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Old 9th May 2010, 11:30 AM   #1
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Default Help with a TDA7240 based amp

Hi,
I would like to build a small amp based on a TDA7240 chip. The circuit can be seen here (page 1).

The first question is: what is the power rating of the two 2.2 Ohm resistors?

The second question is: a 220uF cap (C4) is depicted in the circuit on page 1 but is not present in the design of the PCB on page 3. Is this cap needed or not?
I have seen a circuit (link here) where this cap is a 4700uF one, but I cannot understand what is written. I'm open to suggestion about this cap.

Thanks in advance,
Ralf
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Old 9th May 2010, 11:46 AM   #2
AndrewT is offline AndrewT  Scotland
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C4 <220uF. Datasheet says "Danger of oscillation"
2r2 try 3W to 5W for each.

If you are running off a car battery, that is equivalent to quite a big capacitor at a big distance from the amp. If you are running from a rectified AC supply you will need to attenuate hum and provide an adequately low source impedance to get the amp to work within specification.
On car battery use 220uF to 470uF
On mains use >=4700uF
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Old 9th May 2010, 01:28 PM   #3
infinia is offline infinia  United States
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Use 2.2 ohm 1/4 W and 0,22 uF. These parts are to prevent oscillation, not survive one! If these get very hot, then then you have an oscillation problem. In this case the IC will burn up as well. In normal operation the program level is attenuated, so no high dissipation is required.
Use a HQ bypass cap of 220 or 470 uF plus 0,1uF near the supply pin in all cases. HQ is as an example Panasonic FC series caps specs esr at higher freq.
The problem with larger caps (case size) can be of lower quality, so the esr sometimes rises too high and IC oscillation can occur. In AC operation you will have additional caps to take care of AC mains ripple. If the AC supply cap is located further than 1 meter then add more caps or reduce wire inductance.
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Old 9th May 2010, 02:05 PM   #4
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Quote:
Use 2.2 ohm 1/4 W and 0,22 uF. These parts are to prevent oscillation, not survive one!
Thanks

Quote:
Use a HQ bypass cap of 220 or 470 uF plus 0,1uF near the supply pin in all cases.
I'm planning to use this power supply: 15V DC Switching Power Supply, and I have one of these 12V Switching Power Supply that I can use instead.
In this case is still valid the suggestion to use a good 220 to 470 uF cap?

Thanks, Ralf
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Old 9th May 2010, 02:20 PM   #5
infinia is offline infinia  United States
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Yes use them in all applications, as the IC data sheet shows this.
The SMPS sometimes doesn't like bigger caps on its output like > 470 uF. The supply can become unstable esp for smaller supplies. Remember use a supply that is rated for at least > 2-3 times the Pout shown on the data sheet chart for the IC Po vs supply voltage esp. for 4 ohm speakers. You don't want the PS to be the weak link in the system or ?.

so look at Fig 3 on data sheet > at 15Vdc Pout = 22W so the supply need to be at least 50 watts for each chip used. So for stereo you need a minimum 15V 100 Watt supply. P I would go bigger esp. if you don't have experience with current limiting on SMPS.
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Last edited by infinia; 9th May 2010 at 02:41 PM.
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Old 9th May 2010, 02:47 PM   #6
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Quote:
The SMPS sometimes doesn't like bigger caps on its output like > 470 uF. The supply can become unstable esp for smaller supplies.
So in my case it is safer to use a 220 uF cap?

Quote:
so look at Fig 3 on data sheet > at 15Vdc Pout = 22W so the supply need to be at least 50 watts for each chip used.
My application is mono (one TDA7240), and I know that the first PS I mentioned provides the right power while the second not. I cited also the second less powerful PS only because I already have it and could use it before the "right" one arrives.

Thanks, Ralf
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Old 9th May 2010, 02:53 PM   #7
infinia is offline infinia  United States
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Yes 220 uF
have Fun!


As for some SMPS noise improvement use 2 ferrite cores (dark ones not colored ones) winding 6-10 turns using both pos and neg wires together. ie common mode chokes between any circuits AC input and DC output of the SMPS.
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Last edited by infinia; 9th May 2010 at 03:07 PM.
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Old 9th May 2010, 03:09 PM   #8
infinia is offline infinia  United States
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from above EDIT> As for some SMPS noise improvement use 2 ferrite cores (dark ones not colored ones) winding 6-10 turns using both pos and neg wires together. ie common mode chokes between any circuits AC input and DC output of the SMPS.


Use an IEC AC input filter module in addition to your common mode choke for best performance.
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Last edited by infinia; 9th May 2010 at 03:21 PM.
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Old 9th May 2010, 03:17 PM   #9
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Thanks again!
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