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Old 9th February 2007, 02:33 PM   #1
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Default Feedback resistors with 'drop-in' replacementop-amps

Being relatively new to 'modding' my equipment , my main focus is on new capacitors and op-amp replacement. Most of the op-amps I use are those recommended here as 'drop-in' subs for the basic oem types , ie; 4558,5532,4570 and the like. There is not much that I have read about the peripheral components(feedback resistors,etc) needing to be changed when these subs are used. I am familiar with added bypassing near pin on these new(faster)parts. As I continue to read about which op-amps are 'best' , there are always those who love or hate the same part. Would this have something to do with using the original external components and not using the optimum values for the new replacement? If so , how does one determine the correct resistor values(such as feedback resistors) when doing a swap?
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Old 11th February 2007, 01:07 PM   #2
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Please correct me , but this seems like an important consideration??
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Old 11th February 2007, 01:49 PM   #3
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Hi homemade

The feedback resistors around the op amp define the gain of the circuit.

If the swap of op amps is done with pin to pin compatible op amps , everything is fine ,no need to change the resistors.

You only need to watch is that the substitute op amp is stable with the gain pre-definede by the resistors , because there are some that need a gain or 3 or 5 (minimum) to become stable (no oscilation).

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Old 12th February 2007, 03:57 AM   #4
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Thank-you

Maybe this is an obvious question; Then ALL like pinout op-amps respond the same to a given feedback resistor? I'm trying to understand how this stuff works.
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Old 13th February 2007, 03:19 AM   #5
Minion is offline Minion  Canada
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The Feedback resistors configure the Gain for the Opamp, The same gain calculations work for most standard Opamps so there is rarely any need to change the feedback resistors when swaping Opamps unless you also want to change the Gain of the opamp.....

In inverting opamp configuration you divide the Input resistor by the value of the Feedback resistor to get the Gain, So if the Input resistor is say 10k and the Feedback resistor is say 100k then you divide 10k by 100k and you get 10 which means the curcuit has a Gain of 10, This is the same with most all Opamps in the Inverting configuration and probably the same for all curcuits that use Feedback to create gain....

Go here and read:

http://colomar.com/Shavano/intro_opamp.html


This will tell you the basics of how to calculate gain and all about useing feedback to create gain....


Cheers
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Old 13th February 2007, 11:23 PM   #6
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by Minion
The Feedback resistors configure the Gain for the Opamp...
But having settled on a desired gain, how do you settle on the magnitude of the resistor pair? 1k & 10k or 100k & 1M?
Quote:
From Shavano link
To keep power consumption down, as well as noise introduced by cheap carbon resistors, we will use resistors in a range of 10,000 Ohms thru 1 Meg Ohm.
I was under the impression that higher values introduced more noise?
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Old 13th February 2007, 11:40 PM   #7
Minion is offline Minion  Canada
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Yes, Higher Values will introduce more Johnson Noise....

I believe you select your feedback resistors partially based on the Input impedace of the of the opamp...For instance Fet Opamps like to see a High Input impendance (Higher Value Input resistor) were BJT Input opamps like to see a Lower Input Impedance (Lower Value Input resistor)....You generally should not swap out a Fet Opamp for a BJT or Visa Versa as Fet opamps are sometimes chosen for there High input impedance and BJT"s for there low input impedance depending on the application.....


Cheers
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Old 14th February 2007, 01:43 AM   #8
lineup is offline lineup  Sweden
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High or lower value of feedback resistor

Uusally we try not to use too high value for this FB resistor.
Likewise we, if we can, try to keep input impedance resistor moderately low.
Say, for audio, in both cases like 10-47 kOhm.

There are a couple of reasons for doing this.
Resistor current noise is higher the higher resistance.
At very high resistance capacitances may cause troubles.

Even JFET input Op-Amps will get more noise, the higher input impedance!
People seem not to always understand this fact


So is always good to keep resistance as low as you can
without causing other problems.
Too low can be bad, too.



There is one special case, where we may have to use HIGH feeback resistor:
Op-Amp Inverted circuit, with higher gains.

Say we want input impedance 20 kOhm and a gain = 50.
We would use R1 = 20 kOhm
and feedback resistor = 1000 kOhm = 1 Mohm.
Voltage gain 1000/20 = 50

In this case, if want to use a lower resistor, than 1 Mohm
we would have to add a buffer before Op-amp.
One transistor or Op-Amp with good current drive output
that can feed a lower resistor loading.

After this we may use 50 kohm/ 1 kOhm, and still have Vgain=50.

-------------------------------------------

A very wellknown amplifier with a rather high value feedback resistor
is the Thorsten Loesch original Inverted Gainclone circuit.

Click the image to open in full size.

Thorsten uses 220k at BOTH input pins of LM3875!
To use this gainclone idea, many have added some input buffering
and so allow to have other resistor values.
There are several examples of this.


Regards, lineup
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Old 14th February 2007, 02:32 AM   #9
Pars is offline Pars  United States
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Here's a good article, including links to resistor calculators, etc.:

http://www.tangentsoft.net/audio/hs-opamp.html
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