TDA7265 No Feedback Capacitor Question - diyAudio
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Old 6th August 2015, 03:30 AM   #1
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Default TDA7265 No Feedback Capacitor Question

Hi,

On the TDA7265 datasheet, the following schematic is found.
Click the image to open in full size.

My question is regarding the 18k + 560R resistors to set the closed loop gain.
To my knowledge, usually there is a capacitor in series with 560R when i compare with other chipamp datasheet (LM1875 and LM3886 for example).

Like so..
Click the image to open in full size.

Why is there none on the TDA7265? Isn't this a disadvantage in keeping the output offset minimum? Would it work if i put a capacitor anyway?
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Old 6th August 2015, 03:52 AM   #2
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I'd suggest including the capacitor - since the input offset can be up to 20mV the output offset could be up to 660mV without the cap.
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Old 6th August 2015, 03:56 AM   #3
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Yes indeed..
So, it would still work with the cap, then?

But why would STMicroelectronics suggest such schematic on the datasheet?
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Old 6th August 2015, 04:00 AM   #4
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It would work better with the cap in yes, at least in the worst case situation where you could have 100mA flowing through the speaker voicecoil in quiescent conditions.

Plenty of oversights occur in manufacturers' datasheets.
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Old 6th August 2015, 04:04 AM   #5
sangram is offline sangram  India
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It will work both ways but you will have output offset either equal to the input offset - with cap - or multiplied by gain of the amp, if you do not use the cap.

The schematic is only for reference. The user has to implement the circuit based on their requirements of AC/DC gain, whether other things like servo etc will be installed, and whether they want tone controls in the feedback path...
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Old 6th August 2015, 04:05 AM   #6
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Just the answers i needed!

Thank you all!
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Old 8th August 2015, 09:36 AM   #7
AndrewT is offline AndrewT  Scotland
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One can minimise the output offset by equalising the "resistance" seen by the two inputs.
+IN sees an unknown internal resistor. The value is probably specified in the datasheet.
-IN sees 18k||560r or ~543r when no capacitor is fitted.
Adding the capacitor changes that resistance seen by -IN to 18k

You want to make +IN the same as -IN.
either decrease +IN to 543r, or add that capacitor and change +IN to 18k.
Then measure the actual output offset and trim the resistance to eliminate the remaining offset.

Unfortunately the output offset changes with chip temperature. It will wander as the chip warms up.
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Old 11th August 2015, 03:17 PM   #8
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by abraxalito View Post
It would work better with the cap in yes, at least in the worst case situation where you could have 100mA flowing through the speaker voicecoil in quiescent conditions.

Plenty of oversights occur in manufacturers' datasheets.
I wouldn't call it an oversight in this case. Manufacturer likely did this on purpose to keep the part count low to lure customers. They could have optimized the IC design to keep the output offset minimal as long as customer uses recommended parts.
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Old 11th August 2015, 11:11 PM   #9
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I agree, good point. I've noticed they've done precisely the same thing on other amp parts' datasheets.

So how could they avoid the worst case 100mA through the voicecoil through optimizing the IC design?
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Old 12th August 2015, 12:10 AM   #10
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by abraxalito View Post
I agree, good point. I've noticed they've done precisely the same thing on other amp parts' datasheets.

So how could they avoid the worst case 100mA through the voicecoil through optimizing the IC design?
Tighter DC design. As I heard an TI engineer say about the LM675. "All these are is an LM1875 with tighter DC tolerances."
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