Ultra-cheap CDR-King Digital Car Stereos
I found this out at a local forum and it got me curious. This is just a basic review plus internal pictures of the two units I was able to get my hands on.
They have a variety of car headunits which are very cheap. I was curious as to how it was built and what you can get for that price.
We will start with this one. It lists for about Php680 which is around US$15. I bought it to use as a workshop blaster so I have tunes while I'm working on something. It has been powered up via the alternative power system since I bought it last Nov 2010 and surprisingly, it still works so far. A friend has bought one and used it in their car and reported engine noises when running and also said it drained the car battery when the car wasn't used for a while. Testing showed over 100mA draw even when radio is off. ('real' car stereos have less than 1mA.) On my battery powered system, the only noise I can hear besides the tunes is some digital communications noises coming from the SD/USB reader.
Specifications according to website:
I don't know how they got 4 channels but the unit only has a two channel amplifier IC (TDA2005) probably
both left channels are paralleled internally and so are the right channels
RCA output was measured to have maximum output of 1.2Vrms at 30% distortion. if you want less than 1%, you'll get 0.5Vrms or less.
Here are inside shots of the stereo when I took it apart... Chassis overview:
Back of the front panel:
LCD pins aren't that well soldered. ( I wonder how those will stand to the vibrations it will experience in a car?):
There is no electronic volume IC. Volume is controlled by a pot, balance is controlled by another pot. No bass and Treble or any other form of EQ.
Vehicle interface and Power amp board:
Main MCU board:
System regulator (7805 based):
Nice board to board connector :
AM/FM Tuner module (I suspect it uses the same chip used for personal AM/FM receivers):
RCA jack (who needs shielded wiring even right beside the power inductor!!):
Here is the other unit that is Php580 which is around US$13.20. Dad bought this one so there is music in the livestock building in the farm. After about a few weeks of use, took it back home for me to look at since it suddenly stopped working. (Pic was taken after repair was done.):
Specifications according to website:
When I opened it up, I was in for a surprise...
I wonder how it produces "2x 30W" with two 2.75A fuses in the power wires and two TO220 chip amps (possibly similar to LM383T's), one per channel and heatsinked through the back panel which is steel not high conductivity aluminum. Current draw is about 40mA when ACC is off and goes up to over 100mA when ACC is on. I'm guessing chip amps are powered off the ACC line.
System regulator (also 7805 based):
Bottom of main PCB (which is phenolic based and single sided):
Tuner module. possibly same chip as previous unit:
Muting transistor copper pad torn so the quality personnel added a blob of solder to 'fix' it up:
Back of panel PCB (chip is possibly main MCU as there are no other LSI chips in the radio unit. Also, try counting
the number of interconnecting wires in this unit ):
Back of PCB behind volume pot:
Now there's your problem! A solder bridge across ACC and GND wires caused the fuse to blow and stop the unit from functioning. I have no idea what the quality assurance personnel was doing at the time (at this cost, I guess there is NO quality assurance! ):
It seems the solder bridge is making intermittent contact as it is easily bent over. It is possible that temperature fluctuations made it contact the other path and shorted out. Removed the solder bridge and replaced fuse and everything worked again.
29 Dec 2015:
After 5 years in use, surprisingly, it still works. But it still has its quirks.
The power amp section is basically the same as this except for a few resistor value changes, and the DC blocking caps (C10 and C11) are 470uF instead of 2200uF.
Combine this with the fact that the front and rear speakers are in parallel (2ohms total) and the RCA output is just tapped from a resistor divider across the speaker outputs, you get absolutely no deep bass, even when you add a subwoofer!!!
The power amp section is also run straight off the B+ so it is ON 24/7. No shutdown pin, no muting, no relays. Which explains the massive standby current of the radio.
I had made a bunch of these modules and this one uses the TB2936 4ch power amp IC. It has much better specs in absolutely everything compared to the original TDA2005.
It also has mute and standby pins so there will be lower standby current and no pops.
The power amp module grafted into the unit.
Conveniently, the heatsink has provisions of mounting a different power amp chip and there are holes that are perfect for the TB2936. The original power amp circuit also has the B+ running through a jumper so removing this cuts power out completely on that section.
Lots of trace cutting on the main output connector and adding a transistor inverter circuit for the mute control line and it all worked. The RCA output is tapped directly off the volume pot output for better "sound quality".
It did work pretty well, actually.
Sound quality improved greatly compared to before, much better bass too. And also, no more pops and thuds when switching on/off or changing tracks.
I have yet to measure standby current drain but I'm pretty sure it has dropped considerably.
Oh, and I found the original receipt when I bought this, stuffed underneath the radio.
Dated: Nov 22 2010. I guess it is well past its warranty period.
Page created and copyright R.Quan © 04 Oct 2011.