low value feedback resistors on LM3886

jarthel · 15th October 2007, 02:45 AM

Is there any disadvantage if my feedback resistors are: 75ohm and 1.5Kohm (for a gain of 21).

Thank you

Juergen Knoop · 15th October 2007, 05:32 AM

temperature...power dissipation could reach 0,3W
which increases noise or maybe even cause thermal modulation.
Stick with the usual suspects 1k+22k! Those work fine.

Regards
Jürgen

sangram · 15th October 2007, 09:26 AM

^^ I'd like to know more about this, if you can help me find some references.

I've used 22/1 and 4.7/220 (on different chips, though) and I'm still to figure out the effects and differences of these arrangements. I was going for lowest impedance to lower the output offset, and it did help. That was the only thing I could measure, really.

Would love to read more, though I'm beginning to get a hint of why this is true.

Thanks.

AndrewT · 15th October 2007, 09:32 AM

Quote:

Originally posted by sangram
^^ I'd like to know more about this, if you can help me find some references.

I've used 22/1 and 4.7/220 (on different chips, though) and I'm still to figure out the effects and differences of these arrangements. I was going for lowest impedance to lower the output offset, and it did help. That was the only thing I could measure, really.

22k/1k0 and 220k/4k7 ?
or
22k/1k0 and 220r/4r7?
or
22k/1k0 and 4k7/220r?

sangram · 15th October 2007, 01:38 PM

Sorry - 22K/1K and 4K7/220E.

Edit: Both the combinations were used on two separate amps at two separate points in time, the 'regular' one on all the 3886 ans 4780 amps I built, and the low resistance one on a (now exploded) LM4766.

Juergen Knoop · 15th October 2007, 08:15 PM

the usual values are just convenient. Small enough for noise, offset and parasitic effects.
Large enough for thermal effects and if you want to add some capacitors.
I think the LM chips are sweet tempered and one could choose the feedback resistors over a broad range.
regards

jarthel · 17th October 2007, 01:42 AM

Quote:

Originally posted by juergenk
temperature...power dissipation could reach 0,3W
which increases noise or maybe even cause thermal modulation.
Stick with the usual suspects 1k+22k! Those work fine.
Regards
Jürgen

what if I used 2W resistors? I suppose I shouldn't be concerned with heat?

Juergen Knoop · 17th October 2007, 04:40 AM

those have larger temperature coefficients, larger resistance tolerances and larger dc related noise.
I don't know, if all these theoretical disadvantages matter in a real world application.
But this is also true for the advantages.
What do you expect from lower value feedback resistors?
Regards
Jürgen

AndrewT · 17th October 2007, 08:53 AM

Hi,
a pair of double value paralleled resistors gets upto 1.2W. One to each side of the PCB helps keep them in the air stream.
One gets 50ppm/Cdegree using standard 600mW metal film.
There are sub 20ppm resistors available without going exotic.

Juergen Knoop · 17th October 2007, 09:03 AM

Hi,
yes, one can fight the disadvantages of lower value resistors.
But what justifies the effort?

Regards
Jürgen

15th October 2007, 02:45 AM	#1
jarthel diyAudio Member Join Date: Jan 2002 Location: somewhere in Australia	low value feedback resistors on LM3886 Is there any disadvantage if my feedback resistors are: 75ohm and 1.5Kohm (for a gain of 21). Thank you

17th October 2007, 08:53 AM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, a pair of double value paralleled resistors gets upto 1.2W. One to each side of the PCB helps keep them in the air stream. One gets 50ppm/Cdegree using standard 600mW metal film. There are sub 20ppm resistors available without going exotic. __________________ regards Andrew T.

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15th October 2007, 05:32 AM	#2
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	temperature...power dissipation could reach 0,3W which increases noise or maybe even cause thermal modulation. Stick with the usual suspects 1k+22k! Those work fine. Regards Jürgen

15th October 2007, 09:26 AM	#3
sangram diyAudio Member Join Date: Sep 2002 Location: India	^^ I'd like to know more about this, if you can help me find some references. I've used 22/1 and 4.7/220 (on different chips, though) and I'm still to figure out the effects and differences of these arrangements. I was going for lowest impedance to lower the output offset, and it did help. That was the only thing I could measure, really. Would love to read more, though I'm beginning to get a hint of why this is true. Thanks.

15th October 2007, 01:38 PM	#5
sangram diyAudio Member Join Date: Sep 2002 Location: India	Sorry - 22K/1K and 4K7/220E. Edit: Both the combinations were used on two separate amps at two separate points in time, the 'regular' one on all the 3886 ans 4780 amps I built, and the low resistance one on a (now exploded) LM4766.

15th October 2007, 08:15 PM	#6
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	the usual values are just convenient. Small enough for noise, offset and parasitic effects. Large enough for thermal effects and if you want to add some capacitors. I think the LM chips are sweet tempered and one could choose the feedback resistors over a broad range. regards

17th October 2007, 04:40 AM	#8
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	those have larger temperature coefficients, larger resistance tolerances and larger dc related noise. I don't know, if all these theoretical disadvantages matter in a real world application. But this is also true for the advantages. What do you expect from lower value feedback resistors? Regards Jürgen

17th October 2007, 09:03 AM	#10
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	Hi, yes, one can fight the disadvantages of lower value resistors. But what justifies the effort? Regards Jürgen

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