Bypass Capacitor On Op Amp V+ and V- Pins

[TomCarlos]

Friends....

I have another thread open on my Peavey Ecoustic 112 - Loud Hiss. I want to ask a general question about Bypass Caps on Op Amps and didn't want to bury the answer in that thread. So here it is...

As I trying to clean up some noise and hiss in the amp, I noticed that identical Op Amps (4558) have some instances where the V+ and V- pins have a .047uf bypass cap and others that have a .1uf cap. Other Op Amps do not bypass caps.

If you look at the attached image, you will see the Guitar Channel's first stage Op Amp has a cap on the V+ and V- pins. If you look at the Mic Channel's first stage Op Amp, the V+ and V- do not have a cap. There was space on the board to add one but the designers did not. On the PC board, these Op Amps are somewhat separated from one another- I don't know if that makes a difference.

Could it be that the decision to use a cap is based on proximity to other components or does it come down to electrical characteristics of the circuit? I am looking for some reading material to understand this.

Thanks in advance.

Tom

[Jazz P Bass]

Here is Rod Elliott's take on it.

Power Supply Bypassing.doc


"Bypassing
Although most opamps have a very good PSRR, it is always recommended that the supply be bypassed with capacitors - especially with high speed opamps. Bypassing should always use capacitors with good high frequency performance, and ceramics are probably the best in this regard. It is common for designs to use electrolytic capacitors, themselves bypassed by low value (100nF) capacitors. This ensures that all noise sources are minimised, and helps to prevent oscillation. When this occurs with a high speed (HS) opamp, it will commonly be in the MHz region, and is extremely hard to see on most oscilloscopes.

A sure sign is inexplicable distortion, that mysteriously disappears (or appears) when you touch the opamp, or a component in its immediate vicinity.


Figure 2 - Bypassing The Opamp Supplies

Even with HS opamps, electrolytic capacitors are not needed for each device (generally needed only on each board), but the use of ceramic bypass caps between the supply pins of each device is highly recommended. Figure 2 shows the most common method of bypassing power supplies for opamp circuits (A), but there are others. In some cases, the supplies may not be bypassed to earth (ground), but just to each other. This has the advantage of not coupling supply noise into the earth (ground) system (B).

Claims have been made that supply bypassing ruins the sound (rubbish), or that ceramic caps should never be used in audio, even for bypassing (more rubbish), and even that high value capacitors (> 1uF) slow down the sound (unmitigated drivel). These claims are made by frauds and charlatans, and should be completely ignored - they have no basis in fact whatsoever, and indeed, quite the reverse may be true in each case."
copyright: Rod Elliott

Audio Designs With Opamps

[TomCarlos]

Thanks John...

Coincidentally, I found that same paper after I made my post here.

And thanks for posting from the paper. I am going to take a close look at the other op amps on the two pc boards to see which are bypassed and those that are not.

I am also comparing some of the designs the the Ecoustic to my KB series amps and the XR series mixer amps - interesting how they are similar yet different. Peavey does a nice job of "recycling" designs, boards, etc from one amp to make another. But sometimes, the shortcomings could also be carried over.

I have a handful of Peavey salvage PC boards that I can rummage through for parts (if I am looking for identical caps on all op amps as shown in the photo here). Otherwise, it's easy to find .1uf or .047uf ceramics.

Tom

[J M Fahey]

Claims have been made that supply bypassing ruins the sound (rubbish), or that ceramic caps should never be used in audio, even for bypassing (more rubbish), and even that high value capacitors (> 1uF) slow down the sound (unmitigated drivel). These claims are made by frauds and charlatans, and should be completely ignored - they have no basis in fact whatsoever, and indeed, quite the reverse may be true in each case."
copyright: Rod Elliott

+1

[Jazz P Bass]

First off, I would check the ripple on the opamp supplies.

The statement from the first post is deceiving as the schematic is just that.
A schematic.

The board layout may offer the 'why' as to the lack of a bypass cap on the mic channel.
If it is in close proximity to the other opamp, then there is not really a need to 'do it again'.

Keep in mind that 'bypassing' the opamp supply rails is really critical in the RF spectrum & in high speed configurations.

Not what we are dealing with here at all.

[Enzo]

And schematically, sometimes the bypass caps are drawn on one of the two op amps in the IC but not on the other. Drawing them on both would imply two sets of caps, but in reality there is only one set of caps for the IC.

For instance, the KB300: http://bmamps.com/Schematics/Peavey/...Schematics.pdf

Note U1a has bypass caps, while U1b does not. That is because they are in the same IC, and the caps are only drawn once.