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previous: R.G. Basics of "pop" from all stomp boxes -- 8/4/2000 6:44 PM

Two More...

8/7/2000 1:34 AM
moocow	Two More... I just thought I should add 4th and 5th causes. These have happened to me while using a DPDT switch in some effects of my own design. 4) RF Oscillations: When an effect is switched out of the signal path by a DPDT switch, nothing is connected to the input or output. If you're unlucky, RF can get into the circuit, and there can be a large RF signal sitting there on the output. Once you bring the effect back into the signal path, this oscillation will be killed, but it will briefly be present at the output, causing a loud pop. 5) External DC offset: Suppose an effect happens to be plugged into another device with a DC offset. When the DPDT switch is engaged, the effect's own pulldown resistors will remove the offset from the device, resulting in a pop. An example is the Fender reverb unit. It has a .05 uF capacitor isolating the guitar input from a cathode follower. There is about 120VDC across this cap, and even though there's a 1M pulldown resistor at the input, a leaky cap will cause a DC voltage at the input. I measured about 1.2V leakage at the input one old reverb unit. Now that was a loud pop ! Fixes: For #4, it's a case of inadequate design. The fix is to place a small cap (~100pF) in parallel with the input pulldown resistor. For #5, the effect is not at fault. Use a voltmeter to verify that none of the devices in the signal path are leaking voltage to either the input or output of the effect.

8/7/2000 1:34 AM

moocow

Two More...

I just thought I should add 4th and 5th causes. These have happened to me while using a DPDT switch in some effects of my own design.

4) RF Oscillations: When an effect is switched out of the signal path by a DPDT switch, nothing is connected to the input or output. If you're unlucky, RF can get into the circuit, and there can be a large RF signal sitting there on the output. Once you bring the effect back into the signal path, this oscillation will be killed, but it will briefly be present at the output, causing a loud pop.

5) External DC offset: Suppose an effect happens to be plugged into another device with a DC offset. When the DPDT switch is engaged, the effect's own pulldown resistors will remove the offset from the device, resulting in a pop. An example is the Fender reverb unit. It has a .05 uF capacitor isolating the guitar input from a cathode follower. There is about 120VDC across this cap, and even though there's a 1M pulldown resistor at the input, a leaky cap will cause a DC voltage at the input. I measured about 1.2V leakage at the input one old reverb unit. Now that was a loud pop !

Fixes:

For #4, it's a case of inadequate design. The fix is to place a small cap (~100pF) in parallel with the input pulldown resistor.

For #5, the effect is not at fault. Use a voltmeter to verify that none of the devices in the signal path are leaking voltage to either the input or output of the effect.