[sdiy] Do op amps need bypass caps?

[Roman Sowa]

Good point, but not every circuit drives 600ohm load. Even at 10V 
amplitude on 600ohm, that would cause power line drop by less than 1V, 
still within safe margins assuming it's +/-15V supply. At +/-12 you'd 
have to use R-R opamp anyway to reach that amplitude, so that's still 
safe. And PSRR at low frequency filtered by 47u/45R will make that dead 
silent.

OTOH 100n as the only resort, straight across power rail, has corner 
frequency in range of MHz. So it has no effect on audio frequencies.

I'm not saying this decoupling is good for everything, neither one opamp 
is good for everything. But for most of low power audio circuits this is 
fine.

Roman

W dniu 2014-08-20 08:14, Tim Ressel pisze:
> I have to disagree. If your running an opamp capable of higher output
> currents, say 600 ohm drive capability, you may find those 47R resistors
> causing issues.
>
> If you look at the PSRR curves (power supply rejection ratio) of my
> favorite amp, the LME49723, you'll see it is quite nice at low
> frequencies but drops to about -55dB at 10 KHz. This means high
> frequency crap on the power supply could leak into your nice clean audio
> path. Careful layout and bypass caps in the 10n-100n range are needed
> here. For more filtering consider using inductors and caps in a PI
> configuration to keep out the nasties.
> Tim Ressel
> Circuit Abbey
> 503-750-9331
> timr at circuitabbey.com
>
>
> ------------------------------------------------------------------------
> *From:* Roman Sowa <modular at go2.pl>
> *To:* synth-diy at dropmix.xs4all.nl
> *Sent:* Tuesday, August 19, 2014 1:24 AM
> *Subject:* Re: [sdiy] Do op amps need bypass caps?
>
> 0.1uF alone is tad too small for audio. Good practice is 47 ohms feeding
> 47uF cap optionally together with 0.1uF one. That gives corner frequency
> below 100Hz, which may be considered as DC quiet power line.
>
> If you don't bypass, and 0.1u is like you don't, all those transient
> signals, caused by varying current of opamp's load, are travelling all
> the way via power and ground traces, and are picked up by other parts of
> the circuit, which sometimes makes nice oscillator out of an amplifier.
>
> Roman
>
> W dniu 2014-08-19 08:06, Chris McDowell pisze:
>  > Hi guys,
>  >
>  > I was taught to put 0.1uF-ish bypass caps on every IC, every time,
>  > but recently have come across statements declaring that op amps in
>  > the audio range and below don’t need them. Is this true? Is this
>  > controversial? What’s the real scoop here?
>  >
>  > Thanks, Chris _______________________________________________
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