the 38kHz stereo
carrier. What you are hearing is the sum and the difference frequencies that are
the result of the mixing process. The same occurs with high frequency audio
such as cymbals and when someone says the letter ‘s’.
To help control this problem, this kit incorporates a 16kHz 8th order Butterworth
low-pass filter. This helps to eliminate interference with high frequencies, such
as cheap CD players and other high frequency interference. Some of you may
ask what 8th order means? Well, put simply it means a steep cutoff curve
above 16kHz. The more orders in a filter, the greater the slope of the cutoff
curve. Butterworth is the name given to the description of the curve’s shape. A
Butterworth curve means there are no dips or peaks in the audio response
before the cutoff point’s frequency. This means there is no “coloration” added to
the audio being processed in the circuit.
To prevent overmodulation in your transmitter, this kit incorporates a soft
limiter. This limiter prevents the audio from going over a certain set level. Since
the audio is limited, so will be your modulation. There are indicator LEDs
included so that when the limiter is activated, you will know. When an audio
signal is limited, it introduces distortion into the audio causing the sound to
become rather distorted. The idea is to run your audio just under the limiting
level so that the LEDs blink very rarely. It also is a good way to keep all of your
audio levels close to the same for sound volume consistency.
To add a little more functionality to the STC1, and make it difficult for the
engineer to stuff all the parts on a small, single-sided board with a very few
jumpers, a set of tone controls were added in. This allows you to compensate
for a lack or surplus of bass, treble or midrange before the sound is transmitted.
Some people don’t like a nice flat response in their sound, they like the bass
turned up to give a nice full sound to their listeners. Needless to say these tone
controls allow you to custom tailor your sound to your own tastes.
STC1• 5
HOW DOES IT WORK?
To help you understand where we are in the circuit you will want to look at the
circuit diagram in the center of the manual. We will work from the input of the
left channel to the output of the left channel. We don’t need to look at the right
channel since it is identical to the left half.
The audio signal coming into J1 is a line level signal at about 1 volt peak-topeak, which is a 0dB line level signal (standard). The audio level is cut in half
by resistors R63 and R28. This is necessary due to the lower voltages supplied
in the circuit. This keeps the signal level well inside the operating range of the
opamps later in the circuit to prevent distortion.
The signal then enters into U1:A and surrounding capacitors and resistors. This
is an active low pass filter to prevent any signals from outside the audio range
from entering into our circuit. This keeps high frequency signals from interfering
with our special filters later in the circuit.
The low pass filter output is then fed into what appears to be three feedback
loops in an opamp circuit. Each branch is a different type of filter. One for low
pass, one for midrange, and one for high pass. Yes, you have it, it’s the tone
controls for the left channel of the circuit. This arrangement allows us to boost
or cut frequencies from +/-12dB in three bands.
The output of our tone controls are then fed to the limiter circuit. This is where a
little magic comes into play when you see the two diodes in the feedback path
of the opamp. As you may know, diodes only conduct in one direction, but what
you may not know is that it takes about .5 to .7 volts to begin to conduct in the
forward direction. This is called the forward voltage drop of the diode. This .7
volt drop is used as our limiting voltage reference.
During signal levels under .5 volts peak-to-peak on the output pin 1 of U2:A ,
U2:A acts like a simple inverting amplifier with its gain controlled by R23, R18,
and the feedback resistor of R15. For example if we had R23 set at 7.8K ohms,
the gain of the circuit would be -1 in these signal conditions. The change comes
with signal levels over .5 volts. The diodes D1 and D2 begin to conduct on either positive or negative going signals since they are connected in opposite
directions. This allows R51 to come into the gain equation. When the diodes
are on the gain lowers dramatically. In this case the gain goes to less than -.1.
Now the gain is 1/10 of what it used to be, and it prevents the output of this
stage from going much over .7 volts peak-to-peak.
The output of this limiter stage is then fed to two different stages, one of which
is the clip detector. This clip detector is set to detect any signal over .6 volts
peak-to-peak at the output of the limiter, which is where significant sound distortion begins. D6, C4, R55, and R58 make up a simple peak hold circuit by
rectifying the AC audio output of the limiter into a DC level related to volume.
STC1• 6
When this voltage on pin 5 of U2:B goes higher than 5 volts of the supply on pin
6 of U2:B, the output of U2:B goes high, thus lighting the LED clip indicator.
The other part of the signal goes to U3, a switched capacitor lowpass 8th order
Butterworth filter. These filters are really neat since they don’t need any high
accuracy frequency dependent parts, and don’t require pancakes. All that is
needed is a good steady TTL clock signal to set the cutoff frequency. Internally
these chips have a divide by 100 cutoff in relation to the clock frequency, so to
get a cutoff frequency of 15KHz, we needed a clock frequency of 1.5MHz.
Since 555 timers don’t like to run at this speed, a stable source was needed
that was better than a CMOS oscillator. Well, a little overkill never hurt anyone,
so now there is a crystal oscillator running at 6MHz, and divided by 4 by U6:A
and U6:B to give us 1.5MHz. Now we have a cutoff frequency right at 15kHz
with very little drifting in frequency.
There is also an internal opamp inside of U3, which allows us to filter the clock
frequency out of the audio signal before going out to J2 and then on to your
transmitter. R14,16,17, C9, and C16 are the parts included for this filter (Notice
similar part values around U1:A)
Well, that about sums it all up. Now we will get on to the fun stuff, and make
ourselves a really great kit that we understand!
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.
STC1• 7
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.
Mount all electrical parts on the top side of the board provided. This is the side
that has little or no traces on it. 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).
STC1• 8
RAMSEY STC1 PARTS LIST
Semiconductors
2 LF347 Quad Op-Amps (U1,2)
1 74HC74 Dual type ‘D’ flip-flops (U6)
2 MAX291 8th order Butterworth Switched Capacitor Filters (U3,5)
6 1N4148 small signal diodes (small glass body with black stripe)
(D1,2,3,4,6,8)
1 4.9 to 5.1 volt zener diode (small black or grey body with stripe on one
end) (D5)
2 7805 three terminal regulators (marked 78L05) (VR1,2)
1 2N3904 NPN transistor (Q1)
2 LEDs (D7,9)
Resistors
1 100 ohm resistor (brown-black-brown) (R47)
5 270 ohm resistors (red-violet-brown) (R40,56,57,60,61)
8 1K ohm resistors (brown-black-red) (R19,43,44,46,48,50,51,54)
4 1.8K ohm resistors (brown-gray-red) (R11,12,36,37)
2 2.2K ohm resistors (red-red-red) (R18,27)
4 3.3K ohm resistors (orange-orange-red) (R8,9,31,32)
6 4.7K ohm resistors (yellow-violet-red) (R28,35,38,39,63,64)
12 10K ohm resistors (brown-black-orange) (R4,5,7,15,24,25,26,30,41,
42,52,53)
8 15K ohm resistors (brown-green-orange) (R1,2,14,17,20,21,29,34)
6 47K ohm resistors (yellow-violet-orange) (R3,16,22,33,55,59)
1 100K ohm resistor (brown-black-yellow) (R45)
2 4.7M ohm resistors (yellow-violet-green) (R58,62)
STC1• 9
Capacitors
5 100pF ceramic capacitors (marked 100, or 101) (C1,9,18,27,45)
1 470pF ceramic capacitor (marked 470 or 471) (C46)
4 .001uF ceramic capacitors (marked .001, 102, or 1n) (C6,16,25,32)
4 .0047uF or .005uF ceramic capacitors (marked .0047, 472, .005, or 502)
(C7,10,29,34)
2 .01uF ceramic capacitors (marked .01, 103, or 10n) (C38,42)
1 .1uF ceramic capacitor (no reference designator)
2 .022uF or .02uF ceramic capacitors (marked .022, 223, .02 or 203)
(C8,C31)
2 .047uF or .05uF ceramic capacitors (marked .047, 473, .05 or 503)
(C2,20)
1 .1uF ceramic capacitors (marked .1, 104, or 100n) (C44)
15 10uF electrolytic capacitors (marked 10uF) (C3,4,5,11,12,13,21,23,
26,28,35,37,40,41,43)
2 100uF electrolytic capacitors (marked 100uF) (C36,39)
Miscellaneous
1 DPDT power switch (S1)
1 Power jack (J5)
4 RCA jacks (J1,2,3,4)
2 100K trimmer pots (orange tops marked 104) (R23,49)
2 100K dual ganged potentiometers (R6,10)
1 500K dual ganged potentiometers (R13)
1 6.00MHz crystal (marked 6.000) (X1)
1 12” piece of hookup wire.
Loading...