Microphone / Guitar / Universal Mono / Universal Stereo / Universal Three Independent
Channel Pre Amplifier
By
Steven Stanley Bayes
www.Steven-Stanley-Bayes.co.cc
(Click on the link Pre Amplifier)
Introduction
Audio Pre Amplifier
There are many different sources of audio signals these days: many different microphones,
iPods, iPads, Cellular Telephones, PC Audio Cards, MP3 Players, Guitar Hum Buckers,
Keyboards, etcetera.
The analogue outputs of these are not standardised. As a consequence of which the sound level
differs tremendously as depends on the device used. For example, there are many microphones
which can barely be heard with the same settings with which other microphones perform
normally. Also, there are many devices which have microphone inputs with different parameters.
The same microphone may not be heard when connected to a given device even at highest
possible volume adjustment while this may perform well with other.
The same can be said for amplified PC speakers. Either the PC output or the amplifier of the
speakers may be too weak to perform to the expectations.
I have, therefore, done a simple device which uses simple and basic electronics circuitry which is
a three channel pre amplifier and can be used in many combinations thereof. One of the most
useful application may be as a microphone and amplified speaker pre amplifier.
I have been using this device and the sound performance of the PC has been improved
tremendously.
For those interested, please visit www.Steven-Stanley-Bayes.co.cc and click on the link "Pre
Amplifier".
Schematics
+
9V DC,
≥=250mA
+
1KΩ
1µF
1/4TL084
+
220µF, 25V,
Electrolytic
1KΩ
-
1/4TL084
Microphone
or Guitar
39KΩ
10µF
+
10KΩ
-
47pF
10KΩ
130KΩ
0 to 100KΩ
130Ω
1/4TL084
39KΩ
1µF
+
-
47pF
10KΩ
130KΩ
0 to 100KΩ
130Ω
1/4TL084
39KΩ
1µF
+
-
47pF
10KΩ
130KΩ
0 to 100KΩ
130Ω
All resistors and the potentiometer: ¼ W, 5%. All capacitors: 25V or higher. The resistor
connected with a dashed line is not equipped.
Bill of Materials
Supplier: www.ActiveSurplus.com
Yes
/No
Quantity
Price
Each
1
2
2
2
2
1
1
1
3
4
3
3
3
3
2
1
1
3
1
1
1
4
20’
1’
1
1
1
2
3
3
3
1
1
1
1
5
4.95
0.05
0.05
0.05
0.05
0.05
0.05
0.25
1
1
0.15
0.10
0.25
1
0.10
0.10
0.10
0.05
0.05
0.05
0.50
0.05
0.05
1
4.95
Part Number
TL082/4
Name
Operational Amplifier
Female (panel mount) mono ¼” guitar and/or
microphone jack socket
Female (panel mount) stereo 3.5mm jack socket
PLASTIC Female (panel mount) Power Jack Socket
Perforated Board
Aluminum Enclosure
100K Linear Potentiometer
10K, 1/4W, 5% Resistors
130K, 1/4W, 5% Resistor
130Ω, 1/4W, 5% Resistor
39K, 1/4W, 5% Resistor
1K, 1/4W, 5% Resistor
1µF, ≥25V, Capacitor
10µF, ≥25V, Capacitor
220µF, ≥25V Electrolytic (Polarised )Capacitor
47pF Capacitor
Plastic 5mm LED Holder (2 piece panel mount)
5mm Super Bright LED
Permanent (not momentary) switch
Plastic Stand off’s
Connector Wire
Ground Wire
Ground Hook
Screw
Nut
Washers
Potentiometer Knobs
Potentiometer Washers
Potentiometer Nuts
Corresponding Male Power Jack
9V, ≥250mA DC Power Adapter
Approximately $26 + Tax ~= $30. Can be said to be a $30 development.
Some parts may not be available at Active Surplus. Go to Creatronic Inc., www.Creatronic.com
Considerations
Notes:
Ground the unconnected metal potentiometer enclosure.
Attention with the guitar jack connection: Longer pin is signal.
Use PLASTIC power jack socket ONLY! Must not make any connection with ground.
Ground the aluminium enclosure with a ground hook, screw, nut and two washers. Sand
the paint of the enclosure (in case there is any) off to the bare metal.
5. Do not install the 10KΩ resistor across the microphone for a better performance. In case
of two much noise, or too high volume of the signal, install either this resistor or any
value between 1KΩ and 100KΩ (1MΩ may be possible as depends on the noise).
6. Remember: the lowest gain is a bit higher than 2 and not 1.
7. Connect the center pin of the potentiometer to ground and the right hand side (when
looked at the pins of the potentiometer) to the circuit. This way, when the potentiometer
is turned all the way left, the gain will be the lowest possible (a bit higher than 2) and
when the potentiometer is turned all the way right, the gain will be the highest possible
(14).
8. Remember the “scale” of the potentiometer is not linear.
9. For the stereo INPUT ONLY (not the output), use such a stereo female 3.5mm jack
socket which has the signal inputs connected to extra pins when the jacks are not inserted.
In other words, when the jack is not inserted the signal inputs of the stereo channels must
be grounded but not the outputs. When the jack is inserted the connection to ground is
broken and the signal inputs of the stereo channel gets connected to the signal source
(PC, stereo, iPod, guitar, microphone, etcetera)
10. Take care of the unused operational amplifiers (in case there are any) when these
principles are considered:
 The op amp inputs must be actively held within the input common-mode voltage range of
the device.
 The op amp output must be set within the output voltage swing range of the device.
Here is a correct way to do it: (A better way is to use them for simple filters.)
1.
2.
3.
4.
Applications:









Microphone pre amplifier
Connected to a microphone input of an amplifier
Connected to a line input of an amplifier
Guitar pre amplifier
PC speakers stereo pre amplifier. The output of the preamplifier must be connected to the
input of an amplifier (or amplified speakers) and not directly to the speakers.
Microphone and PC stereo Speakers
Guitar and PC stereo Speakers
Guitar and microphone in different stereo channels
High Gain Mono Pre Amplifier (gain of 8 to 2744, ~7500 gain) by connecting all three
channels in series (one after another). Requires a special cable which is easy to do.
General purpose mono/stereo pre amplifier.
Frequency range: 0 to 25KHz. Remove the 47pF capacitors to achieve frequency range of
0 to ft/G (3GHz/14) which is 0 to 214285Hz, approximately 0 to 215KHz.
Considerations:
The power supply can be of any value between 5VDC and 36VDC. For better
performance, the power supply should be as high as possible. This, however, introduces a
possibility for the output voltage of the pre amplifier to be higher than the supply voltage
of the amplifier thereafter. This may appear to be a problem in case of a CMOS input of
the amplifier. Hence, a protection resistor may be a good idea at the output. This
protection resistor must ensure the maximum possible output RMS current is less than
1mA. In the very worst case of, say 50V at the output (36V non regulated power supply
somehow displayed at the output) and powered off amplifier after the pre amplifier, a
51K resistor will ensure high level of protection. However, this protection resistor forms
a voltage divider with the input impedance of the amplifier and lowers the signal. This is
particularly dangerous for microphone inputs with low value of parallel resistor at the
input. This is why, for the microphone channel of the pre amplifier, a protection resistor
of 130Ω has been chosen to accommodate the lowest cited parallel resistor of 1K at the
input of the amplifier. The protection resistor can be very high for the other inputs but the
will not be changeable, i. e. there will be only one microphone input and the rest will not
be possible to be used as microphone inputs in such cases. Thus 130Ω resistor has been
accepted everywhere. Higher resistor values would introduce more noise, although this is
compensated by the AC connection of one end of the protection resistor to the very low
output impedance of the pre amplifier circuit.
The best is not to put any protection resistor and to take the risk and be careful with the
gain of the preamplifier to never provide higher voltages than the secondary amplifier’s
power supply. These can be adjusted by year. Also attention must be paid to power the
preamplifier after the amplifier.


For those, concerned with protection:
Use supply voltage for the pre amplifier lower than or equal to the power supply voltage
of the amplifier in case you know the power supply voltage of the amplifier. For most car
applications, the car power supply of 12VDC would power the two pieces of equipment.
This is OK when no power conversion to a lower value is carried out in the amplifier. In
case you do not know the values, use 9VDC which should give +/- 4.5V to satisfy even
as low as +/-5VDC power supply of the amplifier. (Be careful, a non regulated power
supply of 9VDC would usually display 9 to 15VDC when the load is very low as is the
case with the pre amplifier but will be reduced in case of a load such as damaging current
to the amplifier’s circuit. However, this will be for a short period only before the voltage
drops below. The pre amplifier power filtering capacitor will also discharge in a
damaging way in case previously charged. The bigger the capacitor the worse.)
Use power supplies with as low output current as possible in order to ensure the voltage
will drop quickly at higher currents in case of a problem. The 250mA has been chosen to
ensure the possibility for the 4 operational amplifiers to work at 50mA outputs which is
theoretically impossible because the three working operational amplifiers must have a
maximum of 50mA overall consumption because the ground amplifier sinks the currents



of all of the rest and this amplifier, being unbuffered by a common collector, can sink
50mA. Thus a power supply of 50mA would cover the maximum, 250mA gives extra
possibility to go over. 25mA power supply would work just as fine provided the current
through the LED is limited to 10mA. Usually, the generally available power supplies
would not go below 100mA.
Leave the 130Ω resistor(s).
Change the 130Ω resistors to, say, 20K for the non microphone inputs.
Reduce the value of the power filtering electrolytic capacitor. This would introduce
ripples but this circuit does not consume a lot, so there would be no problems even at
10µF electrolytic or less. The ripples will have no effect because of the high CMRR of
the operational amplifier.
After the power supply, there are two capacitors: one electrolytic with a value of 220µF,
25V and 1µF. The voltage value of all capacitors must be higher than the maximum value
of the power supply voltage. Use 50V capacitors or higher for 36VDC power supplies.
The electrolytic capacitor must be as high as possible to better filter the power supply.
Too high values may have an adverse effect on the secondary amplifier in case of an
accident. For better performance, however, use as high values as you have room for (and
money). The 1µF capacitor is welcome for filtering of transients. This capacitor may be
omitted. However, one may wish to put several more capacitors in parallel with these two
with different values going throughout the whole range from 1µF to a few pF. Capacitors
have parasitic resistance and inductance which may have an adverse effect in case of
transients or switching power supplies. Capacitors with low values have lower parasitic
resistance and inductance and yet may be able to filter transients and switching noise
despite their low capacitance values. In case you decide to use switching power supply,
ensure the switching frequency is much higher than the audible range maximum of
25KHz.
The LED and the 1K resistor are in parallel to the power supply. The resistor must be
chosen to limit the current to the maximum current of the LED at maximum power
supply. This leads to low currents at low voltage power supply which reduces the light.
Then there is a voltage divider made by 2 resistors of 1K value to provide the middle
point of the DC power supply, thus, theoretically, any power supply can be used to power
the circuit and the middle point will always be a middle point (half of the power supply
voltage.
The middle point is buffered by the very low output impedance (around 1Ω or less) of the
ground buffer non inverting amplifier which is powered directly by the power supply to
where the ground current is sunk by the output of the buffer.
Thus a three point power supply, +, - and ground is made. The limitations of the power
supply are defined by the desirable levels of the outputs of the pre amplifier and the
operational amplifier. TL084 can reach from negative to 1.5V less than positive of the
power supply. The symmetry of the output would limit the amplitude to the range of
negative power supply + 1.5V to positive - 1.5V. The input of the amplifier which is to
take the signal from the pre amplifier will be happy to see 1V average RMS signals and
3V or higher may be needed in some low level applications. Thus the lower end of the
power supply is limited to +/-4.5VDC or 9VDC. However, one may wish to go as low as
5VDC which gives +/-2.5VDC and will allow the output of the pre amplifier to swing
from -1V to +1V which, some people may think is kind of standard for an amplifier
input, others may think is even high. I THINK EVEN 3V OF AMPLITUDE IS NOT
SUPER HIGH. (They probably make amplifiers out of wood these days. )
Also, the higher the supply voltage than the signal, the more away the signal is from the
non linear range of the operational amplifier (low signal performance as opposed to high
signal performance of the operational amplifier).
For the microphone input, a resistor can be put in parallel with the input of a value in the
range of 1KO to 20KO. The resistor will make the coil of the microphone discharge the
energy through. The coil will drive a current through the resistor and create voltage. The
lower the resistor the lower the voltage. The resistor may also have a filtering effect due
to the inductance of the coil.


However, a resistor may be desirable because:
The coil needs to discharge somewhere
Lowers the noise
Without the resistor, the coil will try to discharge through the very high input impedance
of the amplifier and through the coil’s own output resistance which appears to be in
parallel to the coil
The other inputs go directly to the non inverting amplifier circuit.
This starts with a 39KΩ resistor which serves protection purposes which are not so much
necessary and also, at middle gain, equalises the effect of the offset currents which enter
the negative and positive input of the operational amplifier and create a bias. These
currents are incredible low because of the JFET input of the TL084 and are also very low
at the bipolar Darlington inputs. However, as long as the operational amplifier does not
go too much off so the signal or part thereof goes to the non linear area close to one of the
power supply’s rails, the offset of the signal 0 from the circuit 0 is not a problem because
the introduced DC component will be stopped before the output of the pre amplifier by a
capacitor and will have no effect on the amplifier thereof.
This DC filtering capacitor must be as high as possible not to filter the audible range
components. The capacitor forms a high pass filter with the input resistance of the
amplifier. The cut off frequency must be below 20Hz. Thus, in the worst case of 1K input
resistance of the amplifier (microphone input), this capacitor must be:
C=1/(2πRfc)=8µF
Thus an off the shelf 10µF, >=25V big capacitor was purchased for $1 or around. The
capacitor is the size of a matchbox and looks like been made by paralleling three
capacitors by the manufacturer. These are generally available. However, for the expected
standard value of minimum of 10K input resistance 1µF would do just as fine. For the
channels which are to be primarily used for to go to non microphone amplifier inputs,
where the input resistance is expected to be at least 1M, even 1nF would do OK. 1µF
capacitors are not as big as to affect the available room and are relatively inexpensive, so
these are a good deal to be used.
The schematic continues with a simple non inverting operational amplifier with a 100K
potentiometer connected with the middle to the ground. The case of the potentiometer is
not connected to any of the pins and is connected to ground through a chassy wire and the
enclosure. Beware the enclosure connection is unreliable unless the paint is scratched.
Hence unpainted enclosure is recommended or sanding the paint off.
Obviously, the gain is 1+R2/R1. When the potentiometer is 0Ω, a 10K resistor is
available not only to neutralise to some extend the offset current (39K resistor is at the
positive pin) but to avoid “infinite gain” (saturation). R2=130K, R1min=10K,
R1max=110K => Gmin=2.18 and Gmax=14.
The potentiometer adjusts the gain non linearly. Not a big problem because the
adjustment is approximate and mainly by ear.
The capacitor of 47pF in the feedback cannot do a perfect filtering but decreases the gain
of the higher than the audible range frequencies. The filtering effect is different at
different gain levels being the worst at Gmin.
There is not much of requirements for the operational amplifier. Most any operational
amplifier would do. A good thing is to have either JFET or CMOS inputs to consume as
little current as possible from the source thus boosting the performance. In case you are
fans of the parallel resistor to the microphone input, then you should not worry too much.
For signals taken from a low output impedance and sufficient power sources (such as for
example another amplifier) there is no problem at all. The funny thing is the preamplifier
takes signal from an amplifier, amplifies this signal and gives signal to another amplifier
when a two amplifier system is boosted.
Additional 2 Channels for Stereo Amplifier
Two channels of:
1/4TL084
+
39KΩ
TIP 29B
+
-
TIP 30B
47pF
10KΩ
130KΩ
0 to 100KΩ
All resistors and the potentiometer: ¼ W, 5%. All capacitors: 25V or higher. IN CASE OF
MONO (ONE CHANNEL), The same common collector transistor couple TIP29 and TIP 30
must be positioned after the IC pin and before the feedback (the same fasion) of the ground
amplifier which sinks the ground current. In case you have two channels (stereo) you must put
twice as powerful transistors at the ground amplifier than on each of the channels.
Maximum RMS output voltage: 12V at 36V power supply voltage (limited by the operational
amplifier).
Maximum RMS output current: 1A (limited by the transistors). However TIP29 and 30 may not
be able to display this current without overloading the operational amplifier due to very low
minimum hfe.
Maximum RMS output current without transistor limitations:
1.5A at 8Ω speaker
3A at 4Ω speaker
(limited by the resistance of the speaker)
The transistor limitations are easily overcomable by replacing the transistors with Darlington
connected ones as follows: TIP29B must be Darlington connected with T41B and TIP30B must
be Darlington connected with T42B.
With Darlingtons, the speakers should be rated to withstand:
36W for 4Ω speakers
18W for 8Ω speakers
10µF
Without Darlingtons, the circuit should be OK with 36VDC power supply at 12Ω speakers or
higher. For lower resistance speakers, either the supply voltage should be lowered or a resistor
should be put in series to the input (or the speaker) with values:
8Ω at 4Ω speaker and 36VDC power supply
4Ω at 8Ω speaker and 36VDC power supply
With these resistors in series, the speakers should be rated to withstand 12W.
Without the resistors in series and without Darlingtons, attention should be paid not to burn the
transistor by keeping the signal at the output down and good heath sinking of the transistors. The
peakers should be rated:
36W for 4Ω speakers
18W for 8Ω speakers
The transistors may be able to take 1.5A with good heath sinking (8Ω speaker) but may be
difficult to take 3A. One way is to keep the on the outside of the enclosure and check them and
turn the volume down when hot.
Also, again, without Darlingtons TIP29 and TIP30 may overload the operational amplifier
because of a very low hfe of 15. The operational amplifier can provide 50mA at the output thus
the maximum normal working current would be 750mA.
Anyway, the circuit is good enough for 8 speakers at 36V.
In case one wants to replace the transistors with more powerful ones without Darlington, just
straight transistors, pay attention to the MINIMUM hfe: the operational amplifier can supply
only 50mA. TIP41 and TIP 42 have a minimum hfe of 10. Thus the maximum normal working
current would be 500mA. TIP29 and TIP30 have a minimum hfe of 15.
To avoid these limitations, replace the transistors with these Darlington connections. Then the
hfe of each couple becomes 150. The maximum of 3A collector emitter current (output of the
amplifier current or load current) will require 20mA current which the operational amplifier will
be able to display (the very harsh conditions listed in the specification are not considered because
they are believed not to be possible in this case). On the other hand, in the worst case of hfe’s of
TIP 41 and 42 with a value of 10, these transistors will need to supply 3A collector emitter
current and thus require 300mA base emitter current which TIP29 and 30 can supply.
Do NOT forget to put all transistors out of the PCB and bolt them to the chassy with TIP41 and
42 put as far away as possible from TIP29 and 30 because, in the worse case only, all transistors
will worm up although they work far under their upper limits.
Do NOT forget you have to multiply the necessary power of the supply by two for your chosen
combination in case of a stereo output. For example, in case you would like to have 2 channels
(stereo) each of them 36W, you need 72W power supply. Get 75W to be happy.
Also, with this circuit which costs just a few dollars and a pair of old >=36W speakers which you
can find on the street, you can build yourself a nice 72W amplifier to whose stereo input you can
plug your guitar in parallel to the two channels and then you get a 72W guitar amplifier which
costs $150 to $300 brand new at most shops and would go to $75 second hand. You pay less than
$7.50 for electronics. Your problem would be the power supply. To wind one yourself is a
solution or to use some kind of switching power supply in case such exist.
IN CASE OF MONO (ONE CHANNEL), The same common collector transistor quad TIP29,
30, 41 and 42 must be positioned after the IC pin and before the feedback (the same fasion) of
the ground amplifier which sinks the ground current. In case you have two channels (stereo) you
must put twice as powerful transistors at the ground amplifier than on each of the channels but
you must think of the minimum hfe. A better way is to make a Darlington triple: just add another
transistor to the Darlington couple where the emitter of the output transistor of the Darlington
couple goes to the base to the newly added transistor, the collector of all three are connected and
the emitter of the new transistor becomes the emitter of the triple. Remember, the new transistor
must the most powerful of all three, the power of the second must be in the middle and the first
(the one whose base becomes the base of the triple) is the less powerful.
Also, in case you go for 75W power supply, you will be looking for not so conventional one, thus
you better look for one with a middle point. Thus you do not need to use the ground buffer. Also,
in case you want to get stuck with one power supply and to void the flexible power supply option
(different power output, though: can’t skip the Ohm’s law: can’t put more or the same current
through a resistor at lower voltage), then you may use zeners to get the middle point.
You can also take a transformer from an old device or you can purchase 2 standard 18V, 1.85A
AC power supply, rectify each of them with a 2A bridge or four 2A diodes put a huge capacitor
at lower voltage on each of them, then connect – with one with the + of the other and here is the
middle point. These transformers are pretty standard although not as standard as the AC to DC
power supplies. Yet they are possible to be found at a price of $10 to $12.50 a piece + 13% tax.
TIP29B
TIP41B
TIP30B
TIP42B
Put SIMILAR TO this quad (or a couple of Darlingtons) on the output of the ground amplifier
with TWICE THE POWER OF WHAT YOU HAVE ON EACH OF THE STEREO CHANNELS
AND PUT THE SAME FOR MONO.
TIP30B
1/4TL084
+
TIP42B
39KΩ
10µF
+
47pF
10KΩ
130KΩ
0 to 100KΩ
1/4TL084
From the voltage divider of
the power supply.
+
+
-
-
TIP29B
TIP41B
For stereo, use
double power
transistor or a
Darlington triple.
For mono, the same
as the channel.