TDA7273 (for car audio) -- purpose of output caps - diyAudio
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Old 18th May 2017, 08:33 PM   #1
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Default TDA7273 (for car audio) -- purpose of output caps

Here's a link to the ST datasheet for this device [a TDA for car audio].

With respect to the TDA7372 datasheet schematic below...

Click the image to open in full size.

....
A while back, a mild debate arose elsewhere as to what function(s) the OUTPUT CAPS serve (2200uF, C9-12 in schema above).

Are they to BLOCK DC

...and/or ...

(as contended by another) "all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle." ?

In other words, can one REMOVE the 1000uF caps?
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Last edited by hollowman; 19th May 2017 at 03:27 AM.
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Old 18th May 2017, 09:00 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by hollowman View Post
can one REMOVE the 1000uF caps?
Each output is at a DC potential of half the power supply wrt ground.
This is normal in a single polarity power supply amplifier circuit.
Blocking capacitors are necessary at the outputs.

Last edited by rayma; 18th May 2017 at 09:03 PM.
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Old 18th May 2017, 11:18 PM   #3
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yep, the output at idle sits at half the psu voltage. to block this DC the caps are used.
edit* by mistake i was thinking this chip has BTL mode too, but i see it is not easyli done. so you should not remove the capacitors.
otherwise you get an unhappy speaker.

"all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle." this statemant is incomplete , and at first glance looks to be.. not so clever.
however , you "could" if you want to, (but you don't have to, and should not want to since its pointless) consider that for the "negative part" of the sine wave at outut you can sortha assume the capacitor as a kind of voltage source. its not "wrong" to do so, its just... better to look at what it actually does and go by that. witch is blocking dc.
allso does not need to be push-pull output.
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Last edited by Arty; 18th May 2017 at 11:22 PM.
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Old 18th May 2017, 11:36 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by hollowman View Post
can one REMOVE the 1000uF caps?
There are BTL amplifiers that don't need the output blocking capacitors.
TPA3118D2 30-W Stereo (BTL) Class-D Audio Amp w/ SpeakerGuard? | TI.com
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Old 19th May 2017, 03:19 AM   #5
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Yeah -- that el cheapo TDA is used in low-end card radios (class B, I think).

For some reason, I was getting confused with more-respected LM3886 Gainclone that uses +/- PS and no limiting blocking caps on the output:

Click the image to open in full size.

EDIT:
I looked at the LM3886 datasheet a little more carefully and saw an option for a SINGLE POWER SUPPLY [PDF, pg. 6].
That schema seems more complex than the DUAL-SUPPLY setup.
I have no idea what industry (auto, budget home stereo, etc.) prefers what option. The parts count seem similar??
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Old 19th May 2017, 03:30 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by hollowman View Post
I was getting confused with more-respected LM3886 Gainclone that uses +/- PS
and no limiting blocking caps on the output:
Yes, that's a more conventional linear audio amplifier, in a module.
Still, it does need a bipolar power supply in order to not require output coupling capacitors.
With only one power supply, it does need them.
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Old 19th May 2017, 06:41 AM   #7
oohms is offline oohms  Australia
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There are similar amps that are BTL and do not require output capacitors.. TDA7297 (2 channel) is nice, then you have 4 channel ones used in car head units such as TDA7388, TDA7850 - and even TDA7801 which has a digital input
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Old 19th May 2017, 04:23 PM   #8
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To use those output caps (in a single-supply design) is ... ugghhh
Even with boutique caps you'll get issues with temperature, electrolyte drying up (over time).
Maybe many car (or car audio) manuf's think the driver will trade in the vehicle before these caps wear out
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