Resistor or inductor at opamp output?

[JDAPJ]

Hi!
Just wanted to get ur opinion on this:

When an opamp is hard to get stabilized, sometimes people use a resistor from the op:s output to the feedback resistor and then u connect the load between these two resistors.

Allright...

But then there is another way, using a inductor?

Do they both have the same purpose except that the inductor is also frequency dependent?
Or, are they just there to isolate the output from capacitive loads?

Some input would be nice here..

/Jdapj

 

[rickcr42]

many use the resistor as opamp output protection (limits current) and include it in the feedback loop.I use it to isolate the cable from the output which can and will reflect back to the opamp output and will then have to be considered part of the feedback loop which it then becomes part of.This is not a prefect solution having definate effects on the final current delivery and being directly in the circuit path audible so it must be the highest quality resistor yuo can get.

a better solution is here and at under five smackers a real bargain : http://www.jensen-transformers.com/datashts/oli3.pdf

 

[morsel]

You can use inductors between the opamp outputs and the output jacks. Inductors do not raise output impedance at low frequencies and thus may be a better choice than output resistors for stabilizing finicky amps. Several headphone amps use them. Try 10µH for starters.

 

[jcx]

a classic is winding a inductor on the body of a large carbon comp resistor, electrically in parallel

the inductor gives low audio Z, reduces Cload effects, the reisistor damps potential L*Cload resonance

lossy ferrite bead could serve the same function although magnetic materials may be nonlinear - so testing may be in order (not that test results "prove" inaudiblity)

 

[Tomo]

Hi,

I try those at the output before, but they don't seem to work. It may be because I was then too sensitized to that hiss / noise, or something, however.

I think installing stuff like that is all about killing amp oscillation causing radio-interference. (Yep your amp transmitting noise. It happens ... funny huh?)

I heard people talking about installing really high quality caps at the opamp. You will need to install them at the PSU pins of the opamp. They swear by this, but I do it only because I feel better safe than sorry. I never experienced this killing hiss ...

I think you would be better off stumping down the source of the noise.

Tomo

 

[rickcr42]

Quote:

I think installing stuff like that is all about killing amp oscillation causing radio-interference.

More loop stability with some amp/cable combinations and reflections back down the cable which when the feedback loop is directly exposed at the output ends up feeding the signal back to the opamp inverting input.
More prevalant than most realise so they just do not recognise this as the source of a problem and end up attacking it wrong.The simple inline resistor does work but there wll be some losses in the current delivery which is dependant on resistor value.

The other way to "isolate" the loop is to run a buffer open loop after an opamp instead of "multiloop".Hanging a monolithic buffer on the output of a line stage without any feedback to the gain stage is my preferred method when using such devices even though the "specs" say closed loop is better.Just like feedback with a typical gain stage looks great in the specs arena but open loop sounds better and also allows each stage to be isolated AND optimised without effecting any other stage in the signal path.Feedback is evil unless you need the damping factor

I use the LH0033 class-A as my buffer of choice and get way less problems and more real use music recovery though even here a resistor inline is desired if you want to isolate the stage from external events.
Inductors at the supply pins and a 1K resistor at the input plus the output resistor or inductor terminate all entry points to the actual active stage.