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Old 14th January 2003, 12:33 AM   #1
Elkaid is offline Elkaid  Canada
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Smile Question about this 7294 amp

Hi everyone,
I built this TDA7294 amp several weeks ago and I've got some questions indirectly related to it.

1. I'm using a 500VA (2X30V) toroidal tranformer to feed two of these modules. When I switch it on, my fuse keeps blowing up so I replaced them by a Slo-Blo one but is that normal that I must use an 8A/250V one ? Is 2X30V too much considering that the TDA7294 is rated at +-40V ? (I'm simply using the transformer and a diodes bridge connected to two amp modules.)

2. Do TDA7294 have tendencies to blow up or explode for unknown reasons ?

3. I blew up a speaker because I had DC on one of my Output. Do you have any idea on how to protect my speaker from DC ?
(At least, my blown speaker was a test speaker retrieved from my car hehehe)

Well, thanks in advance for your help !
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Old 14th January 2003, 12:42 AM   #2
halojoy is offline halojoy  Sweden
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Question More & Better Information, PLEASE ...

The Layout of Cicuit + Power Supply.
How the parts are placed. Is a PCB-card used?

This is just as Important for function
of an IC AMP.
--------------------------------

So it is not easy answering your questions
by only a schematic.

Better some pictures
and some PCB-layout daiagram.

THen I promise you will get more answers,
that CAN adress your problem

/just a little hint
from halojoy
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Old 14th January 2003, 12:56 AM   #3
Elkaid is offline Elkaid  Canada
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Ooops, sorry !
There you go here's the PCB

Hope this helps !
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Old 14th January 2003, 01:03 AM   #4
Elkaid is offline Elkaid  Canada
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And this is my amp....
Pretty simple.... almost not protected at all (only a fuse on the main). I hope to improve this amp using all of your tips and tricks which could make it better

Thanks !
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Old 14th January 2003, 03:16 AM   #5
Elkaid is offline Elkaid  Canada
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Default How to protect loudspeakers ?

As you can see the Amp is far from finished !

So if you have any comments or suggestions on :

1. How to protect loudspeakers connected to this (or any amp), from DC or any damageable signals

2. Anyone aware of good softstart circuits ? (not a delay for speakers)

3. Is it "normal" that I must use a 8A/250V fuse on the transformer primary? Otherwise, the fuse blow when the amp is powered up. (transformer is 2X30V, 500VA)

4. Finally, should I connect my Amp's ground to the case and consequently to Earth ? (Yeah, I know newbie question !)

Thanks for your help !
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Old 14th January 2003, 03:22 AM   #6
JoeBob is offline JoeBob  Canada
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One way to protect the speakers from DC is a cap on the output, it's seen in many Class-A amps, but with your IC it shouldn't be needed. You said you blew a speaker, what did you change after it blew?

There's a softstart circuit over at sound.au.com .

Usually it's normal to use a slowblow fuse, try using the smalest value that doesn't blow. Charging caps takes alot of current, so switching on causes the most current to be drawn, although a soststart circuit would help that. Maybe use a thermistor like in the pass designs.
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Old 14th January 2003, 03:30 AM   #7
Elkaid is offline Elkaid  Canada
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Thanks !

A cap on the output ? Any idea of which value should I use ?
What will happen upon failure ? The cap will charge until the fuse blow up ?

Also, I'd like to know how to get rid of the little "hum" that I hear from the speaker when there's no input signal. Looks like it's 60Hz (Canada).
How can it be filtered ? Another cap ?

Thanks again !
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Old 14th January 2003, 03:36 AM   #8
JoeBob is offline JoeBob  Canada
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Well, I'd only use the cap if you're excessively paranoid about DC getting to your speakers. The cap will act as a highpass filter, so you want it to be a high enough value so as not to attenuate frequencies you want, somewhere around 5000uF would be fine. DC won't flow through a cap, so your speaker would be protected from it. Again, some say that a cap in the signal path can degrade sound quality, keep that in mind.

As for a 60Hz hum, it could be a ground loop. I'd check there first.
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Old 14th January 2003, 04:23 AM   #9
Elkaid is offline Elkaid  Canada
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Default a Ground Loop ?

Thank you for your help !

A ground loop ?
What can be the causes of a ground loop ?

If I take a look at my picture previously posted, the two module's ground (Green Wires) are connected to my transformer secondaries (Blue & Grey wires) at one point.

Should I connect my speakers' (-) at this point too ? Could it be the cause ?

Thanks again !
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Old 14th January 2003, 08:01 AM   #10
Vivek is offline Vivek  India
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Hi,
I want to know where I can buy TDA 7293 ICs. I am not aware of any good source in my city.
Indians on the forum, please help.
Cheers,
Vivek
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