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Old 16th April 2012, 09:58 AM   #1
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Default Problem with power-on and off

Hi all,
I build a small amp based on a TDA7240 chip. I followed the data sheet and the only difference is I didn't implement the standby switch.
This is the schematic of the circuit:

Click the image to open in full size.

And this is the PCB I made:

Click the image to open in full size.

The circuit is powered by a 9V AC toroidal transformer followed by a bridge rectifier providing thus an unregulated 11-12V DC to the circuit.

When I power-on or off the amp there is a bump not really loud but the woofer's cone moves suddenly by several mm. As can be seen from the schematic there is not a clearly marked + and - output, so depending how I connect the two output wires to the speaker, the movement is out at power-on and in at power-off, or reversed if I reverse the wires.

Apart from this on and off problem, the amp seems to work flawlessly, but I don't want to damage the speaker, what could be the possible cause and how can I avoid it?

Thanks in advance for any suggestions.

The complete TDA7240 data sheet is here: TDA7240, and the power switch is a simple DPST one that breaks both live and neutral followed by a fuse.

Ralf
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Old 16th April 2012, 10:21 AM   #2
AndrewT is online now AndrewT  Scotland
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I don't know how to, or if the amplifier can mute it's input/output when starting up or shutting down.

If it cannot be made quiet during ON/OFF then I suggest you add a speaker protection relay in series with one of the speaker feeds after the output Zobel.

This relay should have delayed ON and instant OFF.
It would be nice if it also had DC detect and trigger OFF.

I recommend adding a 1M0 across input to signal ground. And to parallel this with a 47pF (maybe as high as 1nF) capacitor for RF attenuation.
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Last edited by AndrewT; 16th April 2012 at 10:24 AM.
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Old 16th April 2012, 12:32 PM   #3
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Why not implement the standby switch, that's why it is there, to avoid on/off transients.

You need a simple switch and a 22uf cap connected to pin2

Use the "stand-by" instead of a traditional "power switch"

Last edited by Alvis; 16th April 2012 at 12:35 PM.
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Old 16th April 2012, 12:37 PM   #4
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Quote:
I don't know how to, or if the amplifier can mute it's input/output when starting up or shutting down.
I don't think it can.

Quote:
If it cannot be made quiet during ON/OFF then I suggest you add a speaker protection relay in series with one of the speaker feeds after the output Zobel.

This relay should have delayed ON and instant OFF.
It would be nice if it also had DC detect and trigger OFF.
Can you recommend a circuit? As for powering the circuit I have 9V AC or +12V DC (I don't have -12V DC).

Quote:
I recommend adding a 1M0 across input to signal ground. And to parallel this with a 47pF (maybe as high as 1nF) capacitor for RF attenuation.
Do you mean adding the resistor and cap between pin 1 and 2 on the X1 connector? Pin 1 is signal ground and pin 2 is signal input.

Thanks, Ralf
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Old 16th April 2012, 12:52 PM   #5
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Quote:
Why not implement the standby switch, that's why it is there, to avoid on/off transients.

You need a simple switch and a 22uf cap connected to pin2

Use the "stand-by" instead of a traditional "power switch"
So can I simply add a switch between the leg from pin2 to the cap C2 and the left ground path (referring to the pic of my PCB)?
When is the amp in standby? With this switch open?

Thanks, Ralf
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Old 16th April 2012, 01:00 PM   #6
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The switch will go parallel to the 22uf cap. Both are connected to pin 2 and ground.

I would think the amp is in standby when the switch is open, just guessing. if not, turn the switch 180 degrees.
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Old 16th April 2012, 01:03 PM   #7
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Quote:
Originally Posted by giralfino View Post
So can I simply add a switch between the leg from pin2 to the cap C2 and the left ground path (referring to the pic of my PCB)?
When is the amp in standby? With this switch open?

Thanks, Ralf
Yes.
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Old 16th April 2012, 01:16 PM   #8
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OK - so 2 more holes on the PCB :-)
Thanks again, Ralf
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Old 16th April 2012, 01:19 PM   #9
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No problem.
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Old 18th April 2012, 02:11 PM   #10
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Yesterday I did a quick test using just some wires, alligator clips and a switch, implementing the standby part of the circuit.
I can say it works: I firstly switched on the main power-on switch and then the standby switch, without the bump. Reverse operation for powering off.
I must add that the amp is in standby when the standby switch is closed, so I had to reverse "logically" the switch: now it indicates on when the switch is actually open.
So now it's time to make 2 more holes in the already populated PCB and connect permanently the new switch.
Ralf
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