Keeping things simple with a modular amplifier with separate electret mic input, pre-amp, and power supply.

The recent Classroom column showed how to design a simple transistor amplifier, intended to explain the process rather than to build a single transistor amplifier. This project describes a simple amplifier divided into four stages: the microphone, the pre-amplifier, the power amplifier, and the power supply.

Although most electronics designs are thought up in discrete stages, or blocks, they are usually presented as a single PCB with a sea of components. Often there is no explanation of how any stage works, and usually without mentioning that the stages must feed from one to another.

This project allows flexibility to meet the needs of beginner or experienced makers. To achieve this, the PCB is made in sections that can be easily separated, and the individual stages customised as required. Better still, more stages can be added.

The Initial Concept Defined

My brief was, well, brief. "Make an amplifier for an Electret Microphone." which while quite flexible, requires some interpretation by the designer. It was also requested that the audio signal had sufficient amplitude to be converted by an Arduino (or such) into digital audio, using the on-board ADC (Analog to Digital Convertor).

That sounds like 5Vp-p, but then maybe 3.3Vp-p for some development boards, and of course they don't want it peaking above 3.3V or below 0V. Audio tends to have a lot of peaky sound, which causes distortion, crashes and pops on recordings.

Digital recordings tend to use "Compression" to increase volume of the lower level sounds, while limiting the volume of louder sounds, as if the audio is recorded on a logarithmic scale. Radios often use compression, and a simpler AGC (Automatic Gain Control). Yes we can go there sometime, but not for this simple project. We will simply keep the output well within 3.3Vp-p.

Many audio devices limit the bandwidth, especially for speech only devices, to between 300Hz and 3000Hz, which requires a Low Pass Filter, and a High Pass Filter, which can also be added at some time in the future as we are making this circuit in stages, and adding two more filter stages, and perhaps an AGV or Compression stage remains a future option.

I have also had occasion to play (play with) electric guitars, and have considered adding music effects to our wish list, Fuzz Box, Wah Wah, Reverb etc. These are good projects, and there are hundreds of potential audio projects, but first, the basics!

Four (initial) Stages

I chose to divide the circuit into four stages almost in circuit diagram order, from left to right, as follows, Electret Microphone with bias network, Pre-amplifier, Power Amplifier, and the Power Supply.

Microphone Stage

A microphone is an Electronic Transducer that converts air pressure, especially audio frequency pressure waves into an electric voltage. There are too many types to cover in this article, but the one we have selected is readily available at low cost, producing a reasonable quality audio (AF) signal.

The Electret Microphone uses a capacitor as the transducer itself, with one fixed plate and the other a lightweight, thin and flexible plate, usually of metallised plastic or foil, presented to the source of the sound. As the voltage generated is very small, the Electret has an on-board amplifier. In our 'economy' version the amplifier is a single Field Effect Transistor (FET or JFET) with a single bias resistor, yes, all inside the 6mmØ by 6mm high metal case!

Larger electrets are used inside stage mics, using a much more complicated amplifier, even analog ICs such as operational amplifiers, and may include filters, compression and many more reasons for charging a lot of money! We work on the KISS principle here, but respect that options are available.

For the experimenter, the Electret is placed on a PCB that also contains the bias and interface components, two resistors and two capacitors. In fact the circuit only requires a single resistor for bias, and the Electret could be fed directly onto the base of a transistor.

For impedance matching, the internal resistance of this Electret is ~2kΩ, and the bias resistor should also be ~2kΩ, (2k2). This is what we used in the Transistor Amplifier from Issue 18.

In audio, however, a lot of amplifiers have a balanced input with two wires supplying audio signals of opposite polarity, called a differential signal. External noise is expected to affect both wires in the same polarity, called a common mode signal.

The amplifier, and especially operational amplifiers, have the opportunity and design to accept the differential audio signal while rejecting the common mode signal, thus rejecting interference. Operational Amplifiers in differential mode have a very high CMRR (Common Mode Rejection Ratio).

Therefore our circuit has split the bias resistor into two 1000Ω (1k0) bias resistors R4 and R5, and uses two 'coupling' capacitors, C3 and C4, instead of one. The coupling capacitors allow the audio signal to pass but stop DC voltage levels from appearing on the audio lines.

Ideally, the impedance of the capacitor would not change over the audio frequency range. However, capacitive reactance (impedance) changes with frequency, and is 10 times less at 3000Hz than at 300Hz. The next best thing is to make the impedance as small as possible at the lowest frequency, and much lower than the bias resistance, 1000Ω.

A 10uF would have an impedance (reactance) of 53Ω at 300Hz or about 1/20th the bias value, which would be a good starting point. However, there is another problem; most 10µF capacitors are polarised, and we would prefer non-polarised. We have two options; carefully think out the DC polarity of the circuit, recognising that the DC to the op-amp is in nano-amps, and therefore the voltage drop is minimal anyway, or go and buy the more expensive non-polarised electrolytic capacitors.

We have kept things simple by using two 2.2μF non-polarised electrolytics from Jaycar. You can experiment if you wish to see if there is enough improvement to warrant anything better.

If the microphone is used remotely, a shielded cable would be preferred in order to further avoid noise in the audio. In addition to the two differential audio signal wires, the cable would also require two power supply wires, thus requiring five conductors in the cable, or four wires plus the sheath. Therefore, the connection on the PCB has 5 terminal pads (+ve, AF+, Ground, AF-, and -ve).

THE PCB DESIGN

We have allowed for the PCB to be made in one piece, but easily broken into parts at the 5-pin connectors. The PCB connections are joined so no links are needed if the circuit is built as one PCB. Breaking the PCB at that point is normally very easy, but to avoid this catastrophe, we recommend using a blade to sever the tracks before breaking the PCB apart. Some even prefer to use a Junior Hacksaw to separate any two PCBs.

Pre-Amplifier

The pre-amplifier is typically a stage to match the input impedance to the following amplifier stages, and in this case to match the differential input to a single ground referenced output. The circuit design is therefore made for a differential input on a five pin socket matching the five pin connection from the Mic PCB.

The circuit is a straightforward differential amplifier with two input resistors; R6 and R7, de-coupled by the capacitors in the previous stage, a feedback resistor; R8, and a grounded impedance matching resistor; R9. The only other components are the output de-coupling capacitor; C5, and IC1, an op-amp which may be chosen from any that have the most common pinout for an 8-pin op-amp.

PIN	FUNCTION
1	NC
2	Inverting Input
3	Non-Inverting Input
4	-ve Power Input
5	NC
6	Output
7	+ve Power Input
8	NC

This allows a wide range of IC options for you to try if you have a number of PCB's or use an IC socket. A very basic '748 or '741 may be used as cheap and readily available IC replacement. TL071 or TL082 can be used for FET input devices, or CA3130 or CA3140 for higher frequency circuits. The LF356 or LF358 may be used for lower noise, HiFi style circuits. After working in industry and at concerts, hearing anything may be a blessing! The point is that many different op-amps could be used as many share the same pinout but with different technologies, and 'superpowers'!

For our purpose I have chosen 100kΩ and 1kΩ resistors to give a differential gain of 100, taking an estimated 20mV from the Microphone to 2Vp-p from the Pre-amplifier, less losses of course which seem to be minimal in tests so far.

Greater gain often means greater noise and distortion, so one option would be to use a second amplifier stage using the same PCB. The first stage output, however, is not suitable to a differential input to the next stage, but the same PCB can be made into a single input by not using R7 at all. The gain can be adjusted to a lower gain in each stage by reducing the 100kΩ resistors, R8 and R9, to perhaps 15k, giving a total gain of 225 and an output of 4.5Vp-p.

The gain you want will depend upon your final use, and the power supply voltage used. It's up to you to think about, and calculate, or play with values.

Power Amplifier

This seems a good time to look at the Power Amplifier stage. The sole task of the Power Amplifier is to generate Audio Power, NOT signal. Even Power Amplifiers come in low power, high power, and holy smoke power!

Note: you won't get out 100W from a single little IC, or even a long string of them, as 100W at 12V means passing more than 8 Amps through the final IC! Reality bites!

A quick internet search didn't find any Guitar Amplifiers putting out more than 400W, which seemed strange, but a number of such amplifiers can be wired correctly to double that to 800Watts, and multiply by 4, in Bridge circuits to 1600 Watts, or even more in Matrix circuits to virtually any power of two. A collection of 16 x 400Watt amps can be wired to output 6400Watts if you're really keen on going deaf! Of course that would require carefully loading all that grunt into 64 x 100W speakers!

Oh yeah! In class A amplification you would need almost 100 Amps from your home power supply! Now that's reminiscent of 'Back to the Future' opening scenes, and feedback would definitely be a bad thing!

So physics wins again; you never get more out than you put in.

Our project power amplifier is very much in the low power category, outputting about what 1950s transistor radios put out, under 2W into 8 Ohms before distortion increases dramatically. Mostly, the limit in this circuit is in the cooling capacity of an 8-pin IC. The body is simply too small to allow more power to dissipate.

Power Amplifier Circuit and PCB

The circuit and PCB shown below reveal a circuit with an 8-pin IC, two resistors and three capacitors. The pcb is also modular so it can be plugged onto the previous stage, and the next stage for power.

The circuit is straight from the datasheets, using a supply voltage from 9V to 12V but capable of more or less. The IC may work with as low as 5V, or up to 22V depending on the manufacturer, package type, and quality of the individual device.

The audio signal is fed to IC2 pin 2 from the connector J4 pin 4, via a simple Volume Control Potentiometer, VR1. We used a 100k pot but it may be replaced with a wire link if adjustment is not required, and any pot from 1kΩ to 100kΩ should be quite suitable, especially when experimenting.

The non-inverting input, pin 3, is simply grounded.

A 1kΩ resistor, R10, is placed between pin 1 and pin 8 to limit gain to 10 (without it the gain will be 20). Adding a 10μF electrolytic capacitor between pin 1 and pin 8 will increase the gain at audio frequencies to as much as 200. Refer to the LM386 datasheets online.

The pin 7 capacitor, C6 (100nF), limits the upper frequency, bypassing higher frequencies to ground, assisted by the 47nF capacitor, C7, and 10R resistor, R11, connected to pin 5, the output, to help limit cross-over distortion as well as limit output frequencies.

The 100µF capacitor, C8, passes the output at pin 5 via J5 pin 4 to the next module, in our case the Power Supply and Audio Out.

Power Supply

J6 connects to the previous stages importantly connecting the centre pin, pin3, to all grounds on all boards. Pin 5 connects to the battery or plugpack positive, and pin-1 connects to the negative of that same supply.

An external power supply between 9 and 12VDC is recommended, but the circuit may work on as low as 5VDC or as high as 22VDC, depending on the manufacturer, case type, and the quality of the ICs used. For automotive use, remember that the battery voltage can rise to at least 14.2V, but that is expected to be OK for this project.

The 100µF capacitor, C1, across the input terminals, is also across J6 pin 1 to pin 5. This capacitor is not essential for a good DC input, but helps smooth the current demand on the battery if used, as well as extra filtering for a rectified ac supply.

There is no fuse on this project, so connection leads to a large battery, such as a car battery should be separately fused.

Resistors R2 and R3 form a voltage divider to make a 'middle' or Ground voltage half of the supplied/battery voltage. Capacitor C2, also a 100µF capacitor helps to hold the DC Ground voltage midway between V+ and V-, but also helps make a very low impedance path for the audio ground to the -ve which is the power amplifier ground and speaker ground.

The 8Ω 3W speaker is connected to JP2. Note that higher impedance and/or wattage speakers can be connected, but lower impedance speakers may cause IC2 to fail.

The only remaining part is the Power LED, and it's current limiting resistor, R1. The LED is optional, as a reminder that the circuit is powered, and may be left out, soldered into the PCB, or mounted on the front panel of any case you choose to use.

The resistor is necessary if you use the LED, but the size of the resistor should be altered depending upon the supply voltage you are using. A smaller value of 220Ω to 470Ω would suit 5V supply while 2k2 might help the LED live longer on 18V or more. This is the only component that may need adjustment according to the supply voltage, although it should be noted that all capacitors must be rated for the highest voltage of operation. We specify 16VDC working voltage which should be enough for automotive use, but 25VDC will be much the same cost anyway and is cheap insurance.

The Build:

Experimenters Amplifier

Parts Required:	Jaycar	Altronics	Core Electronics
1 × 10 Ohm 1/4W Resistor*	RR0548	R7510	COM-10969
6 × 1k 1/4W Resistors*	RR0572	R7558	COM-10969
2 × 10k 1/4W Resistors*	RR0596	R7582	COM-10969
2 × 100k 1/4W Resistors*	RR0620	R7606	COM-10969
1 × 100k Logarithmic (A) Potentiometer	RP3618	R2217	CE05225
3 × 100uF 16V Electolytic Capacitors	RE6130	R5123	CE05258
3 × 2.2uF 50V Bipolar Electrolytic Capacitor	RY6804	R6520A	-
1 × 100nF 100V MKT Capacitor	RM7125	R3025B	-
1 × 47nF 100V MKT Capacitor	RM7105	R3021B	-
1 × Electret Microphone	AM4010	C0171	COM-08635
1 × 3mm Red LED	ZD0150	Z0700	POLOLU-1070
1 × LM741 Op Amp	ZL3741	Z2590	-
1 × LM386 Op Amp	ZL3386	Z2556	-
2 × 8 Pin IC Socket	PI6452	P0530	-
1 × 40 Pin Header Strip	HM3212	P5430	FIT0084
1 × Female Pin Header Strip	HM3230	P5390	PRT-00115

*Quantity required shown, may be sold in packs.

Assembly

Decide whether you want one single PCB, or want the modules separated. It's easier to saw the modules apart now with no components to get in the way, but not too hard to do it later, so your choice.

First, always, inspect the PCB to make sure it looks clean and tidy. Modern PCBs are much more reliable than what we had even ten years ago, but still, accidents do happen, tracks get scratched and component suppliers sometimes provide an alternative pinout which is usually not found until the third day of debugging!

As we always say, start with the hardware, then the passive devices, and lastly the active devices. i.e. All sockets, all resistors and capacitors, (then 1st test), then the electret and ICs.

In this case, if IC sockets are used, everything can be loaded up and soldered except the Electret and the ICs should not be inserted into their sockets. The LED should be left until the same time as the actives, but generally they are fairly robust, just be sure of the polarity of all of the electrolytic capacitors, the electret, LED and the ICs.

Once all are soldered, but before the ICs are installed, you can connect the external power supply or battery. If you want to frequently turn the circuit on and off, and rather than have to constantly remove and re-connect the battery, perhaps add a simple power switch in series with the +ve wire. Also consider whether your application requires a fuse.

Another power option is to use a 6V (or other) battery pack from Jaycar, Altronics, Core or other suppliers which comes with a switch built into the case. Double check that you have installed your polarised components correctly oriented.

Testing

Now you have it all connected, except for installing the ICs, enable the power to the circuit and test the voltages everywhere on the PCB. Start with the module jumper connections, making sure you have +ve, -ve and GND at half the supply voltage by measuring with the black meter lead attached to the GND and checking both sides, +ve and -ve.

Then check that you have a voltage across the two terminals of the electret, and that there is a voltage between pin 7 and pin 4 on IC1, and between pin 6 and pin 4 on IC2.

If all of the voltages are as expected, you sound like you are good to go. If you had any capacitors reversed you might have seen the smoke signals by now.

Insert IC1. If you have an oscilloscope, or have uploaded an oscilloscope sketch to your Arduino, you can do the following test once you power the circuit up. With a radio going as a sound source close to the electret mic, check J3 pin 4 to see something moving on the screen, which would indicate you have an audio signal.

If no radio is available, you can whistle or use an old Amateur Radio method of saying "Heeeeeeeello!" for as long as you are able!

If no signal is seen, the good half of a broken set of earphones can be used if a couple of probes, or bits from a paper clip are soldered onto the leads so you can probe across J3 (or J4 if connected) pin 4 to GND pin 3.

You should hear the amplified radio signal or whistle quite clearly. If not double check everything, and put your multimeter into Vac mode on the lowest range you have and measure across the same pins. Any constant noise near the microphone should provide a voltage up to perhaps 70mV RMS.

Assuming you have found it working, remove the power, insert IC2, and add a pot wired as shown, or link or jumper J7 pin 3 to pin 4. Connect an 8Ω 3W speaker to JP2. The polarity markings are only needed if two or more speakers are used together, but by all means connect the speaker '-ve' to ground if your OCD is itching.

Power up with some sound near the microphone, and the speaker facing away from the mic, and any nearby walls, and you should hear success.

If not, using the multimeter on Vac, autoranging or switching down from higher ranges to lower ranges you should see a voltage on J7 pin 3, which should be the same on IC2 pin 2, and a 10 times higher voltage on J5 pin 4 or IC2 pin 5. Finally, the voltage should be on JP2 across the two pins.

Where to from here?

Now it is your turn to decide how you would like to use this mini-amplifier. Here are some suggestions:

Use headphones instead of a single speaker by wiring a stereo audio jack tips across the two pins of JP2. The headphones are in series with a common ground so ignoring the ground results in two series speakers with twice the impedance of each speaker, e.g.16 Ohms.

Hold the electret against your chest to see if it picks up your heartbeat. Try it on the cat, dog, baby etc!

Place it at the focal point of the largest stainless mixing bowl you can find and use it as a long distance surveillance eavesdropper (aka Big Ear Device).

Build two and make a pair of Big Ears for direction finding. YouTube "Big Ears WWII".

Use it as a stethoscope to hear and find issues with engine bearings, exhaust leaks, squeaks and groans on your car or bike.

Instead of the electret, connect your guitar to the inputs via a 6.5mm jack, and connect headphones to the output to use it as a practice amp to hide your poor strumming from the family.

Listen to plants grow!

There are plenty of opportunities, but the exercise is to get you to think of them and use the amplifier modules to get your ideas going.