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Cheap Chinese BT-309A Power Amplifier

I got myself one of those cheap Chinese audio power amplifiers. Here, you can watch the all-important unboxing, a demonstration of the amplifier in use, and a teardown.

 

As mentioned in the video, the bad points of the amplifier are:

  • Indoor Bluetooth range is pitiful (my phone manages about 3-4 metres, can't make it from one side of the living room to the other).
  • Indoor radio reception is non-existant. Outdoor radio reception is rubbish with a lot of hiss and audible hum.
  • And if you were dumb enough to believe the EIGHT HUNDRED WATTS then, well, you got suckered.
I'm not an audiophile, so I'm going to make no comment on the quality of amplification, other than to say that it is much better than the dinky little CD player that I used to use. It may be that there is distortion. I wouldn't know. It sounds okay to me, but given that I mostly play MP3s via Bluetooth (so being screwed twice with the lossy codecs), I wouldn't consider myself adequately suited at judging such things.

The front of the amplifier
The front of the amplifier.

For the plus points:

  • It's cheap. The whole thing cost me €32 from Amazon with Prime delivery. You can probably get it even cheaper off eBay or if you're willing to wait for it to come from China.
  • Reasonably good bass. There's a bass and treble adjustment to fine tune to your preferences.
  • Seems quite powerful. When listening in the living room, I usually have the volume somewhere between 1/3rd and 2/5ths. I've never cranked it up, don't want to risk damaging the speakers (or my ears).
  • Dual phono input (though, note, they're wired together so you can only have one active at a time).
There are also two microphone inputs, an echo feature, and a separate mic volume in case somebody feels the need to try to sing along with Floor Jansen. That might be a plus point for people who can either sing, or think they can...though in the latter case it might be a negative point for everybody else.

The rear of the amplifier
The rear of the amplifier.

Did I mention that it was cheap? If you're looking for an inexpensive way to hook a phone to some big-ass speakers, it's the best option. There are cheaper amplifier boards, but these tend to be literally amplifier boards, leaving you with problems like finding really weird voltages to feed into the thing. Plus a box to put it in.
This, on the other hand, is really simply a "plug it in, hook it up, start playing music".

 

Looking inside, there are three circuit boards.

The three circuit boards
The three circuit boards.

The board at the back (closest in the photo) is for the power conditioning and the primary amplifier.
At the front, the upper board is the processor with USB, SD/MMC, Bluetooth, Radio, and so on.
The lower board is the analogue board used to control volume, the two tone controls, the microphone mixing and echo.

You will notice, I hope, the complete lack of shielded wires.

The rear board caters for the power input and for audio input and output.
The power input can come from two sources. There is a 12V barrel socket (centre positive) intended for connection to a vehicle cigarette lighter. There is also a mains cable where the 230V AC goes through a transformer to be stepped down to 14V AC which is then rectified. It's worth noting that the power switch on the front switches the mains, so it'll do nothing if running the amplifier from the 12V input.
The rectified power is then passed through a 7805 5V regulator to generate the power for the rest of the circuitry.

The audio inputs are two pairs of photo sockets, marked on the board as CD and DVD, but on the back as simply AUX1 and AUX2. These are wired together, by way of 5.6kOhm resistors on both the signal and ground. This is normal, as a line input impedance is usually around 10 kiloohm, so 5.6 twice would be close enough (there's no standard 5kOhm resistor, the next usual level down is 4.7kOhm).
The flipside of this is that you cannot play audio on two devices and use the amplifier's controller to choose between them. You can only play one aux audio source at a time.

The audio output is a CD7266CZ power amplifier.

The CD7266CZ amplifier
The CD7266CZ amplifier.

This is a Chinese clone of the Texas TDA7266, which is a dual bridge 7W + 7W amplifier. David Pilling made that comment on the video, but it's pretty obvious given it's the same number! This amplifier runs on 3V to 18V. I've not looked at the board to see if it is hooked up to the main 5V supply, or if it runs from the higher voltage input (which is either 12V~13.8V when connected to a car, or ~14V from the AC transformer), however given that the amplifier has a massive heatsink attached and the 5V regulator has only the dinky heatsink that is built in, I suspect this may run from the higher voltage input. The two wire links that you can see are the power connections. Looking at the photos, it would appear as if the rectified higher voltage passes through two meaty electrolytic capacitors and then into the amplifier.

A bridge amplifier, by the way, means that the speaker is wired between the two halves of the amplifier circuit (the positive part and the negative part), as opposed to being wired between the amplifier output and ground. This is cheaper to produce, but brings the risk of phase problems (and poor bass) if the speakers are not wired up correctly. To be fair, though, the manual points that out quite clearly.

 

Looking to the front now.

The two boards at the front
The two boards at the front.

The lower board is the analogue board, the upper one is the digital board. Let's start with the digital board.

The digital board
The digital board.

The digital board is based around a JL BP01126-26A4 device. This entire board seems to be similar to the ~€12 2846-MP3FMBT module which appears to be the catalogue number of the item in an Italian version of Maplin (Futura Elettronica).

As is to be expected, there's no information on the chip. I've looked around JL Audio but it doesn't mention parts. It does seem that the (American) company moved production back from China a few years ago, perhaps their Chinese counterpart is still making some of the underlying tech? Or maybe the entire chip is a clone of something?

There's a part of me that would expect to see this containing some sort of 8051 core, however it supports media (FAT16 and FAT32) using SD and USB, as well as Bluetooth. I'm wondering if this is something the 8051 cores can do, or if we're looking at something with a bit more grunt like one of the MIPS cores commonly used in Chinese IP cameras, or some sort of low-end ARM core? Something along the lines of an ARM9 hooked to a custom DSP for the MP3/WMA decoding ought to manage that.

The FM antenna is a simple bit of wire between the digital board and the antenna on the back. It is routed alongside the transformer and all of the AC wires.
The Bluetooth antenna is a simple bit of wire soldered to the end of the printed track Bluetooth antenna. Somebody in China needs a slap because the printed track antenna is tuned, and you can't just deal with the "it's in a metal box" by soldering a piece of wire to the thing. Radio at those frequencies really does not work like that.
That wire was also routed alongside the AC part. I unclipped it and routed it a little bit away from the transformer. Reception is still kind of rubbish, but slightly less so. ☺

There is really only one input from the rest of the unit, that is the AUX inputs which are probably hooked up as the AUX input to the chip. Then, the controller selects which of these inputs is actually in use - the line (aux), the radio, Bluetooth, or the USB or SD devices. Once the selection has been made, audio is routed out of the chip and down to the analogue board.

The lower board handles all of the analogue side of things. A master volume, tone control (split into bass and treble), and all of the microphone mixing. There are three small chips. The first two (4558P and 4558D) are Op-Amps, most likely for the microphones. Oddly, they are made by different companies - the KIA4558P and the JRC4558D, which are otherwise pretty much the same thing.
There's a ridiculous amount of information on the JRC4558 at Electrosmash (note - this may give you "Get Me Out Of Here" warnings due to an invalid certificate).

The other, larger, chip is a CD2399P which is a reverberation processing chip (likely a clone of the Princeton Technology Corp's PT2399), which samples the input audio to digital form (with a 44Kbit memory). The digital part has an adjustable delay time, with the system clock being based upon a voltage controlled oscillator in order to make delay adjustments simple. The digital data is then converted back to analogue, et voilà an instant echo/reverb facility. Delay times range from 30ms to 340ms.
There's a ridiculous amount of information on this device at Electrosmash (note - this may give you "Get Me Out Of Here" warnings due to an invalid certificate).

 

As the heat of the day... Yes, heat. Last week it was thunderstorms and hail and struggling to reach two digits. Today, just a week later, wall to wall sunshine and topping 26°C! ☺
Anyway, as the heat of the day fades and the bitey bugs come out to play, it's time to upload this, feed the cats, and then brew up some tagliatelle.

Whilst I plan to use this amplifier (kinda why I bought it...!), it's interesting to peek inside and see what makes it work.

 

Oh WTF? A bunny just legged it out of the garden, nearly leapt onto my lap, before realising AAARGH! HUMAN!. I'm not sure which one of us was more surprised.
Definitely time to go in. I understand being attacked by bugs as it's that time of day, but being attacked by bunnies is crossing the line.

 

 

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David Pilling, 1st June 2021, 15:51
Hey comments help no? I'd guess TDA was a European prefix and the web points to it being an ST microelectronics part (French/Italian company). Back when I were a lad, power amplifiers had big electrolytic capacitors to block the DC through the speaker. Seems a bridge design removes the need for that. Will be amusing if one side of the bridge ever goes short circuit. 
Class AB design, which is something, not computerized Class D. 
Guess you could check the voltage on the chip or check continuity between power pin and input voltage. Another plus for bridge is ability to work from lower voltages. I'd guess less distortion with higher voltages. 
I'm enjoying the new bold font for comments - making my thoughts emphatic - really in my head they could do with triple bold. 
As I said however nonsensical the comments all count for the algorithm of success or failure. 
Zerosquare, 1st June 2021, 23:49
Besides removing the need for the big electrolytic coupling capactiors, bridge amplifiers have another advantage: compared to the "one side grounded" ones, you get twice the output voltage range for the same supply voltage. 
 
The CD7266CZ is definitely not powered from the output of the 7805 regulator, otherwise it would waste a lot of power as heat and severely limit the output power of the amp for no good reason. 
 
Regarding your speaker's impedance: it's actually frequency-dependent, and the value given by the manufacturer is an average. What you measure with the multimeter is the DC resistance (i.e. the impedance at 0 Hz), which is not the same thing.

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Last read at 05:25 on 2021/06/13.

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