6 Reasons Guitar Pedals Pop When Switched
It is a common complaint that guitar effects pedals make a pop or click sound when the bypass switch is actuated. This can occur with different types of switching systems and there are a number of reasons for it.
A sudden change in voltage level is heard as a click, pop or thud, depending on the duration of the voltage change. If the voltage level at the input jack changes quickly, the sound is more like a click. This is equivalent to a fast rise time for the voltage transition.
A narrow width pulse is heard as a click or tick in the audio. This waveform has many harmonics and even at a low level it will easily cut through the background sounds. If the rise time of the pulse is slowed down, some of the high frequency content is filtered and the noise is more like a pop that is not quite as sharp sounding. Further slowing of the edge of the voltage transition will filter additional high frequencies until it acquires more of a thudding tone.
These rapid clicks and pops are actually used in some drum synths to simulate the click and snap of beaters or sticks hitting a drumhead. However, this is not what you want when you step on the switch of your echo pedal or fuzz box.
6 Reasons Guitar Pedals Pop When Switched
1. Different DC Voltage Levels
Although capacitors are used to block DC voltage, in the real world many capacitors will leak a tiny voltage, which will build up on the input (or output) as a small charge potential (5mv in the example). When the pedal is switched on, the voltage is discharged into the signal path and a ďpopĒ is heard.
The DC voltage that accumulates must be eliminated to stop the popping sound so pulldown resistors are added. The small charge is now leaked off through the pulldown resistors (shown in blue) and the input (or output) will have no voltage to cause pops as it is switched on.
A pulldown resistor (1M or 2.2M) can be added on the signal jacks of Boss, Ibanez or similar pedals to fix the problem. Usually pulldown resistors are not put on the jacks, but this is a special case.
An alternate method of wiring a bypass switch grounds the input of the effect circuit when it is being bypassed. This will keep the DC voltage at the input at ground and though it helps, it is not a complete fix for the problem by itself. The AMZ 3PDT switch boards are automatically wired this way.
2. Different signal levels
Most bypass switches have a rapid action. When signals of different volumes are switched, switch contacts quickly transition and the changing volumes can be heard in the audio as a short, sharp sound. This type of switching noise should only be heard when there is a signal present, such as when you are playing. It doesnít make a switching noise if you silence the strings and hit the footswitch, since the signal level is then close to zero.
This type of transition noise is worse in pedals that have an inverted output since a positive going signal in the input will be negative in the output, effectively doubling the voltage differences. It may also be found in booster pedals where the output is quite a bit larger than the input.
If the audio could be switched at the instant when the audio wave is at zero (crossing from positive to negative) then there would not be a noise since there would be no voltage difference in the audio to produce the impulse. This is impractical in most pedals and difficult to engineer without microprocessor control to detect the zero-crossing point.
This type of noise is less common and there isnít much that can be done about it with true bypass pedals. The jfet electronic switch systems as used by Boss, DOD and others, will not have this noise since there is a slow turn-on/off in the jfet control circuit that essentially fades the audio in and out, avoiding the clicks and pops.
3. Switch contact bounce
Switches do not make a clean single contact when the mechanism is activated because of the mass of the parts and the springy nature of their construction. The switch contacts hit together, then rebound and make contact again. This can happen numerous times from a single switch actuation.
There will be a noise that results from the metal connections inside the switch making and breaking contact as they spring back from the impact. The contacts may bounce 4 or 5 times (or more) during the 10 to 20 milliseconds after being activated. The audio signal path is being opened and closed rapidly by the contact bounce. This series of pulses from the audio interruption is heard as a pop or click, or sometimes as a more subdued thud.
Almost all pushbutton switches have contact bounce, to one degree or another. Some switches are worse than others, and small signal relays suffer from the problem too.
With a guitar amp at a loud stage volume, almost every mechanical footswitch or relay will make some amount of noise when actuated. Pulldown resistors donít help with this problem. Grounding the input of the effects pcb is not going to solve the problem either, though it is a good practice.
The only way to avoid the problem with passive switches is to use a muting circuit that kills the audio signal for a short amount of time until the switch contacts have settled in their final position.
4. Mechanical Noise
Sometimes the noise when a switch is hit can be mistaken for a pop or click in the audio, when actually it is just the mechanical movement of the parts in the footswitch. Some types of the 3PDT pushbutton switches that have stiff activation resistance will make this noise. It is made even more prominent depending on the type of enclosure on which the switch is mounted, and thin metal boxes can emphasize the sound, like a drumhead.
Relays donít have this type of noise, though if you put your ear close to the pedal, you can almost always hear a faint tick sound when the relay activates. Itís not loud enough to be a problem though.
There is not much to be done with stiff switches that make this noise, though I have heard that the white plastic washer that comes with the ubiquitous blue 3PDT can be mounted on the inside of the pedal close to the switch body to help dull the sound. I do this as a standard practice, if only because I donít like the look of the washer on the outside.
5. Microphonic components
A microphonic component is a part that generates an electrical impulse in response to an impact or other physical interaction. This phenomenon is well documented and some types of ceramic capacitors have a piezoelectric effect that makes an impulse when the layers are compressed or flexed.
I was once diagnosing a pop sound in the bypass of a pedal for a customer and eventually discovered that the mechanical action of the 3PDT footswitch was stressing the pc board and one of the MLCC capacitors was generating the noise. The ceramic layers in the capacitor were acting like a piezo transducer each time the switch was activated. I replaced the faulty capacitor with a different construction capacitor and the pop went away.
This problem is easy to detect by tapping lightly on the components with a soft object such as the eraser end of a pencil. If you tap a microphonic component, a noise will be heard in the audio output. Once found, the microphonic part can then be replaced.
Vacuum tubes, inductors and other passive parts can also exhibit this noise effect.
6. LED current draw
When an LED is switched on or off in a pedal, the current it requires from the power supply can change quickly by up to 20 milliamps, though it is usually less. If the power supply is not very stiff (not tightly regulated), then the change in current draw by the LED circuit will produce a voltage drop or spike as it is switched. In some circuits, mostly simple transistor models with high gain, the voltage change can be heard as a pop in the audio output.
This is one of the less likely causes of popping in a pedal, though it is certainly possible and has been found previously by plenty of builders.
There is an article in the AMZ Lab Notebook that shows how to eliminate this type of noise.
There are numerous causes for pops and clicks in footswitch designs. One of the best ways to eliminate the noise is with the jfet switching system used in many Boss, DOD, and Ibanez pedals. The tradeoffs are added circuit complexity and expense. There are also specialized switching ICs that can be used for silent switching but they have other problems, including the need to have different voltages for powering and switching. Another solution is to use a microprocessor driving a relay that has a muting transistor to silence the audio signal path long enough for the relay contacts to settle. There have been complaints of delays in the switching time reported by users of this circuit design.
Noisy pedals can even be used with a bypass box or loop box that has a silent swicthing system, instead of perfroming mods on the pedal. You could even go so far as to include a kill switch on the pedalboard that would be used to mute the audio while several pedals are stomped. There is no perfect solution and some players are more sensitive to switching noise than others.
Update: Aug 2017 - Static pops
I have seen numerous posts in online forums that attribute pops in bypass switches to static electricity that has accumulated on the component. This is not true as there is no static accumulation on the switch since the metal plunger and threaded body are in close contact with the plastic parts. Any charge that tries to form on the switch is quickly drained off because the metal body of the switch is grounded to the enclosure. Not only that, but most switch configurations have a ground wire on one of the lugs that further serves as a path for static to be leaked away.
Static charges are not a source of pops in pedals, and what these players believe to be static is actually the dc capacitor leakage described above. The leakage through the capacitor can quite slow to form and it is mistaken for static since a few stomps on the footswitch deplete the charge on the capacitor, which does not seem to come back. Actually, if allowed to idle for several minutes (or longer), the charge will slowly build up again on the capacitor and make a pop on the next press. Grounding the input of the effect when in bypass will help with this, and is a common way to wire the 3PDT.
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