LM1875 and questions regarding the example application in the datasheet - diyAudio
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Old 3rd November 2008, 12:42 AM   #1
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Default LM1875 and questions regarding the example application in the datasheet

here is the example application for the LM1875 from the nat. semi. datasheet.

Click the image to open in full size.

my question is regarding a few differences between this and example applications of lower power op amps in a typical non-inverting amplifier schematic (an example is either the CMOY pocket headphone amp or on the wikipedia page for operational amplifier applications for non-inverting amplifier schematic).

1) typically with headphone amps and general low power non-inverting amp. setups i see that there is no C2 capacitor as in the above schematic for the LM1875 and instead R3 goes directly to ground. what purpose does C2 serve?

2) what does the R5/C5 on the output pin, parallel to the speaker output, do?

3) i assume that the C1/R2 is for removing DC bias, and aside from a slight change in the corner frequency, moving to C1=0.47uF and R2=100KOhm, what implications/impact does this have on the overall design?

thank you,
christopher abad
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Old 3rd November 2008, 01:14 AM   #2
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Hi,
These are some very basic questions.

C2 will shunt DC in the feedback loop to ground, therefore it will not be amplified. This should be retained if you value your speakers/headphones.

R5/C5 is a Zobel. It is important for amp stability and should be retained. It has no effect on sound quality.

C1 blocks DC on input and with R2 form the input filter. There's a formula, but I'm too lazy to look for it but suffice to say that the values in the ap note are good ones.
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Old 3rd November 2008, 06:54 AM   #3
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Hang on, C2 shunts DC to ground? How does that work......a capacitor, which can't pass DC, between the feedback loop and ground somehow passes DC to ground?

Consider me confused, but interested to learn the answer!
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Old 3rd November 2008, 09:24 AM   #4
jaycee is offline jaycee  United Kingdom
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What he means is that at low frequencies approaching DC, the amplifiers gain is reduced to unity rather than being amplified. This minimises DC on the output.
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Old 3rd November 2008, 12:12 PM   #5
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thank you, that was very clear and helpful to me.
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Old 3rd November 2008, 12:12 PM   #6
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i think what he might mean is that since its the inverting input, it shunts the AC to the ground but the DC goes into the inverting input, therefore removing it from the amplified output of the op amp (although i dont know, im new to this stuff mostly)
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Old 3rd November 2008, 01:16 PM   #7
Ted205 is offline Ted205  United Kingdom
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the capacitor will pass frequencies to ground based on its value.


as i understand it, the 22uf cap will only pass the lower frequencies (approaching DC) to ground.
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Old 3rd November 2008, 05:49 PM   #8
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(Ci + Ri) and Rf form a voltage divider to adjust how much of the output signal is fed back to the inverted input as correcting signal.

The lower the frequency the higher Ci's impedance in Ohm. That means more voltage drops across Ci + Ri and less across Rf. In other words the lower the frequency, the lower the feedback voltage at the inverting input and the lower the gain. The lowest possible gain is unity below the frequency where Ci + Ri = Rf, in this example around 0,38 Hz.

F(-3) for this filter is around 6,55 Hz, where the gain is halved. Not where Ci = Ri, which would be around 7,23 Hz.
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Old 5th July 2009, 06:04 AM   #9
Kinnja is offline Kinnja  United States
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Attached is an image of the example schematic and layout from the LM1875 datasheet.

I noticed on the layout C1 and C2 are both shown as pairs of capacitors with opposing polarities. To me this looks like a mistake, but it is National's datasheet. Are the polarities shown on the layout correct?
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Old 5th July 2009, 06:32 AM   #10
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Yes. By connecting two polarized capacitors in series back-to-back you create a non-polarized capacitor with half the capacitance. They work, as if each of them was by-passed by a diode, so each of them is active for one half of the sine-wave. This design is already old, and when it was made non-polarized capacitors were not as readily available and cheap as they are today. Therefore two polarized types were chosen instead.

If you want to build this amplifier you could (should) replace those two polarized caps with a single non-polarized as per the schematic, e. g. an MKP capacitor to achieve a better sonic performance.
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