Is the 22uf C2 (feedback cap) in LM1875 optional - diyAudio
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Old 13th April 2009, 06:38 PM   #1
mudihan is offline mudihan  United States
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Default Is the 22uf C2 (feedback cap) in LM1875 optional

In the LM3875's datasheet, the feedback cap Ci is marked optional (as does the one in LM3886, I think). But in LM1875's datasheet, the same feedback cap C2 is not. Is there a reason that the National engineers treat the cap in LM1875's datasheet differently? Or were they just over cautious at the beginning?

I of course want to omit it.
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Old 13th April 2009, 07:22 PM   #2
mudihan is offline mudihan  United States
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I know that I should measure to the the DC offset at the output if I choose not use the feedback cap in a LM1875 gainclone. But I just wonder whether the engineers at National intentionally want to treat LM1875 differently in regard of the cap for other good reasons.
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Old 14th April 2009, 06:44 AM   #3
Dxvideo is offline Dxvideo  Turkey
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I have done a lot of chipamps with LM3886, LM3875, LM4780 and LM1875.. And as I measured;
For overture series (3886,3875,4780) <=20K Rb (input shunt resistor) the DC offset value is not more than 50mV which is acceptable.. And for LM1875 its better (~20mV).
So I don't see any reason to put Ci both for 1875 and the others.. Of course, you must consider the sources DC offset first (if your source has 10mV offset then with a 21X gain amplifier you will have 210mV on speakers!)..
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Old 14th April 2009, 11:52 AM   #4
Stream is offline Stream  Russian Federation
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But if we use input capacitor (up to 10uF), does it means that chip shouldn't has dc offset?
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Old 14th April 2009, 12:07 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
make the amplifier DC coupled and put in the protections to ensure reliability of the system.
Alternatively make an amplifier that is AC coupled.

Do not build an amplifier that is mixed AC and DC coupled.
Fitting one of the DC blocking caps and omitting the other is mixed AC and DC coupling. Don't.
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Old 14th April 2009, 12:08 PM   #6
Dxvideo is offline Dxvideo  Turkey
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If an amplifier is AC coupled then its AC coupled. Means you should use both input DC stopper (Cin) and NFB DC stopper (Ci)..
And if its DC coupled then you dont need neither..
In real life, I have tried all combinations for these capacitors;
No Ci but various MKP Cin
No Cin but Elna Silmic II, Elna Cerafine and Nichicon Muse bypassed with low value MKP as Ci
MKP Cin + Elna Silmic II, Elna Cerafine and Nichicon Muse bypassed with low value MKP as Ci
No Cin, No Ci
And I can say the best sounding one is no cap = DC coupled.. But the possible DC offset problems should be considered as mentioned.
But for many modern sources like CDP etc.. this is not an issue.. Most of them has an output cap or a dc servo unit for output..
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Old 14th April 2009, 12:16 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Dxvideo
But for many modern sources like CDP etc.. this is not an issue.. Most of them has an output cap...
This is an issue.
The DC blocking cap in the output of the source acts just like the DC blocking cap at the input of the receiver. You end up with mixed DC and AC coupling. Don't.
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Old 14th April 2009, 12:21 PM   #8
Dxvideo is offline Dxvideo  Turkey
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So we have to make AC coupled amps always!
Because we can't know which source has an output cap and which has a dc servo!
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Old 14th April 2009, 12:22 PM   #9
AndrewT is offline AndrewT  Scotland
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do the homework.
Be informed before you make the decision.

There is little advantage in guessing.
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Old 14th April 2009, 12:25 PM   #10
Dxvideo is offline Dxvideo  Turkey
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Ok... What about, if there will be an input selector before the amp? In this case that means I am planning to use multiple kind of sources, some with output cap and some without...
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