Many of you want to build an audio amplifier with your own hands. A small booster amplifier for a siren too quiet, an amp built into a portable speaker for playing in the street, or a guitar amp or hi-fi for the living room. Very good idea! But where to start? Amplifier with electronic tubes (lamps) or with transistors? The question comes up all the more often as we now find components or modules that seem to make the task easier.
Amplifier with tubes (lamps) or with transistors?
This is one of the questions that come up most often with the beginner, who would like at the same time and if possible, to start with a “simple” circuit. The notion of simplicity is difficult to quantify. From the point of view of the “number of components,” some tube amps are simpler than some transistor amps. In contrast, a tube amp (s) poses constraints that the transistor amp does not have.
- Are you a real beginner? So start with a small transistor amplifier powered by a battery (s). Tube amplifiers require high voltages (between 250 V and 450 V in general) and these voltages are deadly, and therefore dangerous.
- Do you want to play in the street? The amplifier must be able to operate on battery, and for this, the technology with transistors or integrated circuits is indicated. We can certainly make portable a tube amp, but it requires the use of a voltage booster converter that reduces autonomy. Not to mention the fragility of the tubes and the problems related to the vibrations during the transport (the tube that leaves little by little of its support).
- Must the amp be housed in a tiny case? In this case, you do not have much choice, you will have to rely on transistor technology or integrated circuit (CMS if necessary).
For any use where autonomy is the main criterion (mobile system powered by battery or batteries), I recommend a digital amp (class D). We find ready-made amp modules of good price/quality ratio and especially of dimensions adapted to nomadic projects.
“Standard” (analog) amplifier – Where to start?
Many of you want to go headlong into the realization of a tube amp for guitar, or in a transistor amp for the living room (sometimes it’s the opposite, to each his way). I remember my first power amp as if it was yesterday — a “booster” amplifier for car stereo based on TDA2002, which had nothing to do with the current year. It was in a small box Teko aluminum (P1 if I have a good memory), consisting of two half-shells assembled by four screws. The case was the radiator; it was fun! BF 002 Amp.
The funny thing is that my first amp version (two TDA2002 per channel, similar to the one shown above) came out of the sound with a “thing” superimposed on it and not nice at all. For sure, the editing oscillated (I did not know where it came from) and the sound, which was indeed amplified, was completely “crushed.” I’ll talk about this kind of thing in a few lines. In this case, the problem was related to the fact that the mass of the four TDA2002s was connected to the ground track of the printed circuit, and at the same time to the aluminum case, through the metal sole of the integrated amplifiers. Once removed this nice loop of mass and shortened the flying threads, everything worked correctly. All this to say that I left on a 2 x amp [full of watts] without fear of what will be said. So why not you? But if you want to make your teeth on a more modest amp with a single TDA2002 or even a small LM386, I have nothing against.
BF 003 Amp
During a mail exchange with a surfer, I remembered that I had in stock a certain amount of TAA611 (with its pins arranged in staggered rows) and TBA820, which at one time were very used in large circuits. Public. I believe that these built-in amps were among the first audio power amplifiers that were affordable for everyone. Curiously, I did not hang more than that with these circuits and I stayed for a long time with my TDA20xx and LM386. Life is sometimes curious; perhaps my fear was proportional to the number of pins of the circuits in question. If today I am asked for advice on the “best amp to do yourself,” I am unable to answer. At most I could suggest the TDA7293 or TDA7294 which are good animals: they equip a lot of commercial amplifiers, which is reassuring somewhere. The LM3886 is not bad in its category too.
BF 010 Amp
These circuits are suitable for a few tens of watts, the TDA2030 and TDA2040 (admittedly a little less powerful) are also well placed for modest power (up to 10 W or 15 W). BF Amp 008.
You will have noticed that I only talked about integrated amplifiers, while everyone knows (except those who do not know) that we can also make a power amp only with transistors, without any integrated circuit and sometimes with settings that are not properly adjusted, fuses components other than fuses. If we have to talk about these amps, let’s quickly say that there are two types: those with protection and those without protection. Those without protection are easier to build (there are fewer components), but in case of a glitch, the speaker that is connected to it can be as much in danger as the amp.
“Miniature” amplifier for mobile use
You are just as likely to want to realize a miniature and autonomous amplifier, with power on battery or battery, in a separate case, or built-in a musical instrument or a home-made enclosure. To play in the street (amp with HP) or the train (amp with headphones). The main constraints posed by a portable amplifier (without mains socket) are the size, the weight, and the autonomy (compulsory power supply by battery or accumulator).
For such an amplifier, it can quite easily do something very small and have a good autonomy, because the power required is low and the current consumption reduced (a few tens of mW are sufficient). A power supply with a small battery 9 V (the famous rectangular 6F22) is possible, but for regular use, it is still better to opt for a battery (see paragraph Power Source). There are several types of headphone amplifiers, which can be classified into three main families:
– standard amplifiers of standard AOP type, capable of delivering a modest power;
– “power” integrated amplifiers specially designed for this purpose;
– amplifiers all transistors.
I tried these three types of circuits. AOP editing LM741 or NE5532 when I started when I was poor ( example1 or example 2 ), all transistors when I was easy and specialized integrated circuits (style TDA2822) once reached the status of a millionaire ( example). The first advantage of being a millionaire is that one can subcontract the realization of its printed circuits. It’s cool. For quite some time now, there are integrated audio amplifiers “dedicated headphones” that work in class D (digital), G or H, and whose performance is excellent (it allows greater autonomy). Some examples among others: TPA6139 (25 mW), LM4908 (120 mW), LM4980 (42 mW), MAX4410 (65 mW), MAX97200 (45 mW) and MAX9723 (60 mW). Some, such as the LM4980, operate at low voltage (e.g. 3V) and are ideal for portable applications. And as their target is precisely portable devices, they exist in miniature version – which bothers some and delights others.
Headphone amplifier with line output?
If in addition to the headphone output you want to have a line output to connect the gear on a more powerful external amplifier, know that again, it is possible. Two solutions at least:
- The sensor (microphone or piezo for example) is connected to an audio preamplifier, which itself is connected to a power amplifier. In this case, the line output jack is connected to the output of the preamp section, at the volume knob (see also my page Adding a line output ).
- The sensor is connected to an audio amplifier that already integrates the preamp. In this case, there is no intermediate socket available, and it is the amplified output of the amp that can act as a line output. If the amp is intended to work with a low-value load, then one must be provided at the output of the amp (for example 33 ohms) when this output is connected to a line input. This is not always required and you can try with or without to see if it changes anything. Be careful with the use of an amplifier circuit that works at low voltage (1.5 V or 3 V): the output amplitude may be insufficient if the amp that follows has a low sensitivity.
The use of a stereo jack connector is possible as an output, even if the amp is mono. It is indeed enough to connect the two pins of the jack which correspond to the left and right outputs (on the other hand, never do that on an input jack which would receive a stereo source: this would cause an unpleasant short circuit between the G outlets and D of the source).
Amplifier for HP
For a larger amplification intended to power an HP, the autonomy will depend on the capacity of the batteries or the battery, as well as the requested acoustic power. For the latter, it is better to have an HP that has a very good performance, to “hear loud without pushing the electric watts.” Indeed with an HP whose performance is poor and to obtain the same acoustic power, it is necessary to provide more electrical power, so more power consumed and reduced autonomy. For the same given acoustic power, it is preferable to use a 5 W HP that has a 100 dB / W / m efficiency rather than a 10 W HP that has a 97 dB / W / m efficiency. Ah, those watts that are so dear to us… If the needs are modest, we can very well be satisfied with an amplifier of a few hundred mW, which with its 0.5 W HP and its 9 V battery can fit in a pack of cigarettes. 1/4 “jack connector included. To play in the street, it is not ideal it must be recognized, and there must be a little more, hence the need (often) to use a battery if you want to have a comfortable autonomy (lead battery in general because of good ratio capacity/weight/ price). This always surprises when you say that an amp of about 10 to 20 W can be enough to play in front of a few people. If it is a good amp with a good HP, yet it is quite possible.
You have certainly already heard about these famous classes of amplification that we designate with a letter of the alphabet: class A, class AB or class D. In the case of a portable application, we can trust the small amplifiers working in class D, which are the digital type. When I speak of a digital audio amplifier, I am not referring to analog amplifiers with a digital input (so with digital/analog converter) but power amplification in “all or nothing.” Class D offers an excellent energy efficiency (greater than 90%) and allows us to miniaturize the amps considerably because the heating is weak and for the “weak” power can be dispensed with a heat sink (for very high power, such a sink is still required). In short, with a class D digital amplifier, you have the power for lower power consumption (and therefore longer battery life) and less weight/footprint with an equivalent power amp class AB. Here are some examples of low-cost modules that may make you want to try announced powers are unsurprisingly little detail): In the photo sequence, from left to right: 2 * 15 W amp, 12 V power supply (6 V to 15 V): about 4 euros 2 * 20 W amp, 12 V power supply (12 V to 24 V): about 6 euros 2 * 100 W amp, 24 V power supply (12 V to 24 V): about 16 euros It is true that some digital amps found at a low cost sometimes offer a curious sound (So little we have HP good enough to judge). I think we will always find good and bad in analog, as well as digital. But we will debate this another time because it is time to take out your soldering iron!
Source of food
Whatever type of amplifier you decide to make (transistors or integrated circuits, class A, AB, or D), power is of great importance. Its quality directly depends on the quality of audio reproduction. A bad power on a very good amp gives disastrous results. An amplifier works mostly on a relatively low average power but has peaks of vigorous power when it is necessary to reproduce bass drum beats (most of the sound energy is in the low end of the spectrum, at least for music normally constituted because with synths you can find everything). The power supply must be able to provide high current at all times, and it must respond well to both high and low frequencies. Yes, I know, it’s a continuous diet. But the continuous voltage must just be continuous and remain as much as possible. And with poorly designed or unsuitable power supply, the output voltage may make pretty yoyos (brownouts and superoscillations) as soon as it is asked a little more than the average. For a fixed use, the power source is generally the sector, and for nomadic use, it is easier to clutter batteries or batteries.
Fixed / domestic use: mains power
A mains power supply can be of a linear or switching type. For low power audio amplifiers, I recommend a linear power supply with step-down transformer, diode bridge, and (large) filter capacitors. For high power amps (several hundred watts) I recommend a switching power supply, not for its overall quality, but its price, size, and weight. But the idea here is not to debate the best type of power supply. The filter capacitors of a linear power supply (just after the diode bridge) often have a fairly high value (several thousand uF at least, not uncommon to see tens of thousands of us) and are sometimes found in “packets. “
Nomadic use: power supply by a battery (s) or battery (s)
The advantage of a battery (s) or battery (s) is that it is not likely to cause humming due to poor filtering of the residual 50/100 Hz. But most importantly, it allows making portable an amp. A battery power supply ends up being expensive to use, so it is often better to use one or more rechargeable batteries. There are several chemistries (Pb, Ni-Cd, Ni-MH, Li-Po) and the choice will depend a lot on the capacity/time desired, the desired charging speed, and the overall size of the capacity. One important thing: Batteries, if misused or loaded by an improper charger, can explode. It is therefore essential to adopt a charger adapted to the battery used.
It may be interesting, in some cases, to use a DC / DC voltage converter to raise the voltage supplied by the battery or batteries, to have a higher power. An amplifier designed to operate at a voltage of 12 V to 24 V can indeed deliver a higher power under 24 V that can be expected with 12 V. Using a 12 V battery with a DC / DC converter 12V / 24V, we can gain power … but lose autonomy since the output of such a converter is never 100%. Beware of low power converters the first price that does not support high current calls; it will protect and cut the output (loud or strong distortion ensured).
Decoupling, regulation of preamp stages (mains or battery)
The amplification section sometimes requires strong currents and this is not without repercussion on the supply lines, which has a parasitic ohmic resistance, admittedly weak but very present. Voltage variations can thus be observed at various points of “power,” and it is not necessary that these variations are seen by the preamplifier stages, under penalty of “loopbacks” and parasitic oscillations at more or less low frequency. To avoid this phenomenon, we always take care to “decouple” the sensitive stages by adding a filter circuit or an additional voltage regulator ( additional details ). Example with preamp and amp under the simple power supply
Information like that, but do not repeat it especially: some audiophiles feed their preamp and amp hifi lounge (fixed use) with lead car batteries …
Off-topic? Hmm … not so sure. Do you know that an amplifier can behave strangely and deliver a signal that you can not hear because well beyond the upper limit of the audio band (which is 20 kHz when you are young)? Yes, an amplifier cut to reproduce sounds in the audible band can also deliver without our knowledge an inaudible parasitic signal, which will not really be reproduced mechanically by the transducer (HP) connected to it (except possibly a tweeter) but which nevertheless may overheat your device and/or significantly deteriorate the quality of the useful audio signal. Why address this topic? Because you are now old enough to understand. When does this phenomenon occur? Well for example when we experiment with a new circuit layout, or when assembling the components on an experiment board, with long wires. Oh, I’m not saying that you will be systematically confronted with this type of problem. But as Murphy’s law spares no corner of the planet, it may happen to you at least once in your life. So as much as you are warned at the earliest. Is there a way to be aware of such an unwanted phenomenon? Yes: abnormal consumption of the amp without modulation (audio source) at the input, rapid heating, very sharp distortion of the useful signal even at low volume. The safest way to find/confirm the problem is to visualize with an oscilloscope what happens at the output of the amp. A signal of high amplitude at a frequency greater than 20 kHz whereas no signal is applied to the input, shows that a parasitic oscillation has taken its place. Removing it is not always obvious, it is not adding a capacitor of 1000 up to the power supply terminals that everything will necessarily be better (I tell you that because it was a time when I was convinced). This is part of the game. Some amps, especially those based on integrated circuits, are very ticklish, and the track layout (width, length, position) is very important. Whenever possible, use existing CI drawings. Was convinced). This is part of the game. Some amps, especially those based on integrated circuits, are very ticklish, and the track layout (width, length, position) is very important. Whenever possible, use existing CI drawings. Was convinced). This is part of the game. Some amps, especially those based on integrated circuits, are very ticklish, and the track layout (width, length, position) is very important. Whenever possible, use existing CI drawings.