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mrfeedback 1st April 2003 04:12 PM

Opamp Decoupling
 
It is common in many bits of gear to see opamp power supply pins decoupled from the rails by 47 or 100 ohms resistors, and variable value of decoupling cap (electro).

Has anybody done measurements on the effects on distortion, envelope behaviour and dynamics of this technique versus differing values of resistor and cap ?.
Subjective test results are ok too.

Eric.

carlosfm 2nd April 2003 12:36 PM

Hi Eric,

I've never gone to the trouble myself to measure or to listen and comparing to different value resistors/coupling caps.
But I can tell you what I normally do.
First thing I do is to measure the voltage between the supply rails of the op-amp.
If it has a voltage much below +/- 15 volts (that happens very often), I try to find a way or raising the supply voltage to anywhere between around +/- 15 and +/-18 volts.
Sometimes the solution is simple, just lower the value of those resistors.
But sometimes I have to do something to the PSU, or even a new one.
The problem is, and that I tested several times, with a lower voltage than, let's say, +/- 12 volts, the op-amp behaves and sounds much worst.
I've seen cd-players with +/- 5.6 volts on the op-amp!
That's crap, because they share the same PSU for the digital and analogue section.
If the op-amp has couplig caps of 100 uf or larger, I don't touch them (but I bypass them with 100 nf ceramics or polyesters).
If it's lower (sometimes 2.2 uf, far from the op-amp!), I change it for something from 100 to 330 uf (and bypass near the op-amp).
I always, but always, put a ceramic 100nf cap between + and - pins of the op-amp (under the circuit).
Well, this techniques always work very well for me.:nod:

sajti 2nd April 2003 01:02 PM

Hi,

I agree with carlosfm with the solution. I just recommend another one:

In most of the equipment manufacturers use three terminal voltage regulators, for opamps. I often remove this regulators, and apply separeted zeners for each opamps.
I found that 15-18V zener bypassed with 100uF LOW ESR+100nF ceramic gives the best solution.
Of course series resistor with proper value is necessary for every opamp, and in every rails. Use the highest possible current - 1 or 1.3W zener strongly recommended!


Regards: Sajti

mrfeedback 2nd April 2003 01:10 PM

Distorted Decoupling ?
 
Hi Carlos,
Thanks for your experience.
You suggest ceramic caps - these seem to be regarded as distortion sources, even when decoupling.

Regards, Eric.

carlosfm 2nd April 2003 01:33 PM

Ceramics
 
Really?
I heard that about tantalums...
Well...
But the ceramics have good high-frequency characteristics.
Just try not to use them in the signal path.
This may lead to another huge thread, we can all learn from each other.:nod:

mrfeedback 2nd April 2003 02:31 PM

Read These
 
Phred gives some good references to capacitors here - Capacitor Tutorials

Eric.

carlosfm 2nd April 2003 02:43 PM

Thanks Eric,
I read that some time ago.
The main problem I have is that here in Portugal it's very difficult to buy good parts.
You can only find crap things for sale.
I would like to use more good caps, of different types, but it's very difficult to get them here.:bawling:
If I whant something special, I have to buy abroad.
Oh well... sometimes I dream with Black Gates!:bawling:

Steve Eddy 2nd April 2003 04:16 PM

Re: Opamp Decoupling
 
Quote:

Originally posted by mrfeedback
It is common in many bits of gear to see opamp power supply pins decoupled from the rails by 47 or 100 ohms resistors, and variable value of decoupling cap (electro).
I assume you mean the resistor is in series with the decoupling cap and not simply across the supply.

The resistor's just there to damp the resonance of the LC circuit formed by the decoupling cap and supply lead inductance.

Quote:

Has anybody done measurements on the effects on distortion, envelope behaviour and dynamics of this technique versus differing values of resistor and cap ?.
I haven't but, but then I've never had any stability problems due to resonance of the decoupling cap and supply lead inductance so I've never used that technique.

Suffice to say that changing the value of the cap will change the resonant frequency of the LC combination and changing the value of the resistor will change the amount of damping. So unless you've already got a stability problem, I don't see how it would effect the parameters you mention above in any meaningful way.

se

sam9 2nd April 2003 04:27 PM

Ceramic bypass cap
 
"You suggest ceramic caps - these seem to be regarded as distortion sources, even when decoupling.
"

Yup, but the function is to remove AC from the DC power supply by providing a path that looks like a short to AC but an open cicuit to DC. Noise and distortion are AC by definition. The important thing about the cermic caps is that they about the highest bandwidth of any type. If they distort, all they distort is the noise and distortion that is being shorted anyway. I suppose there might be some way they reintroduce some to the rails but as long as the net effect is lower noise, etc, you benefit.

mrfeedback 2nd April 2003 04:51 PM

Decoupling The Decoupling...
 
I mean a resistor connected between positive power rail and opamp positive supply pin, and a cap connected between opamp positive supply pin and ground - duplicated for negative supply.

The purpose is to isolate the opamp supply pins from fluctuations on distributed supply rails, especially in big systems like mixers.

The value of resistor and capacitor will effect the opamp's supplies during audio envelopes.

The question is, has anybody done serious investigation of the sonic effects of the values of supply series resistor and shunt capacitor.

Eric.


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