Can I omit the input resistor for the LM3875?

sardonx · 29th July 2005, 12:02 AM

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?

Peter Daniel · 29th July 2005, 01:20 AM

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.

Leolabs · 29th July 2005, 02:50 AM

Yes,that resistor is not problem if u are not consider about input low-pass filter.

infobot · 29th July 2005, 06:36 AM

Just curious- anyone know what the resistor there does? Kind of wondering how it is with and without.

boholm · 29th July 2005, 12:23 PM

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.

pooge · 29th July 2005, 12:53 PM

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

boholm · 29th July 2005, 01:23 PM

Oops

sardonx · 29th July 2005, 09:18 PM

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

theChris · 31st July 2005, 11:18 PM

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.

29th July 2005, 12:02 AM	#1
sardonx diyAudio Member Join Date: Jun 2003 Location: Canada	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?

29th July 2005, 01:20 AM	#2
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	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. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

29th July 2005, 06:36 AM	#4
infobot diyAudio Member Join Date: Apr 2005 Location: Eastern US	what it does Just curious- anyone know what the resistor there does? Kind of wondering how it is with and without.

29th July 2005, 12:23 PM	#5
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	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. __________________ Best regards Bo

29th July 2005, 01:23 PM	#7
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	Oops __________________ Best regards Bo


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29th July 2005, 02:50 AM	#3
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Yes,that resistor is not problem if u are not consider about input low-pass filter.

31st July 2005, 11:18 PM	#9
theChris diyAudio Member Join Date: Dec 2001 Location: SIUE, Illinois, USA	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. __________________ if only it could be used for good, not evil...

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