Detailed look at the Auna CD-708 amplifier - with circuit overview
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- čas přidán 17. 07. 2021
- This 2014 Auna CD-708 amplifier has quite an interesting circuit - but it is broken because this circuit might not be as stable as it should be...
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CORRECTION: After taking out the main circuit board and investigating, it turns out that the main amplifier is NOT based on parallel bridge circuits. Instead of two parallel bridge circuits for two channels, there are four simple bridge circuits for four channels. There are four speaker outputs, pair A and B. Even though this is a stereo amplifier, every output has its own dedicated amplifier channel. This is another strange design choice, as most of these amplifiers will probably never have any speakers connected to the pair B outputs. The headphone output is connected to the pair A outputs, so really the two extra amplifier channels are somewhat of a waste. Knowing that pair A and pair B have independent amplifiers, I tested the pair B outputs, and as expected, there is no fault in these channels.
Excellent amplifiers I have had two for 10 years nearly with no problems and no noisy standby or any other problems powerful smooth and reliable and very very cheep as for running hot most amplifiers that I have had run hot as well in hot weather obviously they feel slightly hotter to the torch because of the ambient temperature me personally living in the UK 119lbs will not buy you a better amplifier on the planet unless you have one givien you or you inherit one
🤗🤗
Be good to see an update on this when its made
If you remove one of each pair of parallel ICs (one push and one pull) and the amp should still work, although with less ability to deliver current. Doing this first with the first two then the other two should narrow down which pair is bad then substitute one from the bad pair for the good pair and you should find out by elimination which IC is bad. Then just install one single 1875.
There is no technical reason why you can't bridge-parallel these chips. Well, the authentic ones anyway. The manufacturers would rather you move of to the next powerful IC instead. Not sure why they didn't just use an LM3886 instead of all these chips. They probably know the counterfeit ones are junk.
If you decap one of those 1875 chips, the die size for an authentic LM1875 is 3.7x2.7mm.
Contrary to that I can perceive a certain risk..in this setup..
In one paralled bridge
1) all 4 power modules have their own , indipendent negative feedback paths..so 4 sets of f.b. components..& all may be minutely diff. from each other....
IMHO it would be better to have some common f.b. path taken from post current limiting resistor ( spkr. terminal ) and then split into the individual f.b. paths....
In conventional amps , there are discrete parallel transistors but with only one ( common) f.b. path..and overall lower f.b. gain...( even if f.b. loop is open).
In case of power amp modules..
it is in reality an opamp..
Ich erinnere mich an einen Test eines AUNA-Verstärkers (keine Ahnung mehr, wo das war), bei dem Testgerät war nur die Hälfte der Leistungstransistoren angeschlossen. Also nicht schlampig verarbeitet, sondern die waren tatsächlich überflüssig. Wenn ich die Innereien deines Verstärkers sehe, könnte es gut sein, daß man die Kühlkörper schon vor der Montage voll mit Transistoren bestückt hat und bei der Montage nur die benötigten angelötet hat. Seltsame Lösung, aber vermutlich so eine Kuriosität, bei der die Transistoren einfach zu billig waren, um die Kühlkörper individuell zu bestücken.
If you wanted to make a working integrated amp, you could pull that hinksink out, drop a class-D amp board in there, and bypass the existing amp circuitry. I don't think that would be too difficult but given everything else in the preamp section I don't think I'd bother. As you state in your video, the parts individually are more interesting / useful.
I´m used to repair cheap amplifiers that uses ICs like this one here in Brazil, where I live. In my experience, in most cases this type of noise is caused by bad soldering on the IC´s contacts. As theses chips usually works at high temperatures, as time goes by the solderings become less and less reliable. So, when noise appears, the first thing I do is to remove all the soldering of the ICs and replace with fresh soldering. Chances are great the noise will go away.
Check my comment above, I have Same noise (but not so loud) on my Marantz amp. I will try to disassemble it and check for soldering. Obrigado, cumprimentos de P
Portugal!
@@Gajodareactividade use a heat gun on the soldering points see if it fixes your problem.. if so resolder all points to fix it long term
i thought it was a vintage amp from the video thumbnail. it looks pretty cool though
It was interesting to see Texas Instruments IC's in it, and not just one, but a bunch.
Yes, some of the ICs were good brand name components, others however, like the amplifier ICs, were dubious rip-offs. Strange combination...
That looks like a prime candidate for a 'FrankenAmp!' 🤣🤣
I almost thought that it was a discrete amp, which would be pretty crazy for that money. I would also reuse the face plate and motorized vol pot.
Just put a home made, better amp in there.
What's the purpose of those relays before speaker connectors? I have a Sony amp and from time to time, the sound is cut off following a click sound from these relays. I have to turn the amp off for some minutes and then it works again. Thanks.
speaker protection in case the amplifier goes bad and sends a constant current to speakers. Other purpose is to prevent speaker pop noise at start up.
Finaly video about this brand who scared me....AUNA.....i was always mixted whit Skytec and other cheap bad low amplifier....
At first glance, I liked the esthetics. But then I noticed the black circles for the remote control and the headphones...
Sometimes heating up a component with a hair drier and then freezing can help identify a troublesome component... has worked for me once or twice...presumably you tapped around the board for dry joints also?
Heating the chips is not necessary, they get really hot just with the amplifier turned on already. I have not really checked very much for bad solder joints yet.
One giant aluminum heat sink, Sony did the same thing if I am not mistaken mounted to a flimsy sheet metal heat sink.
Why not replace the power amp with a class D amp module or a circuit based on the LM3886 and use the existing preamp (which should be decent enough) and power supply? Remove the branding and you'd have a nice half-DIY integrated without having to re-cover the faceplate and with plenty of useful features.
I have thought about replacing the power amp, but it is also tempting to see what happens if I replace the bad chips with some genuine LM1875s. These cheap amplifiers certainly have a lot of potential for all kinds of DIY projects.
That's actually not to bad for an amplifier on a budget!
It would be interesting if you figure out what is actually wrong with it even if you decide to pull it apart!
The only time that I have heard such a noise like that is from a noisy resistor which can be found via an oscilloscope or even better a signal tracer!!!, I have heard some single transistors make that noise! And bad solder joints!!!! At least have a look for broken solder joints...
This thing working might be worth something to someone somewhere especially if it's advertised with a picture of it's insides! Even though it's cheap, there's a lot worse out there! Dare I say it! It would probably do well with a recap LoL.... Just kidding 😂
And dirty switch’s… I had cassette recorders making that noise, coming from the record/play switch and spraying it with contact cleaner and repeatedly pressing record fixes it 😉
@@samuelfellows6923 oh of course but in this case it's not a switch or potentiometer as we saw in the video that the input to the power amplifier was disconnected and the sound was still there!
So basically the challenge is still there and it's in the power supply section! I would love to see it fixed, just the process of going through it to find the problem is very enjoyable! Another enjoyable thing to watch is testing bad capacitors! I don't know why, it's rather pointless but people love watching it being done!
Guitar amplifiers that take 12 volts will have switching power supplies to boost the voltage of soft in a good design I don't think of switching power supply should be a problem
auna and electronic star are both brands of Berlin Brand group in Berlin.
I was just about to suggest to gut it out and put an entirely new amp in it.
LM386 with a negative feedback to the buffer chip could demonstrate excellent results.
Looks like the speakercircuit is defect. Maybe the speaker relays need to be replaced or the capacitors. I had the same problem on an AKAI amp.
Did you finish watching the video before commenting? I think I gave a pretty clear explanation of cause of the problem in the video.
I would be tempted to make it into a Wolf in Sheeps Clothing.
Put a nice big class A amplifier inside the case.
Cheaper to just buy a room heater!
Such noise is most likely from bad/ dry/ loose solder joints....include liberal spray cleaning of front panel switch selectors switches also.
I would recommend desoldering one of the parallel outputs on both channels..and use at lower output & so thers a higher temperature margin...besides 4 spare 1875..
I have not yet checked for bad solder joints. As the noise is not affected when disconnecting the signal to the main amplifier, there are no contact problems. Removing one cheap from each parallel circuit to improve stability is indeed an option.
@@DrCassette I would like to add one instance of such
a dry & loose joint issue on my own amp...This bothered me hell of a lot @ times it played quite as a mouse.. , other times it was irritating..after redoing all suspect joints it still persisted ...until finally one day on suspecting an adjacent component. the real culprit was found..though it joints looked good..one lead was dry....now no such problems ever since..
Nice case for a diy amplifier.
I think the same
My marantz PM6006 started to develop same noise, although not so loud. as I increase the volume, the noise disappears, at level 0 is where it's more noticeable. I can only hear the noise without any source on playback, and on quieter music parts. Quite annoying, actually...
The problem on your Marantz amplifier will be different though, likely more complex, as your Marantz is much better built than the amplifier seen in this video.
Hello...
I'm your old subscriber.. could you plz use TDA2030 or TDA2050 ic's in this amplifier.. it could more powerful than before 🙏
Most TDA2030s are fakes these days. 7/8 watts if you are lucky.
I remember Auna. They used to make horrible PA systems with massively exaggerated ratings, yet they sold hundreds of thousands of them. I've noticed they rebranded their products to 'Malone', presumably in an attempt to shake the bad reputation.
Interesting, I have not yet seen any Malone branded equipment...
what is the job of the microprocessor ?
The microprocessor basically converts the operations of the momentary pushbuttons on the front and the operations of the remote control into control signals for the electronic circuitry.
The exterior doesn’t look bad at all. Why not built a new amplifier and reuse the good parts of it, like all the input jacks and the controls on the front. The knobs look quite nice too.
Re-using all the existing control buttons on the front would pretty much mean to also re-use the microcontroller circuitry. At that point it would be easier to leave everything as it is and somehow repair the main amplifier. The knobs are made from plastic, they look good on camera but in real life they are quite cheap.
Of course the Performance of These device is mit very high. The supply ans capacitors are looking very small dir an amplifier. But it will be also interesting what the Problem ist. It looks like a Bridge solution and a compare between the Others or Others half Channel should be easy.
Also, the heat sinks are basically useless. The web the the ic's are mounted to will do 99% of the cooling. The heat has to travel down to the bottom then to the successive webs. Very inefficient. Basically useless.
Tragedy, but for someone may be is perfect.
Hallo, ich würde da auch keine Kosten und Arbeit mehr reinstecken.
Manchmal muss man eben nein sagen....
Danke, und nette Grüße aus dem Oldenberger Land ☺
Wzmacniacz jest naprawdę słaby marny obudowa jest fajna to fakt wsadzić coś w klasie T albo jakiś moduł na układzie TAS 5630 będzie super wzmacniacz
a u n a
Ill bet its dry joints on the ic's. A very cheap fix....i should know as just fixed one for free. Lead free solder doesnt like too much heat
I never checked the underside of the board for bad solder joints, but I still have this amplifier in storage, so one day I certainly will :)
AUNA by Electronc-star (.hu) (in Hungary)
Thanks for the info!