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Chip Amps Amplifiers based on integrated circuits

TDA7293 distortion/power supply issue
TDA7293 distortion/power supply issue
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Old 22nd December 2017, 10:51 AM   #11
Mark Whitney is offline Mark Whitney  Netherlands
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The onboard capacitors look too small for the current demand of a 4 Ohm load. What is the power rating of the transformer?
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Old 22nd December 2017, 01:27 PM   #12
Auzman is offline Auzman
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Quote:
Originally Posted by scottjoplin View Post
See my edit above, there is no need to remove the rectifier and capacitors.....
Ok, I will try it as soon as I hook another rectifier and capacitors.

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Originally Posted by Mark Whitney View Post
The onboard capacitors look too small for the current demand of a 4 Ohm load. What is the power rating of the transformer?
The transformer is rated 300VA. I know that is not sufficient for the whole 8 channel to work, but the problem is that with only 1 channel (for which the trafo would be sufficient) the issue remains.
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Old 22nd December 2017, 01:55 PM   #13
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by Auzman View Post
Here is the link of the item.

I "listened" to all the boards and they behaved the same. I also supposed that the TDA7293 was fake ones or similar, so I bought 2 parts from RS and replaced them. Same thing...
To start, 7293 is an analog chip, NOT digital. So the seller is either a moron or a bald faced liar.

I wondered how these hucksters could build these boards cheaper than the cost of the parts, so I bought a few. The answer - used parts, cutting corners, parts so cheap I wouldn't bother picking them up off the floor. I had to massage the boards and add parts to get them to even work, and I can design and build amplifiers from scratch.

It's a shame that these are marketed towards amateurs, who would have limited resources to figure out why they don't work as advertised.

I couldn't even salvage $5 worth of usable parts from the boards I bought. Waste of money and effort.

In contrast is our own member Tom, whose boards are pricey but reflect the actual cost of research and production. No free lunch.
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Old 22nd December 2017, 02:07 PM   #14
asuslover is offline asuslover  Romania
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TDA7293 distortion/power supply issue
We all bought stuff off ebay, but this is how we learn (not to buy again), if the amp is 5$ but an tda7293 by itself is more than that , then how do you think is going to sound? Parts are not good for sure, I would remove all the components from one boards and replace with new ones.
See what's happening.
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Old 22nd December 2017, 02:22 PM   #15
Fast Eddie D is offline Fast Eddie D  United States
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Originally Posted by asuslover View Post
We all bought stuff off ebay, but this is how we learn (not to buy again), if the amp is 5$ but an tda7293 by itself is more than that , then how do you think is going to sound?
Exactly. It is all a learning experience.

To be fair, I use a lot of used parts. But I would never misrepresent what they are. I almost never sell stuff anyway.

I took some 7293 chips out of a dumpster boombox and built a very nice power amplifier with them. I used nice capacitors, metal film resistors, a used transformer and rectifier. The performance still surprises me. Less than $20 worth of new parts.

Much more satisfying than buying junk off of ebay.

Quote:
Parts are not good for sure, I would remove all the components from one boards and replace with new ones.
See what's happening.
That's what I had to do to get the fleabay board to work. The chips worked fine. Not worth the effort, at all.

Last edited by Fast Eddie D; 22nd December 2017 at 02:24 PM.
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Old 22nd December 2017, 02:30 PM   #16
Auzman is offline Auzman
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Thank you all for the suggestion.

As I said, I tried to replace the TDA7293 with a supposed genuine one, since RS Components is IMHO a certified seller. Price is less than 4$ for one piece. If you go to the source (that PCB costs 0,1$, I wouldn't spend more for 1000 pieces...) the cost could be like that.
Philosophycal discussions apart, fact is that this is an incredibly simple board designed according the manufacturer specifications.
I embrace scottjoplin suggestion regarding the rectifier and I will proceed on the replacement. There are no other active components or semiconductors excluded the IC itself, the rectifier and a 1N4148 on the mute/off circuit.
I wrote here to understand if there was something I didn't considered.
On ebay there is plenty of junk and plenty of almost good things, I do know it.
These boards have been bought bearing in mind that the only useful thing I need was the PCB even if the TDA7293 was a fake one, so I proceeded like that.
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Old 22nd December 2017, 03:47 PM   #17
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by Auzman View Post
These boards have been bought bearing in mind that the only useful thing I need was the PCB even if the TDA7293 was a fake one, so I proceeded like that.
A used, but genuine, chip is almost certainly better than a fake. This assumes that it hasn't been damaged.

A used, genuine chip will show evidence of being washed. A fake will probably have at least one glaring defect. The most obvious defect is that the heat sink tab will not be flat.

I don't know how long these chips can last, but mine came out of a unit that was manufactured 18 years ago.
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Old 22nd December 2017, 03:50 PM   #18
Fast Eddie D is offline Fast Eddie D  United States
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You can damage the chip with haphazard disassembly. Short all the leads together before removing or installing the chip, or before removing ANY component from the board. The shorting wire should be the first thing you install, and the last thing you remove. 24-28 gauge wire threaded through the pins should be fine.
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Old 23rd December 2017, 06:25 PM   #19
phase is offline phase  United States
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Thanks for the tip, would explain one of my past experiences with a chip amp.

I actually enjoy swapping out the garbage caps and tuning up the sound on lo-buck stuff, is frustrating though when the boards have botched circuits however. I got one where the polarity was marked wrong for the incoming power. I guess that cost extra to do correctly!
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Old 23rd December 2017, 07:03 PM   #20
Fast Eddie D is offline Fast Eddie D  United States
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The TDA chips are particularly vulnerable to damage (even from cold solder joints) due to its feature of having the voltage supply of the output stage and the voltage supply of the driver stages separated. Power up one without the other and it's toast.

Used correctly, they've very robust. No thermal runaway is a nice feature.
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