Budget low-frequency multi-zone amplifier

You can’t do without your favorite music for a minute? But what if you need to do a variety of household chores, and your favorite tunes are already hard to hear from the kitchen or bathroom? Make it louder? All the same, the sound quality will suffer, and the neighbors will start banging on the batteries and calling on the door.

A similar problem may exist in the office. Programmers and other mental workers usually need silence to concentrate, while in a warehouse or in a production room, the volume level should be blocked by the increased level of background noise.

The solution is simple - to make a speaker system that will sound in every room with the desired volume without loss of sound quality. At first glance, this task seems costly and intractable. In fact, everything is very simple. In the proposed article, I will talk about the budget approach for busy people. The budget both for the spent funds and the time of implementation (since it is known that time = money).



A multi-zone amplifier can be assembled from off-the-shelf cheap units that are sold on eBay. You can also connect to it a motley pair of speakers accumulated over the years of operation of a variety of audio systems. Here's what a block diagram of such a device might look like.



Parallel connection of four power amplifiers to a sound source at once can greatly affect the signal level and quality. To isolate the linear output of the sound source and distribute it over four channels, the CE Labs AV 400 distribution amplifier was used. On eBay, you can pick up a second-hand device of this type worth about $ 10. Despite the fact that the new one is noticeably more expensive, there are many offers of devices in excellent condition on the market. The device is equipped with conveniently located RCA type connectors and is made in the form of a flat panel that is easy to screw to the wall. If you want 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 cu	Class *	THD,% **	A photo
TDA7297	2x15W	2.76	Ab	0.1
TDA7377	2x20W	9	Ab	0.03
TDA2030	2x18W	10	Ab	0.1
PAM8610	2x10W	4	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	thirteen	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

** The harmonic distortion coefficient is given for the operating range of the amplifier output power and frequency 1 kHz. Information is extracted from the corresponding datasheet chips.

Pay attention to the TA2024 amplifier. In a way, the 1998 release of the all-digital TA2020 amplifier by Tripath was a breakthrough in the circuitry of low-frequency switching 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 line of class “T” amplifiers was continued. The four round blue details in the image are output filter coils.

Each of the readers who want to implement the described system at home can choose an amplifier to their taste guided by the criteria:

• Cost. Our project is a budget one, therefore, we have to think about the price of components first of all. This parameter is best for 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 table above shows the “RMS” rating of amplifiers (“Root-Mean-Square”), i.e. the actually average power of the undistorted sound. If the amplifier is driven into the limiting mode by supplying an input signal too strong, it can give out 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, it is recommended to choose a column with doubled 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, AB amplifiers have traditional analog circuitry, and all the rest have digital or pulse. Connoisseurs are constantly in search of a certain “warm” and “tube” sound and do not recognize any analog signal transformations.
• Profitability. Operating a multi-zone system does not have to be expensive either. If this parameter is more important for you than sound quality, then choose a Class D, T amplifier. Their efficiency is close to 90-100%, that is, not a single watt of electricity will be wasted.
• The presence of volume control. Of the many options for ready-made modules, only those that already have a sound control installed are suitable for us. Otherwise, it will not be possible to adjust the individual sound volume level in each room.

For my project, I decided to choose TDA7297. It has the lowest cost, high enough sound quality, it is compact and equipped with a sound level control. The power of this amplifier is enough to sound my small rooms (10-15 square meters). All necessary terminals are installed on the board and connecting the amplifier does not require the use of a soldering iron. The only drawback is that a 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 from the project as a whole. For me, this drawback 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 obsolescence of modern personal computers is much faster than their physical deterioration. Old power supplies can no longer be used to power new generation motherboards. But throwing them away is too wasteful. Amplifier TDA7297 at maximum power requires a current of about 2 amperes. Thus, the average power supply for the PC is enough to power up to 8 amplifiers.

The power supply is built on a pulsed circuit and operates at a high frequency. To reduce power crosstalk, it is necessary to install an additional shunt capacitor of relatively small capacity at the output of the order of 500 -1000 microfarads. It should be connected to the terminal block inside the case of our device as close to the amplifiers as possible. The larger the capacitor, the better, fewer pickups and less negative effect of voltage sagging at peak current consumption.

All components are mounted in a box from a telephone service. It is convenient in that it provides easy access to the insides, and when closed, it completely screens the interference to the amplifier. From the previous contents, only the terminals to which the outputs of the amplifiers and their power supply are connected are left. Several holes were drilled in the box lid for mounting amplifiers. The housing contact must be “grounded” at -12V (Common black wire from the power supply). Here's what the whole system looks like:

The whole structure is hiding in some secluded corner (for example, in the pantry). After that, we adjust the sound level for each room and enjoy your favorite sound atmosphere. This is what the system looks like in working, that is, closed state:

To summarize. As a result of simple manipulations with ready-made inexpensive modules, a convenient and fairly high-quality acoustic design system for four zones of the living room was obtained. The work took several hours, but now the pleasure is enough for a long time.

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