Low-cost, low-bandwidth multi-zone amplifier

You can not do a minute without your favorite music? And what if you need to do a variety of household chores, and from the kitchen or bathroom you can hardly hear your favorite tunes? Make it louder? Anyway, the sound quality will suffer, and the neighbors will start knocking on the batteries and calling on the door.

A similar problem may exist in the office. Programmers and other knowledge workers usually need silence to concentrate, while in a warehouse or in a production room, the volume should cover an increased level of background noise.
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The way out is simple - to make such a speaker system that will sound in each room at the required volume without loss of sound quality. At first glance, this task seems to be costly and intractable. In fact, everything is very simple. In the article proposed for reading, I will talk about the budget approach for busy people. Budget as for the spent funds, and for the time of implementation (since it is known that time = money).



Multi-band amplifier can be assembled from ready-made cheap nodes that are sold on eBay. You can also connect to it mixed pairs of speakers accumulated over the years of operation of various audio systems. Here's what the block diagram of such a device might look like.



Connecting four power amplifiers to a sound source at once can greatly affect the level and quality of the signal. For decoupling the linear output of the sound source and distributing it to four channels, an amplifier CE CE Labs AV 400 amplifier is used. On eBay, you can pick up a used device of this type costing about $ 10. Despite the fact that the new one is much more expensive, there are many offers of devices in excellent condition on the market. The device is equipped with conveniently located RCA connectors and is made in the form of a flat panel that is easy to fasten to the wall. If you need to make more than 4 zones, there is also a 7 channel version of CE Labs AV 700.

The following ready-made modules available for purchase on Ebay can be used as a low-frequency power amplifier:

Chip	Power	Price in USD	Class *	Nonlinear distortion factor,% **	A photo
TDA7297	2x15W	2.76	AB	0.1
TDA7377	2x20W	9	AB	0.03
TDA2030	2x18W	ten	AB	0.1
PAM8610	2x10W	four	D	0.1
TPA3123	2x25W	7.4	D	0.08
TDA7492	2x50W	9	D	0.05-0.1
TA2024	2x15W	eleven	T	0.04
TDA7498	2x100W	13	D	0.03-0.1

* AB - analog linear, D and T - pulse / digital amplifiers.
More information about the classes of amplifiers can be found here:
Classification of electronic amplifiers

** Nonlinear distortion factor is given for the working range of the output power of the amplifier and frequency 1 kHz. Information extracted from relevant datasheet chips.

Pay attention to the amplifier TA2024. In a sense, the release of the TA2020 all-digital amplifier in 1998 by Tripath was a breakthrough in the circuitry of low-frequency pulse amplifiers. The developers managed to combine high sound quality and economy. Despite the fact that Tripath could not stand the competition and left the market, the T-class amplifier line continued. The four round blue details in the picture are the output filter coils.

Each reader who wants to implement the described system at home can choose an amplifier according to his taste, guided by the criteria:

• Cost Our project is a budget, therefore, we have to think about the price of components first of all. This parameter is best of all at the amplifier on the TDA7297 chip.
• Output power. It should correspond to the size of the room and the power of the selected speakers. Exceeding the permissible power will lead to distortion of the sound or even burnout of the speakers. The above table shows the “RMS” amplifiers rating (“Root-Mean-Square”), i.e. the actual average power of the undistorted sound. If the amplifier is driven into the limiting mode by applying too strong a signal to the input, it can deliver twice as much power. Therefore, the RMS speaker rating should be no less than the corresponding indicator of the amplifier. If there is a lot of bass in the content being played, then it is recommended to choose a column with a double RMS value.
• Sound quality. Generally speaking, this characteristic is quite subjective. The amplifier based on the TDA2030 chip is positioned by Chinese manufacturers as “Hi-Fi”. The amplifier class is also a conditional concept. It is important to understand that Class A and AV amplifiers have traditional analog circuitry, and all others are digital or pulse. Experts are constantly in search of some kind of "warm" and "tube" sound and do not recognize any analog signal transformations.
• Efficiency. Operation of a multi-zone system should not be expensive either. If this parameter is more important to you than sound quality, then choose a class D, T amplifier. Their efficiency approaches 90-100%, that is, not a single watt of electricity will be wasted.
• The presence of volume control sound. Of the many options for ready-made modules, only those that already have a sound control are suitable for us. Otherwise, it will not be possible to adjust the individual volume level of the sound in each room.

For my project, I decided to choose TDA7297. It has the lowest cost, high sound quality, it is compact and equipped with a sound level control. The power of this amplifier is enough for the sound of my small premises (10-15 square meters). The board has all the necessary terminals and the connection of the amplifier does not require the use of a soldering iron. The only drawback is that the 3.5 mm headphone jack is used to connect the input signal. It is not very reliable and can become a source of interference and dissatisfaction with the project as a whole. For me, this disadvantage is not so important because I plan to experiment with this amplifier and will continue to solder some components.

To connect the distribution amplifier with 4 power amplifiers, we need 4 cables that have a 3.5 mm stereo plug on one side and 2 x RCA connectors on the other. Another such cable is needed to connect the input of the distribution amplifier to the signal source.

A power unit from an old computer was used as a power source. The moral obsolescence of modern personal computers is much faster than their physical deterioration. Old power supplies can no longer be used to power a new generation of motherboards. But throwing them away is too wasteful. Amplifier TDA7297 at maximum power needs a current of about 2 Amps. Thus, the average power supply for the PC is enough to power up to 8 amplifiers.

The power supply is built on a pulse pattern and operates at high frequency. In order to reduce power interference, an additional shunt capacitor of relatively small capacity, of the order of 500-1000 microfarads, should be installed at its output. It should be connected to the terminal block inside the enclosure of our device as close as possible to the amplifiers. The greater the capacitance of the capacitor, the better, less noise and less negative effect of voltage dropping at peak current consumption.

All components are mounted in the box from the telephone service. It is convenient in that it provides easy access to the entrails, and in the closed state, it completely shields the guiding on the amplifier. From the previous content left only terminals to which the outputs of the amplifiers and their power supply circuit are connected. In the box cover are drilled several holes for mounting amplifiers. The contact of the case must be "grounded" at -12V (Common black wire from the power supply). Here’s what the complete system looks like:

The whole structure is hidden in some secluded corner (for example, in the closet). After that, adjust the sound level for each room and enjoy your favorite sound atmosphere. This is how the system looks in its working, that is, closed state:

Let's sum up. As a result of simple manipulations with ready-to-use inexpensive modules, a convenient and sufficiently high-quality acoustic system for four areas of the dwelling was obtained. The work took several hours, but now the pleasure will be long enough.

If readers are interested, the next article will discuss how to modify the amplifier to control it from a phone or other gadget with the Android operating system.