Opamp power supply bypass caps always necessary?
« on: December 07, 2008, 01:21:05 AM »
HI all,
I made a limiter using AD713 quad opamps in my line amps and didn't use the recommended power supply bypass caps that the manufactorer recommends.  I actually just forgot to put them in (hand wired so now it would be very difficult to go back).  anyways, it sounds really good, so, what am I missing?  How does it effect the sound when you don't have them in place? etc. 

Re: Opamp power supply bypass caps always necessary?
« Reply #1 on: December 07, 2008, 06:45:39 AM »
A lot of vintage synths that use op-amps that only have a fraction of the bandwidth today's equivalents have omit bypass caps. All the restoration manuals I've read suggest installing bypass caps when you service the synth...

I've found that op-amps can spring into oscillation retroactively; i.e. they work fine for a while, and one day you find them picking up Radio Moscow and discover it's oscillating at 20mHz...

For what it costs to implement them, why leave them out?

Bypass caps also prevent crosstalk between stages (small film caps) and help immunise stages from the loading effects of others (larger electrolytic caps).

The AD713 is a pretty fast op-amp - I wouldn't use one without bypass caps.

Prepare yourself. You are about to become the voice of Interplanetary Parliament.

Samuel Groner

    Z├╝rich, Switzerland
  • Posts: 2935
Re: Opamp power supply bypass caps always necessary?
« Reply #2 on: December 07, 2008, 07:08:02 AM »
I suggest using one 100 nF between the rails (instead of the more usual pair from the rails to ground) for each opamp in this case--should be much easier to install and will be sufficient if it is stable even without. But surely decoupling should not be considered optional and the decoupling caps are the first thing I place on a breadboard (or PCB layout) as they should be close to the opamp.

« Last Edit: December 07, 2008, 07:09:33 AM by Samuel Groner »


Re: Opamp power supply bypass caps always necessary?
« Reply #3 on: December 07, 2008, 09:54:37 AM »
As a default  , i can't see it hurting anything
so other than a manufacturer's cheapness , why not ! ?
GKB Audio / Greg Boboski


Re: Opamp power supply bypass caps always necessary?
« Reply #4 on: December 07, 2008, 07:14:57 PM »
Always include them.
As suggested a couple of 100nF caps up close to the power pins of the op amp usually suffice from small signal amps and buffers. If you are driving a low impedance line (eg 600 ohms) then you probably need to use 10uF low ESR caps as well as the 100nF caps.

The purpose of the caps is to present the power pins of the opamp with a low impedance. In an ideal world the power supply would have zero impedance, however in the less-than-ideal real world it does not. Add some impedance due to inductance of PCB traces and wiring as well and you need to compensate for this at the op amp power pins. When the op amp has to switch suddenly (eg transient short signals) then it needs power quickly to dump into the load. Any supply impedance will degrade the transient performance since the supply will appear to sag at the op amp power pins. Adding low ESR caps at the op amp power pins prevents this from happening.


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