Opamp Decoupling - Page 2

Steve Eddy · 2nd April 2003, 09:10 PM

Quote:

Originally posted by mrfeedback
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.

Yes. The local supply will be modulated to a greater degree by the local opamp's current draw (i.e. the audio signal). The smaller the capacitor and/or the larger the resistor, the greater the local supply modulation. How much of that modulation that gets mixed in with the signal will depend on the opamp's power supply rejection.

Quote:

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

You originally seemed interested primarily in measurements. Bascially measurements would simply show the modulated signal added to the output. No different than how supply ripple gets added to the output.

se

mrfeedback · 3rd April 2003, 01:53 AM

Steve I well understand PSRR, and the fact that the supply pins will be modulated, and according to the level of PS modulation and the degree of opamp PSRR a valid audio output will result.

Quote:

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

Eric.

Steve Eddy · 3rd April 2003, 02:40 AM

Quote:

Originally posted by mrfeedback
Steve I well understand PSRR, and the fact that the supply pins will be modulated, and according to the level of PS modulation and the degree of opamp PSRR a valid audio output will result.

Ok. So you weren't interested in measurements. Sorry, I must have been seeing things.

Quote:

Has anybody done measurements on the effects on distortion, envelope behaviour and dynamics of this technique versus differing values of resistor and cap ?.

There it is again.

Time to go get my eyes checked.

se

mrfeedback · 3rd April 2003, 02:56 AM

Steve, it seems that you have not done measurements or else you would be presenting them I expect.
Ditto subjective sonics testing too or else you would be presenting your finding in this area too.

Has anybody any subjective data here ?.

Eric.

Steve Eddy · 3rd April 2003, 03:48 AM

Quote:

Originally posted by mrfeedback
Steve, it seems that you have not done measurements or else you would be presenting them I expect.

Nope. I've never built any large multichannel mixers with gobs of opamps so I've never been compelled to resort to such a drastic measure as that.

Quote:

Ditto subjective sonics testing too or else you would be presenting your finding in this area too.

What I stated above notwithstanding, I tend not to say much about my subjective experiences because I don't feel they have any particular value to anyone but myself. If they did, then they must not be very subjective in the first place.

se

carlosfm · 3rd April 2003, 07:48 AM

eric,
Even on cd-players, preamps, etc. sometimes I see resistors on the power supply lines before op-amps.
They are there to try to isolate the component from other components sharing the same PSU, and to reduce the current entering the op-amp.
I change their value only if the supply voltage is low on the op-amp pins (it happens very often, and you can get more 2 or 3 volts).
As for subjecctive results...
Well, everytime I get the voltage as I like, I get better results.
The sound is faster, cleaner and more dynamic.
I don't see a necessity to limit too much the current of a 7815 or similar. They won't hurt an op-amp.

mrfeedback · 3rd April 2003, 12:42 PM

Quote:

The sound is faster, cleaner and more dynamic.

Hi,
Thanks that is the sort of reply that I was seeking.
Can you give values for particular opamps ?.

Thanks, Eric.

carlosfm · 3rd April 2003, 01:02 PM

Eric,
In general, OPAs (2604, 2132, 627/337, 228/2228) like to be driven above +/- 10volts (this applies to most other op-amps too).
Below that you notice immediately they don't play as they should.
Maby some people don't bother with this, but it's really important.
An OPA2604 works until +/- 24v, but I never tried to go over +/- 18 volts.
I say that generally for any op-amp the optimum is between +/- 15v (the standard normally) and +/- 18v (the limit for the majority of them).

3rd April 2003, 02:56 AM	#14
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	I Did Say 'SONICS' too.... Steve, it seems that you have not done measurements or else you would be presenting them I expect. Ditto subjective sonics testing too or else you would be presenting your finding in this area too. Has anybody any subjective data here ?. Eric. __________________ I believe not to believe in any fixed belief system.

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3rd April 2003, 07:48 AM	#16
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	eric, Even on cd-players, preamps, etc. sometimes I see resistors on the power supply lines before op-amps. They are there to try to isolate the component from other components sharing the same PSU, and to reduce the current entering the op-amp. I change their value only if the supply voltage is low on the op-amp pins (it happens very often, and you can get more 2 or 3 volts). As for subjecctive results... Well, everytime I get the voltage as I like, I get better results. The sound is faster, cleaner and more dynamic. I don't see a necessity to limit too much the current of a 7815 or similar. They won't hurt an op-amp.

3rd April 2003, 01:02 PM	#18
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	Eric, In general, OPAs (2604, 2132, 627/337, 228/2228) like to be driven above +/- 10volts (this applies to most other op-amps too). Below that you notice immediately they don't play as they should. Maby some people don't bother with this, but it's really important. An OPA2604 works until +/- 24v, but I never tried to go over +/- 18 volts. I say that generally for any op-amp the optimum is between +/- 15v (the standard normally) and +/- 18v (the limit for the majority of them).