The tale of the development of active wireless speakers HiFi. And about. Part 1

Prehistory

From time to time I practice walking around electronics and audio stores. It is interesting to see what progress has reached, what is being done new. Well, they don't do much new things. I mean a fundamentally new. Improved element base. There are new chips in digital processing. Well, in general, circuitry has remained the same circuitry. Only here our audio equipment is completely gone. Japan, USA, Korea, China, Italy ... but not Russia.

Why?

I was engaged in monitoring Runet open spaces in order to understand what we are doing from audio in general. As it turned out, we are doing a little and mostly expensive. It is expensive not in the sense of "not worth the money", just this technique is designed for special connoisseurs. There are no quality speakers for ordinary workers and peasants.

Well, the second point. Behind the window is the age of wireless technology. Everything that is possible to transmit without wires is transmitted without wires. The audio signal (analog) is not transmitted to me as a lossless quality. And the code is not a problem at all. It will only be necessary DAC code in the speaker itself.

Formulation of the problem

Definitely need to do something.
To understand what exactly to do and how - a specific task is needed. If you want - a technical task.
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1. The entire analog part is done according to the classics. Well, respectively, is taken unless the modern element base. Well, a little circuitry is sucked;
2. The audio signal is generated from the code directly by the speaker itself. Those. concept - sound card inside. This gives complete independence of the sound quality at the speaker output from what it has at the input. Dear PC or half-dead laptop. The sound depends only on the quality of the DAC (and its implementation) in the AU.
3. Wireless code transmission. You can create a radio channel with a bandwidth for transmitting sound Fs = 44.1 kHz and a 16-bit stereo width using the CC8520 chip. It remains to calculate the antenna at 2.4 GHz.

It seems something like that.
We do.

Codename IO

"Io" is the satellite of Jupiter. One of the three. Well, there was such a goddess in Greek mythology. Well, now and this acoustics will be.

In general, there is, of course, an idea to do something super-compact and cheap, and vice versa. Honest three-strip. Let's see how it goes.

After setting the task and making a decision, a rather lengthy and tedious process of constructing a functional circuit (although, no, the functionary is done quickly) and selecting electronic components, finding circuit solutions, drawing circuits, wiring boards, etc., takes place. and so on (and this is really a long time).

In short, during this process I understood for myself.

1. We have practically no integrated circuits. As for DACs, high-quality opamps, power amplifiers, transceivers, everything is sad;
2. But it’s kind of like we can do “loose” - capacitors, coils, transformers, transistors, etc .;
3. Well, we also have our own dynamics. Joyfully;
4. By organizing the development process - nothing can ever be done on those dates that you plan ahead. Any time needed to double.

I will give a functional diagram (I’ll omit all the principal ones, there are quite a few of them, they are big, and I think they will change too):


Figure 1. Schematic diagram of the AU

A little explanation for each node

Linear power source. Toroidal transformer type TTP100. Two secondary windings. Gretz diode bridge. Well, plus the capacitor to earth (common point) for each output voltage. All the classics. This is the primary source that connects to 220 V. It feeds the power amplifiers (for amplifiers just below).


Figure 2. Linear power supply

The source also includes two down stabilizers.

The first to power the preamplifier is ± 13 V. The stub is also made according to the classical scheme from a textbook on npn and pnp transistors K815G, K814G and zener diodes KS213B. Below is an approximate diagram of a stabilizer of a positive voltage of 30 V - 13 V. In the branch of the negative, the circuit is similar, it differs only in the orientation of the Zener diode and the diode. Well, the npn transistor is replaced by pnp. The output voltage is regulated by resistors R1 and R2.


Figure 3. 13 V Stabilizer

The second stabilizer is +3.3 V. Represents an LDO of type LT3062 or equivalent. The output voltage is regulated by a resistive divider. We feed the DAC and transceiver. Typical scheme of "datashit", I will not give.

Receiver and transmitter do on CC8520. The heart of wireless transmission. Without it, the figure would have to drive over USB. This is a high quality integrated audio transmitter for the 2.4 GHz ISM band.
The textiles instruments PurePath wireless platform is the thing for wirelessly delivering high-quality digital audio.
At the same time, due to the internal signal processing, there is no interference between the audio stream and the WiFi data stream (transmitted on the same carrier frequency).
The transmitter supports 2 audio channels. The CC8520 works autonomously and can be used without or in conjunction with an external MCU. The external processor connects via the SPI interface and controls some operations.
The CC8520 chip connects easily with other audio chips and DSPs (using I2S and DSP / TDM). You can transmit a signal with a sampling frequency of up to 48 kHz and a bit depth of up to 24. This is quite enough for the transmission of most of the flags and other lossless ones. Well, audio cd is also quietly placed in this framework.
To view the scale of disasters of what can be stowed on cs8520, attached the picture below.


Figure 4. Application of the CC8520

The scheme of the transmitter - is done in the form of a flash drive. Installed in a USB computer or laptop.


Figure 5. USB transmitter

Below is a diagram of the receiving side. Located in the AU


Figure 6. Receiver

Matching circuit / external amplifier of radio-frequency signals is not used (crossed out with a red marker in the diagram). Those. The matching circuit with the antenna will of course be, but not in the form of a separate microcircuit / power attendant.

DAC WM8940 - Wolfson sigma audio DAC. Quite a common thing, widely used in portable devices and external sound cards Fiio x3, ibasso dx50, irirver astell kern, etc. If you believe the audio forums - DAC is very good. The sound is transparent, analytical, even, smooth, clear, “adult”. I used to use large audio players based on it. Fiio and iHiFi. Sounds of course very cool. Not soapy. Purely. Clearly. Recently, however, tired of carrying bricks. Went to the side of the portable.
The inclusion scheme of the DAC is close to the standard.

Pre-amplifier , also known as a low pass filter. Made on a two-channel opamp texas burn brown opa2134.

Power amplifier TDA7293. Also quite an interesting chip. Can give up to 40 ohms load up to 40 W with almost no distortion (SOI <1%). And the theoretical maximum is 100W.

Columns - two-way. Volume slightly less than 40 liters. The case is a tree.


Figure 7. Speaker appearance

Summary of the first part

At this stage, almost all electronic components are assembled (those who are not assembled are ordered). Divorced almost all printed circuit boards. And some of them have already been collected and tested. Now we order the rest.
Next step setting, testing, bringing to mind.

Well, in general terms, I think that the very idea of ​​a wireless speaker with a built-in sound card is what the audio industry of multimedia and relatively inexpensive high-end will come to (sooner or later) (although they may not be the only ones). IMHO. Everything in this world tends to innovate in one way or another. Wireless technology is what the audio equipment lacks at this stage.

There are many unsolved problems in audio at the moment:

1. Creating small-sized speakers that play the entire audible frequency range. In my opinion, the future in this regard is behind the technique of predistortion and the technique of compensating for elastic vibrations arising inside the box (if we are talking about speakers). Pre-emphasis is a purely digital theme. Compensation - the revision of the constructive box. Well, in general, it seems to me that in the future evolution should go not along a diffuser path, but a hybrid between electrostatics and iso (or ortho) speakers. Still, from the speakers already almost everything that could be pulled out, in terms of quality, was pulled out. And the film still has potential. And of course, ionophones are also a direction in which little has been done.
2. Well, if now the speaker rules the world, then you need to fight with its principal "sores." Inertia of the diffuser and coil. Well, narrowband there too.
3. Disposal of all signal wires, since in fact, they do not carry any function other than transport. For this, for example, you can make a receiver and a DAC in each column;
4. Wireless power - I don't even know yet;
5. Transformation of voltage from 220V to lower voltage. Here, I think, the future belongs to semiconductor circuitry.

We just decided to combine the three systems into one, and at the same time make an attempt to solve one of the above problems.

But it all starts small.