Nota Bene: Pops when switching, even with pulldown resistors

[R.G.]

There is a weak link in the power/ground setup in most effects using stereo jack switching. You can get pops when switching the relatively high LED current even if you have pulldown resistors on both input and output caps.

The culprit is ground noise. The ground noise happens when the sudden start and/or stop of current in the LED makes the signal ground voltage change suddenly.  +9V decoupling may not help, and may make it worse.

The ground noise comes from our friend, the stereo jack ground switch.  If your jack is new and the plug is new, things work great. When dirt, oil, crud, bar funk, and other nasties build up on the ground ring of the input jack and the second-channel contact that supplies ground through the plug into the input ground ring, and thence to the effect board, the resistance through that path can get high enough to make a noticeable pop when the LED current starts and stops.

This sudden transient is worse with mechanical switches because they really do go from fully off to milliohms of resistance in zero time when the metals contact. The sudden "ground bounce" can flow right into the input of your effect, and it doesn't take much to hear a pop from the input of a gain-of-a-zillion MegaBlaster.

What to do, what to do??

(1) Clean your plugs and jacks. Yep, and do it again. And again, every so often.
(2) Use the OUTPUT jack for power switching - it's less sensitive by the gain of the pedal.
(3) Use the GEO scheme for cold-switching +9V by using a PNP transistor with it's base tied through a resistor to the stereo lug on the input or output jack, and the battery minus tied directly to the board. The PNP transistor only lets +9 through when its base is pulled down by the plug in the jack, but this current does not change noticeably when the LED is started, and so any ground bounce is only through the hard-soldered milliohms of the battery lead, and is much, much smaller.

See the power supply switch in the Neutron filter at GEO or at GGG, or see the positive 9V switch in http://geofex.com/FX_images/oaspltr.gif for how to hook up the PNP.

[Hal]

good to know!

Peter - can u send that to FAQ?

[Jim Jones]

Thanks, RG!

Jim

[erio fraga]

Thanks, RG. really very interesting and usefull info

Erio

[Phorhas]

Is there a way to incorporate this cold switching with your MOSFET polarity protection to kill two birds at once?

http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

[R.G.]

To continue the analogy, no, you'd have to toss two stones with one throw to kill both birds. A second MOSFET (or bipolar) switch is needed.

The MOSFET setup for protection is not oriented correctly for cold-switching power, as the intrinsic body diode is backwards for this purpose. If you tried to switch the thing with the gate-source voltage, you'd find that the body diode lets through the battery supply less one diode drop.

However, a MOSFET will do a dandy job of switching battery power - it's just more expensive and no more effective than a bipolar. A bipolar will NOT do the polarity protection job in this circuit.

[Fp-www.Tonepad.com]

Why was the stereo input jack power switching implemented as a standard instead of using the output jack?

[smashinator]

Why was the stereo input jack power switching implemented as a standard instead of using the output jack?

Just seems logical!    I mean, plug guitar in, effect comes on.  Attach effect to amp, effect comes on makes MUCH less sense to me.  

[puretube]

if you`d pull out the output-plug while having the amp still turned on...
whereas pulling out the input-plug, a contact would switch the effect-input to ground;
also, in the beginning, you`d only use one pedal - and when you pack your guitar away, you usually pack the cord away with it (wether you`re using your own amp at rehearsal, or a rented amp at the venue);
when more than one band plays the same backline, the effect that`s not in use will be un-powered, when the player that used it has left the stage;

(there may be more reasons...)

[Satch12879]

Don Tillman suggests using a switched jack instead of our usual scheme to turn power on in his preamp-in-a-plug project.  I'm not really sure what he means but then again he seems like the's got a subtle distain for our ilk.

Then again, heresey, but why can't we just use electronic switching and not worry about this? There's a circuit on J.Phillpott's site for a bypass switch based on a logic controller and a flip flop.  Maybe this could be implemented to provide non-mechanical, quiet bypass.

[Gilles C]

Maybe the same True Bypass Mojo.

Most people prefer the mechanical contacts to the electronic one.

[R.G.]

Then again, heresey, but why can't we just use electronic switching and not worry about this? There's a circuit on J.Phillpott's site for a bypass switch based on a logic controller and a flip flop. Maybe this could be implemented to provide non-mechanical, quiet bypass.

This is a fairly deeply plowed field. There is a lot of discussion of bypassing as a technology at GEO, as well as relay, FET, and CMOS switching bypassing, information and some designs with CMOS logic and microcontrollers to do the heavy lifting.

Besides, bypassing wasn't the question - power switching was. Electronic switching requires some constant power, and that will eat batteries. The whole reason for power disconnect is to avoid draining batteries, which is what the stereo switching jack does, as well as the variants at GEO with cold switching to avoid ground current problems.

[RDV]

All my LED equipped boxes pop even with pulldowns, except for the ones I used the Millenium 1+ with. They pop not.

RDV

[Phorhas]

Hey R.G. - I saw your "Cheap and Intelligent" polarity protector - and I've two questions:
1. Does the 4M7 resistor introduce much thermal noise to the circuit?
2. Can amy implamentation of a cold swithing arrangement be used with this design?

(I know- two dirds... a couple of stone...oh well... if I won't ask I won't learn...)

[aron]

Thanks R.G.!

[R.G.]

Hey R.G. - I saw your "Cheap and Intelligent" polarity protector - and I've two questions:
1. Does the 4M7 resistor introduce much thermal noise to the circuit?
2. Can amy implamentation of a cold swithing arrangement be used with this design?

1. any thermal noise from the 4M7 resistor does not matter. I realize that you may be confused by the way I labeled the drawing. "Vin" is the battery, not an audio signal. As it is set up, the 4M7 resistor is used to pull the PNP fully off when there is no pull down from the NPN. When the PNP base is pulled down, the PNP is saturated, and the 4M7 resistor cannot cause noise; when the PNP base is NOT pulled down, the PNP is off, so any thermal noise stays there, because the PNP cannot conduct it into the circuit, which is not working anyway because the PNP is not giving it power.
2. I want to say that you could simply connect the emitter of the NPN to the stereo second channel lug, grounding it that way. That will work except in exceptional circumstances, but I still need to think through what happens when the battery is reversed, no signal, etc.

[Satch12879]

Besides, bypassing wasn't the question - power switching was. Electronic switching requires some constant power, and that will eat batteries. The whole reason for power disconnect is to avoid draining batteries, which is what the stereo switching jack does, as well as the variants at GEO with cold switching to avoid ground current problems.

No absolutely, of course, R.G.  But in the end aren't they (bypassing-power switching-LEDs and popping) intimately connected?

Gross over simplification on my part methinks...:mrgreen:

[brett]

Thanks for another excellent post RG.

Why I use a regulated supply;
- grounding is usually better (reduces switch "popping")
- they don't need power switching
- gives more space inside the box
- they don't go dead or weak, except for exceptional circumstances
- they cost almost nothing to maintain and feed
- they are cheap (an LM317, a couple of caps, etc in a box)
- lots of power lets you use cheap/multiple/coloured/flashing LEDs
--> they save the environment from waste batteries

There's probably more reasons.
cheers
Brett R

[Phorhas]

. I want to say that you could simply connect the emitter of the NPN to the stereo second channel lug, grounding it that way. That will work except in exceptional circumstances, but I still need to think through what happens when the battery is reversed, no signal, etc.

I thought this would be the way... but, what are exceptional circumstances?

[petemoore]

Thanks for another excellent post RG.

Why I use a regulated supply;
- grounding is usually better (reduces switch "popping")
- they don't need power switching
- gives more space inside the box
- they don't go dead or weak, except for exceptional circumstances
- they cost almost nothing to maintain and feed
- they are cheap (an LM317, a couple of caps, etc in a box)
- lots of power lets you use cheap/multiple/coloured/flashing LEDs
--> they save the environment from waste batteries

There's probably more reasons.
cheers
Brett R

You start with  a Wall Wart and ... []