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Old 3rd May 2011, 07:19 PM   #1
Ivchikx is offline Ivchikx  Latvia
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Default Need help with Power Supply for TDA7294

Got 2 pairs of TDA7294 bridged with single reticifier bridge and 10000uF on + & - rails in the same pcb (all, two 10000uF caps, four TDA chips etc on the same PCB - so I can only feed all theese four chips from one PSU). Sounded ok, but got also transformer hum audible trough speakers. Transformer is torodial 600VA 2x24V secondaries. Then I removed existing bridge and wired up external recitifer with two diode bridges. Sound changed to muddy trebles and mids, but transformer hum weren't there any more. This got me tottally confused...

I'm a noobie to electronics, so I ask you for dumbed down version for good TDA7294 bridge power supply. How mutch what, why and where etc... Thank you in advance.
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Old 5th May 2011, 07:02 PM   #2
Ivchikx is offline Ivchikx  Latvia
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Two days, no answers?
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Old 5th May 2011, 07:28 PM   #3
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first look for traditional problems, like RCA connectors not being isolated etc. two small cables with aligator clips on each side can be used to connect various earth points until a quiet configuration is found.
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Old 5th May 2011, 09:18 PM   #4
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Possibly the ground (or V+/V-) are taken from a point on the PCB between the rectifier and 10 000 uF capacitors, as opposed to after the capacitors?
Picture of the PCB?
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Old 7th May 2011, 07:27 AM   #5
Ivchikx is offline Ivchikx  Latvia
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As you can now see (check atachments), original power supply for theese 4 TDA's exist from 25A bridge (not seen, but you can see where it was) and 2 10000uF caps. I have a 600VA 2x24V transformer, so I connected transformer as seen in diagram (see atachments). As I searched trough varios posts here, I made a conclusion, that this crappy PSU may be the reason why I could hear transformer hum from speakers.
Also I tried 2 brige config with more caps between transformer and pcb, but sound was muddy and terrible in result (that is why you can't see in pictures original bridge but see soldered vires instead, AC vire is there to supply with power speaker protection ciricuit, that has it's own one diode reticifier).
Attached Images
File Type: jpg frontPCB.jpg (314.0 KB, 412 views)
File Type: jpg backPCB.jpg (248.5 KB, 350 views)
File Type: jpg trnsfrmr.jpg (13.0 KB, 337 views)
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Old 7th May 2011, 12:00 PM   #6
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something is pretty messed up if you had dramatic sound change just by wireing an external diode bridge. Please do post picture of the toroid + wireing of the bridge too.
i see the schematic, but real life picture "as it is" helps a lot more.
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Old 7th May 2011, 01:57 PM   #7
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Default something to try:

In the image below, the purple tracks or areas show where the 50/60 Hz ripple currents are. These tracks will radiate mains noise and need to be as short as possible and run in tights pairs with the corresponding current return track.

Blue shows how the power should have been taken from the filter capacitor. Possibly similar issue on other side.

The board uses a ground plane, but segregated to form a kind of star ground. The signal ground should also be on this star, not on the leg of one filter capacitor as it is at the moment.


So many things not ideal and possibly a few errors that should be fixed. But to answer your current question, you can try to put the original bridge back and solder the transformer wires directly onto the green dots. Bring all four wires to the board, tightly twisted together, and put the 2 grounds on the original ground screw terminal.


PS. No sign of a Zobel network? What is the red wire (hope it is not the 4th transformer wire)?


EDIT: Bridged amplifier, so please ignore the corresponding yellow lines from the outputs back to the star ground shown on the image.
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File Type: jpg backPCBmarkup.jpg (175.0 KB, 333 views)

Last edited by discrete; 7th May 2011 at 02:10 PM.
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Old 8th May 2011, 05:58 AM   #8
Ivchikx is offline Ivchikx  Latvia
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Yesterday soldered and wired all back as originally was (reading your replies only now). As It turns out, the messed up thing was the speaker I used for testing - somehow it lost its tweeter (will open up speaker and check why today, it's a 8ohm PA speaker with 800W max rating), so only lows and mids were audible, so all my testing with different configurations was pointless (tried to add more caps between bridge and chip, got 4x 6800 uF 80V laying around).

Quote:
Originally Posted by discrete View Post
PS. No sign of a Zobel network?
Sorry, but what is a Zobel network?

Quote:
Originally Posted by discrete View Post
What is the red wire (hope it is not the 4th transformer wire)?
The red wire is AC only to supply the onboard speaker protection circuit, wich has its own one diode recitifer.

So... What can and should I do to get rid of transformer noise?... I'll try to record sample of it today and post here.
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Old 8th May 2011, 12:09 PM   #9
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Try this as a first step:

Quote:
Originally Posted by discrete View Post
But to answer your current question, you can try to put the original bridge back and solder the transformer wires directly onto the green dots. Bring all four wires to the board, tightly twisted together, and put the 2 grounds on the original ground screw terminal.

The idea is to minimize the length of tracks that carry 50 Hz current pulses. (Between the transformer and the big filter capacitors).

Make sure to keep the transformer wires away from the circuit board, the speaker wires and the input wires. Speaker wires and input wires should also be twisted together (each input with its own ground wire up to the circuit board).

The zobel network is a kind of snubber to suppress high frequency oscillation and noise pickup on the output terminals. It is shown on the 7294 datasheet for a standard amplifier (ST calls it a boucherot cell). ST does not show it for the bridged circuit on the same datasheet, and mentions it is optional for the standard circuit, so you may be ok without it. Without an oscilloscope, you may never know though.
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File Type: png tda7294.png (64.2 KB, 306 views)
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Old 9th May 2011, 06:32 AM   #10
Ivchikx is offline Ivchikx  Latvia
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Haven't got a chance to try anything yet. Posting the noise sample.
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File Type: zip AmpNoise.zip (135.3 KB, 84 views)
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