Detailed look at the Auna CD-708 amplifier - with circuit overview 

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.

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.

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 😂

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.

Finaly video about this brand who scared me....AUNA.....i was always mixted whit Skytec and other cheap bad low amplifier....

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.

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.

That looks like a prime candidate for a 'FrankenAmp!' 🤣🤣

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.

At first glance, I liked the esthetics. But then I noticed the black circles for the remote control and the headphones...

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.

I would be tempted to make it into a Wolf in Sheeps Clothing. Put a nice big class A amplifier inside the case.

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.

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

Nice case for a diy amplifier.

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

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.

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 🤗🤗

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 was just about to suggest to gut it out and put an entirely new amp in it.

auna and electronic star are both brands of Berlin Brand group in Berlin.

i thought it was a vintage amp from the video thumbnail. it looks pretty cool though

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.

One giant aluminum heat sink, Sony did the same thing if I am not mistaken mounted to a flimsy sheet metal heat sink.

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...

Be good to see an update on this when its made

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.

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.

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.

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?

Hello... I'm your old subscriber.. could you plz use TDA2030 or TDA2050 ic's in this amplifier.. it could more powerful than before 🙏

a u n a

It was interesting to see Texas Instruments IC's in it, and not just one, but a bunch.

what is the job of the microprocessor ?

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 ☺

Tragedy, but for someone may be is perfect.

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

AUNA by Electronc-star (.hu) (in Hungary)