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previous: Scott On /Off pop from high end stomp boxes...why? -- 8/4/2000 9:22 AM

Basics of "pop" from all stomp boxes

8/4/2000 6:44 PM
R.G.	Basics of "pop" from all stomp boxes Ok, you forced me to put this out early. This material is taken from a new update to the Bypasses article at GEO, as yet unpublished. =================================================== Whether there is a pop when you bypass or un-bypass a box depends on whethere the internal circuits have been carefully designed to prevent it. What causes the pop? All pops from bypass switching come from a sudden change in voltage level on the signal line going into the amplifier from the effect. This sudden change can come from three and only three things, assuming even marginally competant design: 1. the input and output capacitors on the effect will always leak a little bit of DC, no matter how good they are. When the pedal is bypassed in the "classic" true bypass circuit (see "The Technology of Bypasses" at GEO) both capacitors are open circuited. While they are open, the leakage will shift their DC voltage a little, and when the pedal is un-bypassed again, the capacitors have to charge back to their in-use voltages. That sudden voltage difference and the charging current that brings the capacitors back to the working voltage is heard in the amplifier as a pop. If the effect has any gain, the input capacitor pop is further amplified by the gain of the pedal into a bigger pop. 2. for non-mechanical switches the switch elements may couple a little of their control signal into the signal path. This is what happens with JFET, CMOS and relay bypass switches. If any of these are driven with a sudden-change control voltage, the unavoidable capacitances inside them can couple the fast wavefront into the signal lines, and this makes a pop. Some CMOS switches couple only a nanosecond or two of sound in, but several volts of it. The amplifier could not normally respond to this, but it overloads other circuits in the effect, and that overload is then heard as a pop. 3. finally, for true-blue mechanical switches, if the signal itself is not at exactly zero volts when the mechanical contacts touch (and bounce a few times!) this makes an instantaneous change of signal level just because the signal is suddenly connected. This tends to be smaller, but some people hear this as a pop. Here are the fixes: 1. For mechanical switches, put a 100K to 4.7M (exact value does not matter) resistor from the "outboard" end of both the input and output capacitors to ground. This keeps the ends of the capacitors pulled to the right voltage all the time, and cures the leakage problem. No capacitor clicks. 2.Ramp the control voltage up/down slowly (over a mS or two) to keep it from being coupled through the small capacitances. 3.No help here. Live with it, only switch when your guitar is quiet between notes, or use #2. So - why do your expensive boutique boxes pop? Since I think they use mechanical switches, you're seeing either #1 or #3. Be sure that you have the "pull-down" resistors connected from the input/output capacitors to ground on the effect board.

8/4/2000 6:44 PM

R.G.

Basics of "pop" from all stomp boxes

Ok, you forced me to put this out early. This material is taken from a new update to the Bypasses article at GEO, as yet unpublished.
===================================================
Whether there is a pop when you bypass or un-bypass a box depends on whethere the internal circuits have been carefully designed to prevent it.

What causes the pop? All pops from bypass switching come from a sudden change in voltage level on the signal line going into the amplifier from the effect. This sudden change can come from three and only three things, assuming even marginally competant design:

1. the input and output capacitors on the effect will always leak a little bit of DC, no matter how good they are. When the pedal is bypassed in the "classic" true bypass circuit (see "The Technology of Bypasses" at GEO) both capacitors are open circuited. While they are open, the leakage will shift their DC voltage a little, and when the pedal is un-bypassed again, the capacitors have to charge back to their in-use voltages. That sudden voltage difference and the charging current that brings the capacitors back to the working voltage is heard in the amplifier as a pop. If the effect has any gain, the input capacitor pop is further amplified by the gain of the pedal into a bigger pop.

2. for non-mechanical switches the switch elements may couple a little of their control signal into the signal path. This is what happens with JFET, CMOS and relay bypass switches. If any of these are driven with a sudden-change control voltage, the unavoidable capacitances inside them can couple the fast wavefront into the signal lines, and this makes a pop. Some CMOS switches couple only a nanosecond or two of sound in, but several volts of it. The amplifier could not normally respond to this, but it overloads other circuits in the effect, and that overload is then heard as a pop.

3. finally, for true-blue mechanical switches, if the signal itself is not at exactly zero volts when the mechanical contacts touch (and bounce a few times!) this makes an instantaneous change of signal level just because the **signal** is suddenly connected. This tends to be smaller, but some people hear this as a pop.

Here are the fixes:
1. For mechanical switches, put a 100K to 4.7M (exact value does not matter) resistor from the "outboard" end of both the input and output capacitors to ground. This keeps the ends of the capacitors pulled to the right voltage all the time, and cures the leakage problem. No capacitor clicks.
2.Ramp the control voltage up/down slowly (over a mS or two) to keep it from being coupled through the small capacitances.
3.No help here. Live with it, only switch when your guitar is quiet between notes, or use #2.

So - why do your expensive boutique boxes pop? Since I think they use mechanical switches, you're seeing either #1 or #3. Be sure that you have the "pull-down" resistors connected from the input/output capacitors to ground **on the effect board**.

Replies:

Michael Fuller Nice work RG, another thing though.... -- 8/5/2000 2:45 PM
moocow Two More... -- 8/7/2000 1:34 AM
Sweetfinger Re: Basics of "pop" from all stomp boxes -- 8/9/2000 1:27 AM