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TDA7293 distortion/power supply issue
TDA7293 distortion/power supply issue
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Old 22nd December 2017, 08:40 AM   #1
Auzman is offline Auzman
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Default TDA7293 distortion/power supply issue

Hi everyone.

I built a 8ch amplifier with TDA7293 boards from ebay. My idea is to drive a 3 way system with a digital crossover and in the future add a 2 way central speaker. The board from china is the one that integrates the rectifier and the capacitor, so it can be connected directly to the transfomer output.
I choose a 2x30V toroidal transformer since the TDA7293 is specified to work up to +/-50V.
There began the issues.
I tested the amplifiers directly on a normal full band speaker and it sounded noisy on lower frequencies.
Then I started checking what was going on with the oscilloscope.
With no load on the output and with a sine applied at the input, the amplifier works well and clips at 30Vrms. It shows some distorsion from 20Hz to 50Hz but this is not the main issue.
Testing it with a 4 ohm resistive load, the amplifier seems to go in protection mode when the output goes over 6Vrms with frequencies from 50Hz up to 20kHz. From 20Hz to 50Hz it seems to start oscillating either!
I tried a 8 ohm resistive load and things went better. The amplifier showed the same little distorsion on low frequencies (20-50Hz).
For further test I used a variac to lower the main supply to 20-0-20V and the amplifier went stable also with a 4 ohm load.

I tried to add more caps on the rails (it has 2200uF normally fitted) but no luck.
The schematic is the same as the application note from ST, apart the input cap that is 1uF.

My conclusion is that the TDA7293 cannot work with power supply more than +/-27V max +/-30V but many projects use it with higher voltages!

Is there anyone that can give me a suggestion on it?
The only idea is to use a 18-0-18V transformer... But that's not worth!

I will try to post some pictures later...

Thanks!
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Old 22nd December 2017, 09:36 AM   #2
sevenup2278 is offline sevenup2278  Netherlands
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Do you use a big enough heatsink?
Also having several bridge rectifiers in parallel could cause noise issues.
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Old 22nd December 2017, 09:55 AM   #3
Auzman is offline Auzman
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There are two heatsinks, made of standard aluminium 250mm x 100mm x 40mm, each one with 4 boards. Since it will be used on a multiamplification system, I considered that only the two boards (one for each heatsink) connected to the woofers as high heat producer. There is also a forced cooling system with 60C threshold.
By the way, the tests mentioned above are with only one board at once.
The issue is there with only one board connected or with all boards.
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Old 22nd December 2017, 10:01 AM   #4
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Auzman View Post
the amplifier seems to go in protection mode
What happens exactly?
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Old 22nd December 2017, 10:10 AM   #5
Auzman is offline Auzman
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Quote:
Originally Posted by scottjoplin View Post
What happens exactly?
The sine amplitude is reduced, but not switched off. I believe that is related to a current limitation. Also temperature is rising quickly. With a thermal gun I saw that the external case arrives at 95C - 100C. The isolation with the heatsink is guaranteed by a mica washer foil and silicone paste has been used.
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Old 22nd December 2017, 10:22 AM   #6
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Auzman View Post
By the way, the tests mentioned above are with only one board at once.
The issue is there with only one board connected or with all boards.
Is every board behaving the same way individually? Can you provide a link to the boards you bought?
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Old 22nd December 2017, 10:28 AM   #7
Auzman is offline Auzman
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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...
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Old 22nd December 2017, 10:36 AM   #8
scottjoplin is offline scottjoplin  Wales
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I have a board with those chips, it also had rectifier and smoothing capacitors on board, I didn't have a problem with overheating, but it was a bit noisy until I removed the rectifiers and capacitors and used an off board supply. I suggest you try that. As a quick test there is no need to remove the rectifier and capacitors, just connect your DC supply to the AC supply inputs
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Last edited by scottjoplin; 22nd December 2017 at 10:40 AM.
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Old 22nd December 2017, 10:39 AM   #9
Auzman is offline Auzman
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Well, that is an interesting suggestion! I will try that and report back.
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Old 22nd December 2017, 10:42 AM   #10
scottjoplin is offline scottjoplin  Wales
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See my edit above, there is no need to remove the rectifier and capacitors.....
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