Can I omit the input resistor for the LM3875? - diyAudio
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Old 29th July 2005, 01:02 AM   #1
sardonx is offline sardonx  Canada
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Default Can I omit the input resistor for the LM3875?

Is it safe to not use a resistor at pin7 (1K) for the LM3875?

The LM datasheet show it there but even just looking at the resistor on the schematic is making me wish it wasn't there. I'm using an OTL pre-amp right before the amp.. so it would be nice to be able to go straight into the chip with no dirty resistors in between. Should I build the amp with or without the input resistor?
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Old 29th July 2005, 02:20 AM   #2
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I'm not using a resistor there, and the sound seems to be more "transparent".

I tried it in a few different systems and the lack of resistor does not create any problems. The performance will also depend on the source's output.
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Old 29th July 2005, 03:50 AM   #3
Leolabs is offline Leolabs  Malaysia
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Yes,that resistor is not problem if u are not consider about input low-pass filter.
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Old 29th July 2005, 07:36 AM   #4
infobot is offline infobot  United States
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Default what it does

Just curious- anyone know what the resistor there does? Kind of wondering how it is with and without.
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Old 29th July 2005, 01:23 PM   #5
boholm is offline boholm  Denmark
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It reduces DC output voltage towards zero.

The trick is to calculate (Rf x Ri)(Rf + Ri) = RB for minimum DC output.

In this case the 1 kOhm is apparently considered close enough to the calculated 952 Ohms.

All this is only correct, if Ci is omitted.
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Old 29th July 2005, 01:53 PM   #6
pooge is offline pooge  United States
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Quote:
Originally posted by boholm
It reduces DC output voltage towards zero.

The trick is to calculate (Rf x Ri)(Rf + Ri) = RB for minimum DC output.

In this case the 1 kOhm is apparently considered close enough to the calculated 952 Ohms.

All this is only correct, if Ci is omitted.

(Rf x Ri) / (Rf + Ri) = RB
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Old 29th July 2005, 02:23 PM   #7
boholm is offline boholm  Denmark
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Oops
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Old 29th July 2005, 10:18 PM   #8
sardonx is offline sardonx  Canada
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Quote:
I'm not using a resistor there, and the sound seems to be more "transparent".

I tried it in a few different systems and the lack of resistor does not create any problems. The performance will also depend on the source's output.

Exactly what I wanted to hear .. thanks Peter.


One step closer to having a functional amp.....


Sardon
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Old 1st August 2005, 12:18 AM   #9
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the reason for the resitor is in the datasheet and is related to reliability. apparently there is a potential condition where high current would be allowed (if possible) to flow into the inputs and destroy parts of the amplifier during turn on or turn off.

in memory no one on these forums has ever had this issue with the IC.

the resistor also appears in the DC offset due to input bias current mismatch equation.
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