Ramsey Electronics SHA2 User Manual

SUPER STEREO

HEADPHONE

AMPLIFIER KIT

Ramsey Electronics Model No. SHA2

Want to listen to your audio source with headphones but the
manufacturer provided no headphone output? No problem;
simply connect the line level output to the input on the SHA2,
connect your stereo headphones, and you’re all set! This is the
next generation headphone amp with the ability to connect
multiple outputs and with more control options than our SHA1.

• Separate volume, treble, bass, balance and loudness

• Signal level clipping indicator LED

• Stereo loop-through for multiple feeds with separate controls

• CD quality audio

• Runs on a 9 volt battery or 12 to 20 VDC external power

SHA2 • 1

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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.

SHA2 Super Stereo Headphone Amplifier

Ramsey Electronics publication No. SHA2

March 2006

COPYRIGHT ©2005 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.

SHA2 • 2

Ramsey Publication No. SHA2:

Manual Price Only $5.00

INSTRUCTION MANUAL FOR

SUPER STEREO

HEADPHONE AMP

TABLE OF CONTENTS

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

“Learn as you Build”............................. 8

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

Parts Layout Diagram........................10

Assembly Steps .................................11

Schematic .......................................... 12

Setup and Testing..............................19

Troubleshooting .................................21

Warranty ............................................23

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RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone [phone]

Fax [phone]

www.ramseykits.com

CIRCUIT DESCRIPTION

Let’s take a look at the schematic diagram, we will follow through from input
to output to get a general idea how this kit works, and why. We will look
mainly at the left channel circuitry starting at J1. Items in parentheses ,
‘(xx)’, refer to the right channel signal path. Standard configuration settings
will be assumed. See the optional configurations section for additional
operation information.

The nitty-gritty of it all

A line level audio signal is connected to J1. Line level means an audio signal
of around 1V peak to peak, will give a reading of 0dB on a VU meter. The
audio passes through C9 (C10), a coupling capacitor. This capacitor prevents
DC from entering the circuit from external components and interfering with
audio quality. The capacitor lets the audio pass through to pin 19(2) of U3, an
LM1036, which is the tone/volume/balance control IC.

This IC can be thought of as the heart of the SHA2. It provides control of all
the major functions of the unit including volume, tone, balance and loudness.
Without this IC there would be a large quantity of resistors, capacitors and
amplifiers required to achieve the same functionality it provides. By the way,
remember that it is actually two sets of these controls since this is a stereo
system. R9, R10, R11 and R12 are the volume, tone and balance controls.
These functions are controlled by U3 with a DC voltage applied to the
respective control pins. This voltage is supplied by a built in reference source
on pin 17 and each control varies the voltage on the control pins which is
applied to its control pin. The loudness control, S1A, is used to provide a
slight amount of bass boost when the volume is set to lower settings.
Because your ear is less sensitive to low tones at low levels this provides a
more robust sound at these levels. Capacitors C15 and C21 determine the
way the treble control operates. Capacitors C19, and C20 are for the bass
control while C16 and C18 are used both treble and bass.

After being processed the signal is sent from U3 pin 8(13) to the input of the
output amplifier, U2(U4) pin 3 through C10(C17), another coupling capacitor
like C9. U2(U4) is a fully integrated audio amplifier, capable of driving low
impedance loads. It requires very few external components, runs very
efficiently, and has great fidelity. U2(U4) amplifies the signal to drive a low
impedance speaker like those in your headphones.

R7(R14) and C14(C27) on the output side of the LM386, U2(U4) is for
preventing oscillations due to the inductive nature of a speaker coil being
driven by the LM386. This makes the load of the speaker look more resistive
rather than inductive which prevents “motorboating” of the audio signal.

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C12(C24) is another coupling capacitor, and it serves the same purpose as
C9 at the start of the circuit. This prevents the DC portion of the signal on the
output of U2(U4) from being sent to the headphones.

The clipping detector circuit

The clipping detector circuit watches the headphone output signal level and
flashes an LED when the output signal comes close to the maximum level
available from the LM386. This level is determined by the voltage divider
formed by R15 and R17.

U1C and U1D form comparator circuits for the left and right channels. The
output signals from U2(U4) pin 5 is fed to U1C(U1D) pin 10(12), the ’+’ or non­inverting input, of the comparator. If this signal level exceeds the level on pin
9(13) the output, pin 8(14), will go to a high state about equal to the supply
voltage of 9 volts. This forward biases diode D2(D5), causing it to conduct
and apply a positive voltage through R16 and light the clipping LED D3. R16
is simply a current limiting resistor to prevent D3 from conducting too much
current and ’burning out’. Because the outputs of the two comparators are
basically at ground under non-clipping conditions D2 and D5 are needed so
that if one comparator output goes high and the other is still low the current to
light D3 is not grounded through the low output.

The looping buffer

The purpose of this circuit is to provide a signal which is identical to, but
isolated from, the input signal to feed other devices such as additional SHA2
units, your tape recorder, etc. The isolation feature prevents the additional
devices from interfering with the original input signal. For example if the other
the device were to develop a short it will not affect the unit that it is being fed
from.

U1A(U1B) are used as a unity gain amplifier or buffer. The input signal is feed
through C1(C2), a coupling capacitor, to the buffer amplifier input pin 3(5).
The output of the buffer then goes through C31(C32) to the buffered loop
output. R1(R4) and R2(R3) form a voltage divider which sets a bias level on
the ‘+’ input of the amplifier which is half of the supply voltage. This allows the
input signal to swing around this point and produce an output signal which is
between ground and the power supply voltage. This is basically the same
way the clipping detector works except the output of U1A(U1B) is made to
swing between 0 and the supply voltage as long as the input level does not
exceed a peak-to-peak level greater than the supply voltage. Capacitor C31
(C32) removes the DC level of the output and presents a signal which is
centered around 0 volts to the loop output jack. R19(R18) Provide a path for
C31(C32) to discharge and also sets the impedance of the output. This low

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impedance makes it possible to connect multiple devices, like the SHA2, with
high impedance to the output without having any significant effect on the
quality of the signal. This is called bridging the output.

Wondering what impedance is? If you know about resistance you know, or I’ll
tell you, resistance is a measurement, in ohms, of the amount of resistance to
current flow in a DC circuit. Like a garden hose that is crimped. There is less
water (current)) flowing in the hose because of the crimp (resistance)).
Impedance is the same thing but is used to describe resistance to an AC
current flow. Why the difference? The resistance (impedance)) of an AC
circuit is affected by the frequency of the AC signal but there is no frequency
involved with a DC signal. The impedance of an amplifier input will be
different for every signal frequency applied to it but a circuit which has only DC
current will always be the same. When an impedance is specified it is usually
for a frequency at the center of the range expected.

What about H1 and H2?

Notice that the input to the looping buffers have a set of jumpers, H1(H2),
feeding them. These jumpers allow you to choose between the direct input
feed to the SHA2 or the output of the LM1036 processor. By connecting a
jumper between pins 1 and 2 of the headers the buffered output is simply
whatever the input is. If the jumper is placed between pins 2 and 3 the
buffered output becomes the processed signal provided to the headphone
amplifiers.

Let’s say that you want to use your SHA2 to listen to the record level output of
your amplifier because it does not have a headphone jack but you also want
to be able to record the program you are listening to. Yes you could simply
connect a ‘Y’ adaptor to the output and plug in the recorder. But what would
happen if the cables to the recorder became shorted to ground. Now you
loose the signal to everything connected to the recorder output. The looping
buffer in the SHA2 will prevent the recorder form affecting your headphones if
the recorder is defective.

Now let’s consider a situation where you have several power amplifiers that
are connected to speakers in different areas. You want to control all the levels
from one central location. By connecting pins 2 and 3 on the headers you can
feed all the amplifiers from the SHA2 and have your monitor headphones too.

How about feeding one signal to a string of monitors for your band. Simply
daisy chain the SHA2’s using the looping buffers. The possibilities are nearly
endless. The choice is yours.

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The power supply

VR1, C29 and C28 supply the regulated DC voltage for the circuit. VR1
regulates the input, which should be around 12VDC input to J5, to a level of
10VDC for use in the unit. Yes the internal supply voltage indicates it is 9VDC
and here is the reason why. You will notice that diode D1 is connected
between VR1 and the power switch. This diode is to prevent voltage being
applied to the output of VR1 from battery BAT1. Diodes are neat things
because they will only conduct current in one direction such as in this case
when a positive voltage is applied to the anode of the diode with respect to the
cathode. (OK… the cathode is the side of diode to which the arrow points in
the schematic or the end with a band or ‘mark’ on it in the actual device. The
anode is the other side. The cathode likes to be negative with respect to the
anode.) Anyway, you never get something for nothing and in the case of the
diode when it is conducting there is about .6 volts lost across it so the voltage
supplied to the circuitry is around 9.4 volts but we’ll just call it 9VDC.

You will notice that the power jack, J5, has a switch contact which disconnects
the battery from the circuit when external power is being used. This prevents
the battery from attempting to charge when you are using an external power
supply. It is not a good idea to attempt to charge a battery, in fact it can cause
a battery to explode, if the battery is not designed to be charged. NEVER
attempt to charge a standard alkaline or carbon/zinc battery.

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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 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 topside of the PC board. This is the side that
has few or no traces on it, the side with the silkscreen writing. 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 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 close up, clean
and smooth with no holes or sharp points (6).

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