Can I omit the input resistor for the LM3875?

[sardonx]

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]

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]

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

 

[infobot]

what it does

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

 

[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.

 

[pooge]

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]

Oops

 

[sardonx]

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]

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.