QUASAR KIT No.

1052
3-INPUT MONO MIXER

General Description

This is an easy to build, inexpensive but nevertheless fully featured MONO mixer. It will
prove itself very useful in amateur FM transmitters, for mixing the output of musical
instruments, in Disco’s etc. The circuit allows for individual setting of the input sensitivity of
each input and also has separate BASS and TREBLE controls.

Technical Specifications - Characteristics

- 3 inputs having separate sensitivity settings each.

- Input sensitivity 150-300 mV pp.
- Maximum input signal before distortion 1 V pp.
- Output signal level 2.5 V pp
- Maximum output level before distortion 15 V pp
- Separate BASS and TREBLE controls.
- Supply voltage 18 VDC.

How it Works

The circuit is very simple. The three inputs are voltage amplifiers built around the
transistors Q1,2,3 and having adjust able input sensitivities by means of the
potentiometers P1,2,3.
The collectors of the three transistors are connected together and the signals are then fed
to the base of Q4 which mixes the three signals. The output signal from Q4 is taken to the
tone control which is a classic Baxandall tone control circuit and finally enters the output
stage which consists of two transistors Q5 and Q6. The output is taken from the emitter of
Q6 through the coupling capacitor C13.

Construction

First of all let us consider a few basics in building electronic circuits on a printed circuit
board. The board is made of a thin insulating material clad with a thin layer of conductive
copper that is shaped in such a way as to form the necessary conductors between the
various components of the circuit. The use of a properly designed printed circuit board is
very desirable as it speeds construction up considerably and reduces the possibility of
making errors. Quasar Kit boards also come pre-drilled and with the outline of the
components and their identification printed on the component side to make construction
easier. To protect the board during storage from oxidation and assure it gets to you in
perfect condition the copper is tinned during manufacturing and covered with a special
varnish that protects it from getting
oxidised and also makes soldering easier. Soldering the components to the board is the
only way to build your circuit and from the way you do it depends greatly your success or
failure. This work is not very difficult and if you stick to a few rules you should have no
problems. The soldering iron that you use must be light and its power should not exceed
the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose
come very handy specially made sponges that are kept wet and from time to time you can
wipe the hot tip on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it.
There are many different types of solder in the market and you should choose a good
quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too
much flux can cause many problems and is one of the main causes of circuit malfunction.
If nevertheless you have to use extra flux, as it is the case when you have to tin copper
wires, clean it very thoroughly after you finish your work. In order to solder a component
correctly you should do the following:
- Clean the component leads with a small piece of emery paper.
- Bend them at the correct distance from the component’s body and insert the component
in its place on the board.
- You may find sometimes a component with heavier gauge leads than usual, that are too
thick to enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes
slightly. Do not make the holes too large as this is going to make soldering difficult
afterwards.
- Take the hot iron and place its tip on the component lead while holding the end of the
solder wire at the point where the lead emerges from the board. The iron tip must touch
the lead slightly above the p.c. board.
- When the solder starts to melt and flow wait till it covers evenly the area around the hole
and the flux boils and gets out from underneath the solder. The whole operation should not
take more than 5 seconds. Remove the iron and allow the solder to cool naturally without
blowing on it or moving the component. If everything was done properly the surface of the
joint must have a bright metallic finish and its edges should be smoothly ended on the
component lead and the board track. If the solder looks dull, cracked, or has the shape of
a blob then you have made a dry joint and you should remove the solder (with a pump, or
a solder wick) and redo it.
- Take care not to overheat the tracks as it is very easy to lift them from the board and
break them.
- When you are soldering a sensitive component it is good practice to hold the lead from
the component side of the board with a pair of long-nose pliers to divert any heat that
could possibly damage the component.
- Make sure that you do not use more solder than it is necessary as you are running the
risk of short-circuiting adjacent tracks on the board, especially if they are very close
together.
- When you finish your work cut off the excess of the component leads and
clean the board thoroughly with a suitable solvent to remove all flux residues that may still
remain on it.
The circuit of the mixer is a very simple one, and this together with the use of a printed
circuit board with all the components marked on it makes construction very easy. Start
building the circuit by soldering the pins, continue with the resistors and the capacitors
taking care to insert the electrolytic correctly and finally solder the transistors and the
zener diode in their places making sure that they are not over heated during soldering. The
potentiometers can be soldered directly on the p.c. board or alternatively can be
connected to it using short pieces of shielded audio cable. This is up to you and it will
depend on the layout of the case that you are going to use to house your project. The
inputs are across points 1-2, 3-4, 5-6 and the output is taken from points 9 and 10. Points
1,3,5 are common and you should connect the shielding of the cables there while the input
signals should be applied at points 2, 4, and 6. Similarly point 10 of the output is common
(earth) and point 9 is where the output signal will be taken from. The power supply will be
connected at points 7 (+) and 8 (-). The power supply must be regulated a nd its output well
filtered to avoid introducing unwanted noise and hum from the mains to the system. The
three potentiometers P1,2,3 will be used to adjust the sensitivity of the three inputs
separately and P4,5 control the bass and treble respectively. If the mixer is going to be
used for the modulation of an FM transmitter the output from the preamplifier of each
source, (microphone, cassette, Pick-Up) will be connected to a different input of the mixer,
and the mixer’s output will be used to modulate directly the transmitter. For use with
musical instruments, and provided they have a minimum output of 150 mV, the output of
each instrument will be connected directly to an input of the circuit and the mixer’s output
will be connected to the power amplifier’s input.

Adjustments

This kit does not need any adjustments, if you follow the building instructions.

Warning

Quasar kits are sold as stand alone training kits.

If they are used as part of a larger assembly and any damage is caused, our company
bears no responsibility.

While using electrical parts, handle power supply and equipment with great care, following
safety standards as described by international specs and regulations.

If it does not work

- Check your work for possible dry joints, bridges across adjacent tracks or soldering flux
residues that usually cause problems.
- Check again all the external connections to and from the circuit to see if there is a
mistake there.
- See that there are no components missing or inserted in the wrong places.
- Make sure that all the polarised components have been soldered the right way round.
- Make sure that the supply has the correct voltage and is connected the right way round
to your circuit.
- Check your project for faulty or damaged components.
If your project still fails to work, please contact us for information about our Get-You-Going
service.
Schematic Diagram

Parts List

All components including printed circuit board, assembly instructions including schematics
and detailed parts list are supplied when you purchase the kit.

Ordering

For pricing info and online ordering please visit:

http://www.quasarelectronics.com/1052.htm

For further info please contact us by e-mail:

mailto: sales@QuasarElectronics.com

COPYRIGHT © 2003 Quasar Electronics Limited. All rights reserved. Reproduction of this document in
whole or in part in any form or medium without express written permission of Quasar Electronics Limited is
prohibited.

E&OE