Ramsey Electronics PPM3 User Manual

PHONE PATCH

MIXER KIT

Ramsey Electronics Model No. PPM3

Ever wish you could broadcast on-the-air telephone calls like
the commercial radio stations? Well now you can! The PPM3
gives you the ability to broadcast both sides of a phone
conversation without having to change any connections to
return to your music or other programming. Perfect for the
budding talk radio star in your house!

• Operates on 8 to 12 volts DC

• Automatic noise gating

• Automatic variable gain and compression

• Allows both sides of a phone conversation to be broadcast over-

the-air

• Connect your line level audio source to the PPM3 and use the

loop output; no separate mixing board needed

PPM3 Page 1

RAMSEY TRANSMITTER KITS

• FM10A, FM25B FM Stereo Transmitters

• FM100B Professional FM Stereo Transmitter

• TXE433 or 916 Transmitter & Encoder Module

• RXD433 or 916 Data Receiver& Decoder

• RR1 Wired remote repeater

• RRW1 Wireless remote repeater

RAMSEY RECEIVER KITS

• FR1 FM Broadcast Receiver

• AR1 Aircraft Band Receiver

• SR1 Short-wave Receiver

RAMSEY HOBBY KITS

WEB1 Walking Electronic Bug

•

• LEDS1 LED Strobe

• BE66 Blinky Eyes

• EDF1 Electronic Dripping Faucet

• TFM3 Tri-Field Meter

• LC1 Inductance-Capacitance Meter

RAMSEY AMATEUR RADIO KITS

• HR Series HF All Mode Receivers

• QRP Series HF CW Transmitters

• CW7 CW Keyer

• DDF1 Doppler Direction Finder

• 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.

PPM3 PHONE PATCH MIXER KIT MANUAL

Ramsey Electronics publication No. MPPM3 Revision 1.3

First printing: January 200 GMG

COPYRIGHT 2002 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. Pri nted i n the United States of America.

PPM3 Page 2

Ramsey Publication No. MPPM3

Price $5.00

KIT ASSEMBLY

AND INSTRUCTION MANUAL FOR

PHONE PATCH MIXER KIT

TABLE OF CONTENTS

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

Theory of Operation ..............................5

Learn As You Build ...............................8

Parts Placement Diagram ...................11

Parts List .............................................12

Schematic Diagram .............................14

Assembly Steps ...................................15

Testing your PPM3 ..............................21

Troubleshooting Guide ........................23

Specifications ......................................25

Warranty ..............................................27

PPM3 Page 3

RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone [phone]

Fax [phone]

www.ramseykits.com

PPM3 INTRODUCTION

We’ve all heard those awesome call-in programs on the radio. You know, the
ones where the host lets the average Joe give his opinion on-the-air to all of his
listeners . . . regardless of intelligence level or speaking skills! Yes, we’ve all
been embarrassed for poor old Joe when he puts his foot in his mouth and
we’ve all thought that we could be much more brilliant and interesting than the
high paid host we’re listening to. Well, here’s your opportunity to try your hand
at talk radio on a micro scale. You FM and AM micro or hobby broadcasters
know what fun it is to decide the programming on your house station and be
able to listen to all your favorites out in the yard or garage. And your close
neighbors have been listening too, as you’ve heard over the back fence. Then
there are those of you who use our hobby transmitters at swap meets or flea
markets and hamfests to tell the happy customers how to find the best deals
and where the doughnuts are. We designed the PPM3 to add a whole new
dimension to your transmitting/broadcasting experience. Now you can allow the
locals to call in and voice an opinion and hear it broadcast on your micro
station. Hey, we all know how much people love to hear their own voice on the
radio. They’re probably listening to you already and wondering how come you
get to have all the fun (We know it’s because you’re spending all the time and
money but they didn’t think that far!) . . . now you can let them have some fun
of their own! Have your listeners let you know what they’d like to hear on your
station, let them tell people in your listening area about upcoming events in
their own words, start a new talk radio program to deal with community
issues . . . the possibilities are limited only by your imagination . . . and your
callers diction and phone etiquette. No, you probably won’t become the next
Rush Limbaugh but you can enjoy that “in control” feeling in your own
“broadcast booth” taking those phone calls just like the big boys. Thanks to all
of you who called and emailed to let us know that you wanted a kit like this. We
know you’re going to love using the PPM3 with your hobby broadcast
equipment. Let’s stop talking and start understanding and building your new
Phone Patch Mixer!

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PPM3 CIRCUIT DESCRIPTION

The PPM3 is essentially a stereo audio mixer, with one stereo line input and
two specialized mono inputs, each feeding into the stereo line output. The
phone and mic inputs each have a pre-amp stage that acts as a complete sig­nal conditioner. The mixing core is done with two op amps, U2:C for the left
channel and U2:D for the right channel.

The line inputs are AC coupled and volume adjusted by R2 and then fed into
the mixer. The maximum voltage gain from the line inputs to the line outputs
for each channel is 3.2, or 10 dB. This is set by the ratio of R22/R23 for the
left channel and R21/R24 for the right channel. The maximum voltage gain
can be calculated for each channel by:

G(left) = R22 / R23 = 15K / 4.7K = 3.2
G(right) = R21 / R24 = 15K / 4.7K = 3.2

If you are interested in the gain in dB format, it can be converted by the follow­ing:

G(dB) = 20 * Log (G) = 20 * Log (3.2) = 10 dB

These are the maximum gains that are achieved when the line volume pot is
set to its full (clockwise) position.

The microphone input has a DC phantom power of +5V which is required for
some microphones. If not needed, this can be eliminated by not installing
R13. If you’re not sure what your mic needs, install R13. The signal is AC
coupled by C33 before it goes into the pre-amp. A value of 5.6nF was chosen
because with an input impedance for the pre-amp of 180 K, a high-pass filter
is utilized that cuts out frequencies below roughly 150 Hz. In effect, this is
desirable for a vocal microphone because it will eliminate some annoying
“pops” and other low-frequency noise like 60 Hz AC line hum, for example.
U3 is the pre-amp that performs a few useful functions all in one part. First, it
acts as a noise gate which simply means that it effectively “turns off” the mic
when only background noise is present, and then automatically turns back on
when there is a minimum signal. It also acts as a variable compression ampli­fier, which means that it will amplify a quiet whisper more than it would a loud
shout. Its compression ratio is determined by R9, which with the 39K resistor,
works out to be about a 3.5:1 compression ratio. C6 controls the release time
of the compression and the real-time rms averaging for the internal level de­tector. This capacitor can range from 2.2uF to 22uF. The 10uF value seemed
to give a good compromise. Too small of a value and the gain will change two
fast, two large a value can result in slow response times to signal dynamics. If
other compression ratios are desired, this chart can be used:

PPM3 Page 5

The microphone pushbutton switch (S3) is used to turn on the mic channel by
pushing it in, and to mute the mic channel by leaving it out. The mic LED is
powered on when the mic is on by using the other half of the switch.

The phone line input is meant to monitor both ends of a phone conversation.
An external local phone connected to the output phone jack is needed to per­form dialing and to take the phone line “off hook” in order to answer a call.
The phone input can be disconnected from the phone lines and muted by us­ing the phone pushbutton switch. Like the mic switch, mute is “out” and on is
“in”. The phone signal is filtered for unwanted high frequency noise by C3 and
C4; this is called low-pass filtering (LPF). It is also filtered for unwanted low
frequency noise by C5, which is called high-pass filtering (HPF). This cap is
also used to block the high DC voltage on the phone line from passing through
the isolation transformer, T1. Because of this DC voltage which norm a lly is
50 VDC off-hook, and can even have spikes upwards of hundreds of volts, the
first three caps in the circuit (C3, C4, C5) were chosen to be able to handle 1
kV each. R30 performs some signal attenuation and isolation, and then the
isolation transformer T1 further helps prevent unwanted interference between
the phone line circuit and the PPM3 circuitry. T1 also takes the differential
phone audio signal and converts it to a single-ended signal (which means one
side is grounded). C31 performs some more LPF, and C16 performs some
more HPF. R31 is one more protection against the PPM3 circuit interfering
with the phone line circuits and vise-versa. The two diodes, D1 and D2 pro­vide a final protection against high-level signals reaching the PPM3 circuitry
and potentially doing damage. They provide an upper voltage limit of 0.7 volts
and a lower voltage limit of –0.7 volts. Any signal outside these limits will be
shorted to ground. At this point, the signal is amplified with the same type of
pre-amp that mic circuit used, but with a different compression level. The
problem with monitoring both sides of a phone conversation is that the signal
from the local phone is usually much higher is amplitude than the signal from

PPM3 Page 6

the far-end phone. So the solution is to use a high amount of com pres s ion in
order to amplify the far-end, weaker signal much more than the local stronger
signal. In this way both sides of the conversation are closer to each other in
signal strength when applied to the main mixer. A 100K ohm resistor was
used to set the compression level around 7:1. The same chart that was used
for the mic circuit can also be used with the phone circuit if a different com­pression ratio is desired.

The clip detect circuit that powers the clip LED is basically constantly com­paring the signals from both the left and right channels against a set threshold
that is close to the mixer amp’s upper voltage rail, which is where it clips.
There is a little amount of hysteresis to enable the LED to be lit up long
enough to visibly see it during short signal spikes. The core of the peak detect
circuit is D3, D7, R25, and C24. Basically, an averaging DC peak voltage
sample is created, held with a charge on C24, and discharged with R25. R20
and R17 control the charge time of C24.

PPM3 Page 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 to 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 to 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. To install
parts, the component should be placed flat to the board, and the leads
should be bent on the backside of the board to prevent the part falling out
before soldering (1). The part is then soldered securely to the board (2-4),
and the remaining lead length is clipped off (5). Notice how the solder joint
looks close up, clean and smooth with no holes or sharp points (6).

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Although we know that you are anxious to complete the assembly of your
phone patch mixer kit it is best to follow the step-by-step instructions in this
manual. Try to avoid the urge to jump ahead installing components.

Remember that all of the components will be mounted on the component side
of the circuit board and soldered on the solder side of the circuit board (the side
that contains the printed circuit traces). Have a look at the parts layout diagram
to help with your assembly.

Use the boxes to check off your progress.
Check all received parts against the parts list. The parts list describes the

various markings that may be found on the kit parts. Carefully sort the parts into
small piles to aid in finding the correct part at the required time.

Proper Component Installation:

Surface mount component soldering instructions:

The SMT components will be affixed to the topside of the PC board in the U1
and U3 positions. Patience is the key when installing surface mount
components. Typically, the first step (after identifying the component) is to “tin”
one of the PC traces that will connect to the part. This may go a little “against
the grain” at first, but it is by far the easiest method. Notice also that when
reheating the solder that the iron tip does not come in contact with the “tab” on
the body of the chip component. Over heating of this solder tab can cause a
fracture of the bond to the component causing an intermittent connection.

PPM3 Page 9