Microphone preamp circuit for your home recording studio

The microphone amplifier circuit, described in this article has two little secrets that allow you to record vocals in your own home studio with almost the same quality as in expensive, professional studios.

Just follow the text.

A microphone for a home studio

Cardioid polar pattern

There is a simple and very effective solution for recording voice or vocal in your home studio – using a dynamic cardioid microphone. Just because:

  • Firstly, you will have no need to take special measures for noise insulation of your apartment;
  • Secondly, you will have no need to soundproof the rear space behind the microphone for getting rid of the sound reflections of the room, because the dynamic cardioid microphone suppresses all the side and rear sounds;
  • Third, you will have no need to organize additional power sources as in the case of condenser microphones.Микрофон shure sm58

For our purposes, Shure sm58 (or something similar) will be O.K. For example, I have used for years a perfectly working microphone Beyerdynamic Opus39s.

Of course, there exist a large number of various solutions for voice recording. For example, you can make a good soundproof in our apartment, buy expensive condenser microphone with a large diaphragm, but this solution is not so simple, and much more expensive. In addition, a microphone amplifier for condenser microphone needs to be a little different, and we'll talk about in another article.

Buy or make by your own?

A microphone preamp, made ​​by your own hands, has three main advantages over those models that you can buy in the appropriate store:

  • Price.
  • Perfect adaptation to the specific task.
  • The quality of the sound.

Price

So, the price of the products, sold in the store, except for the cost of the components, includes a fee for brand, advertising expenses and profit for manufacturer, wholesaler and retailer, plus shipping costs. It turns out that in the shop version of an amplifier only the case will cost more than the entire microphone amplifier, made by your own.

In addition, there is a number of consumer qualities, which almost all the manufacturers follow in order to achieve a certain universality for microphone preamps possible uses. After all, the developers face the task to achieve a maximum compatibility with all the microphones and the equipment, which they will work with.

This leads to the fact that the microphone amplifier circuit acquires considerable redundancy, like various modes, protection, controls and indicators. The more details has the device, the more they influence on the sound quality, and not always in a good way.

Adaptation to the specific task

In a home recording studio, the microphone amplifier usually works with one particular microphone, in constant conditions, and always performs the same task. This means that the majority of universal possibilities of the purchased preamps are not needed. But we can focus on the highest quality is what we need, ideally adapted own design for a specific task.

Sound quality

What is the difference between a good microphone amplifier for recording vocals from the usual one? First, a good preamp does not give any distortions to the sound, and at the same time it creates the most optimal matching for the microphone to obtain the highest possible quality of the sound conversion into an electrical signal.

It is difficult to hear with a normal preamp. To hear the quality of the microphone amplifier, you need to work with it in the real world, applying a wide variety of vocal processing to an already recorded sound. All the flaws appear at the high levels of compression while attempting to put the vocals into a dense mix.

The sound quality of modern microphone preamps, especially made by the brand marks, usually causes no specific complaints. However, the natural tendency of all the manufacturers to reduce the price of the product leads to the fact that all the characteristics correspond to the formally declared ones, but the components can be inexpensive, just for a marketing purpose.

It is not always possible to check the components of the preamp before you buy it.

Since you did not buy a preamp and did not work with it properly, you do not know its quality. As for your own made preamp, you can easily make changes if something is not pleasant.

And more.

What you should not do

The thing you do not need is a microphone amplifier with some kind of a "special" sound, which is often touted by many manufacturers. This is for a lot bigger budgets for special occasions.

In practice, the specific coloration is not always needed, but to get rid of it, if it exists, is very difficult. And the possibilities of modern DAW (digital audio workstation) are enough to ‘color’ the sound.

A microphone amplifier circuit on the IC

A microphone amplifier circuit is shown in a picture. There are two secret, which were mentioned at the beginning of the article - matching of the microphone with the microphone amplifier and the circuit of the operational amplifier.

Схема микрофонного усилителя на ОУ

Matching

The input resistance of the microphone preamp is much lower, than the accepted standards are. From the general theory of electrical engineering, we know that the maximum power transfer between the generator and the load occurs with the equality of their resistances. So let us provide the input resistance of the microphone amplifier equal to the resistance of the microphone. However, we will use no coupling capacitors, to not cause a signal asymmetry, phase shifts and additional sources of distortion.

To get rid of all kinds of interferences, including interferences from mobile phones, we need a symmetrical microphone connection, which means that the microphone amplifier should have a balanced input.

Differential amplifier specifically designed for such occasions is an ordinary operational amplifier. Its input is symmetrical, differential, with the spreading input impedance of 600Ω. 3Ω resistor R2 does not matter, it is put in here mostly for correct image of the differential amplifier.

You can connect any DYNAMIC microphone. However, the higher quality is better. Normally, the resistance of such microphone is from 200Ω to 600Ω, and you can make the sum of R1 + R3 equal to the microphone resistance (when R1 = R3).

The most important thing is that such an inclusion through damping of the mobile system of the microphone eliminates the sound coloration with parasitic resonances of the microphone itself, allowing to obtain a clean, smooth sound. Then, while processing vocals, you can do everything with the sound. It is supple; you have no need to fight it, eliminating any overtones.

In addition, a low-resistible input’s noise immunity is simply gorgeous! I had no problems recording vocals in a small room where there were more than 20 mobile phones!

We should pay attention to the fact that the matching, according to the book, is just the measure of the parameters and the noise, first of all. And we do not care about the noise. Using the IC with the parameters below 10nV / √Hz, we can forget about the noise. Noise did not disturb me, even when I was using an IC TL071, which made noises 18nV / √Hz. In actual work, the noise of the room is even bigger, and everything depends on the skill of the sound engineer.

However, TL071 sounds pretty good, unlike the world-known NE5534.

The operational amplifier circuit

The second secret of this design is a diagram of the operational amplifier, which has a very large impact on the sound.

This microphone amplifier has a chip OPA604.

The best sound is when you do not think about the sound at all, thinking only of voice and music. Here it comes with OPA604.

It is so clear, that even with the wildest levels of compression, no artifacts get out.

The secret, apparently, is that the OPA604 is a single-cascade operational amplifier, specially designed for professional audio applications. (OPA604 PDF) The number of cascades directly affects to the transient characteristic and the sound in general inversely proportional. The more cascades - the better are the objective characteristics and sound is worse.

It remains to supple the microphone amplifier circuit with controls, and provide the amplifier with the normal, clear power.

Thus, we put the amp control to the feedback circuit. This allows you to keep a zero output resistance of the microphone amplifier, which allows eliminating the impact of the connecting cable from the preamp to the computer on the sound.

For power organisation there is a marvelous voltage regulator TL431. Absolutely clean, with a differential resistance of about 0,2Ω. I like it very much. With it there are no problems. Set and forget.

That's it, the circuit is ready.

I put the ordinary "Jack", although the XLR Input is more correct.

The body has no special requirements. Thanks to the balanced input, compactness and low-resistance mounting strap, this microphone amplifier does not require careful screening.

You just have to solder this microphone amplifier, turn it on and forget that you have ever had a problem of getting a high-quality sound from a microphone in your own home studio.

Sergei Shevgota