My Phase 90 Pops When Engaged--Can This Be Fixed?

[Big Al]

I've got an MXR Phase 90 pedal. I love the thing, but today it started making a loud pop when I engage the footswitch. It is under warranty, and I emailed Jim Dunlop support, but in the meantime, is there a way to fix it? Can I replace the footswitch part? Can I order a replacement footswitch that's smoother or made better?

[Dissentience]

Its probably just a true bypass pop. If you want to go about fixing it yourself, read this: http://www.geofex.com/article_folders/bypass/bypass.htm

[andreas]

The Phase 90 isn't true bypass, and isn't usually prone to popping. Did you change anything else when this started? For instance, did you add (or remove) another pedal to/from the chain when this happened?

 

Often times, the cause of the problem is actually another pedal or the amp, while the symptom (the popping) shows up in the one with the mechanical switching.

 

/Andreas

[Dissentience]

I thought the new Dunlop MXR pedals were supposedly TB

[Big Al]

I tried everything--different power supplies, different cables, and of course isolating it. It even pops when nothing is plugged into the input side. I don't know if it is true bypass or not, but it will allow signal through even when it's not getting power.

[andreas]

 

I thought the new Dunlop MXR pedals were supposedly TB

Some of the newer designs are TB (Carbon Copy, Silicon Fuzz, Distortion III, Fullbore Metal, the CAE boosters etc), and apparently the latest revision Dynacomp and Super Comp. But most of them still use the "hardwire" system.

 

/Andreas

[whiteop]

if the cables are good, open the pedal up and make sure everything's grounded correctly. If that's good then read the following article which should help explain why your pedal is making the dreaded 'popping' noise and how to fix it (replace leaky capacitors or solder in a resistor in the right place of the circuit off the switch):

 

Why does my stompbox pop when I hit the bypass switch?

 

Copyright 2000 R.G. Keen. All rights reserved. No permission for serving from sites other than http://www.geofex.com.

 

Why does this *(#$ ^#^ thing pop when I hit the bypass switch??

 

Whether there is a pop when you bypass or un-bypass a box depends on whether the internal circuits have been carefully designed to prevent it, or if there is a flaw in the circuit that makes a formerly-quiet box now pop at you.

 

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 in non-defective pedals, assuming even marginally competent 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. If the switch bounces a bunch, there is a whole train of these on-again, off-again signal bounces, so there's a "crackle/pop" sound if it's really bad.

 

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.

 

It is also possible that your stomp boxes pop from an assortment of defects in the box, or from causes outside the box itself. If anything puts DC on the signal line from outside the stompbox, the stompbox output cap will be dutifully pulled to 0V by pulldown resistors when the effect is bypassed, and you'll hear a find [pop] when the effect is brought back in as the capacitors are asked to charge to some other voltage suddenly. No amount of fixing on the box itself will make this go away. Fortunately, you can use a DVM to measure the DC on the signal lines and figure out whether it's the stompbox itself or something outside that's causing this to happen. Just measure the DC voltage between the tip and ring of a cord plugged into the input and output of the stompbox with the effect NOT bypassed. If the stompbox has an internal problem like a leaky coupling cap, there will be DC. If there's no measurable DC voltage, the popping is something outside the stompbox.

 

So - why do your expensive boutique boxes pop? Since I think they use mechanical switches, you're seeing either #1 or #3, maybe something outside the stompbox. Be sure that you have the "pull-down" resistors connected from the input/output capacitors to ground **on the effect board**. If they used to be quiet, and are now starting to pop, an input or output capacitor may have partially failed, and have become so leaky that the pull down resistors that are there can't pull the leakage to ground level anymore.

[andreas]

 

I tried everything--different power supplies, different cables, and of course isolating it. It even pops when nothing is plugged into the input side. I don't know if it is true bypass or not, but it will allow signal through even when it's not getting power.

The Phase 90 is not TB, but it still switches the output signal. So if something after it is leaking DC voltage (it could even be the amp), you might get pops. Of course, if the phaser circuit itself is leaking DC, you'll get pops whatever you do.

 

Try it as the only pedal to the amp. Does it still pop? If not, it's not the Phase 90. If it does, try sticking a buffered (Boss-type) pedal (any pedal will work, if it's turned on) between the Phase 90 and the amp. Make sure to try several pedals as the "shield" between the phaser and amp input. If this stops the popping, the problem is with the amp. If it doesn't, simply replace the Phase 90 (it's still under warranty, as you mentioned).

 

Don't try to repair it yourself, as that will only void the warranty.

 

/Andreas

[Big Al]

 

The Phase 90 is not TB, but it still switches the output signal. So if something after it is leaking DC voltage (it could even be the amp), you might get pops. Of course, if the phaser circuit itself is leaking DC, you'll get pops whatever you do. Try it as the only pedal to the amp. Does it still pop? If not, it's not the Phase 90. If it does, try sticking a buffered (Boss-type) pedal (any pedal will work, if it's turned on) between the Phase 90 and the amp. Make sure to try several pedals as the "shield" between the phaser and amp input. If this stops the popping, the problem is with the amp. If it doesn't, simply replace the Phase 90 (it's still under warranty, as you mentioned). Don't try to repair it yourself, as that will only void the warranty. /Andreas

 

 

I think you might have just solved the problem. It started popping when I changed the order of my pedals. It went from being in between other pedals to being last in line. Maybe it has done this all along and I never noticed because it was being shielded by a Boss Chorus. I will move it and see what happens. It's an acoustic guitar rig, so it's going to an LR Baggs Para DI and then to the mixer.

 

THANK YOU!

 

After reading these articles, I think I understand things much better.

[andreas]

That's weird. But somewhere in the chain there's a DC voltage leak on to the signal path. It could be the pedal that now sits immediately in front of the Phase 90 that's leaking at the output, but the only way to find out for sure is to use a patch cable and measure for voltage between the tip and sleeve on each pedal (when powered up). The pop occurs because the Phase 90 has a mechanical, rather than electronic, switching system.

 

List the entire chain from guitar to DI (both before and after the change), and I might be able to point to the most likely suspect.

 

/Andreas

[Big Al]

Drat--I moved it to a different location in the chain, but it's still popping.

 

Should I try using the effects loop built into the Para DI?

 

Here is how it's hooked up:

Korg Tuner > Fishman Acoustic Chorus > Boss Delay > MXR Phase 90 > Boss Chorus > LR Baggs Para DI > XLR cable to Yamaha Mixer > Yorkville Powered Speakers

 

I don't think it's the other pedals because it pops even when it's the only thing being used. Maybe the current is coming from the mixer?