Why does popping happen when pedalboard is going directly to PA system?!?!

[Paul Marossy]

So I've been playing with some old friends for the last three weeks and they don't do amps, everything goes straight into the PA system thru DI boxes and we have monitors and/or headphones to hear ourselves. That gave me a good excuse to break out my old Bulldog Speaker Cabinet Simulator, which works well for this purpose.

So far so good, except for whenever I switch any true bypass pedals, there is a loud pop in the PA system. The FET flip-flop bypass type pedals don't do this (I could now make an argument for NOT doing true bypass, but I won't go there). I know someone is going to ask me what kind of DI boxes they use and the answer is I'm not sure, I'll have to check that this Thursday. I do know that they have a pad (which I think had three different settings), are most probably active and of course they have a ground lift. My guess is that they are using 48V phantom power to power them. My pedalboard is powered by an ungrounded DC wallwart that goes into a well filtered built-in voltage regulator set to 9.5 volts. My pedalboard has been bulletproof until confronted with this situation.

So my question is why the heck does this happen?! It doesn't happen with any of my amplifiers. I've tried it with DI ground lift on and off, no change either way. Is there a way to get around this problem or do I just have to live with it?

[Mark Hammer]

My gut says "Because the inputs to the mixer are predicated on the assumption that the connection to that input will NEVER be interrupted, so they have unterminated input caps with 'free' ends that pop for the same reason that a pedal with similar input caps pops".

Make sense?

I'm gonna suggest that you stick a 1M cap on the output jack of the last pedal in line.  Where the resistive termination of the effect circuit board prevents audible popping within the pedal, that additional resistor across the output jack will drain off the input cap on the board, whether the effect is engaged or not.

[Paul Marossy]

Ah, that makes some sense. Did you mean a 1M resistor?

[joegagan]

sounds like poor design in the DI boxes to me. all DIs are not created  equal, and they are like alarm clocks, a consumer commodity item.

[Mark Hammer]

Ah, that makes some sense. Did you mean a 1M resistor?

Um, I meant a 1M cap, but like, if you want to "experiment", I suppose a 1M resistor could probably be used.  I mean, if you insist.

[joegagan]

of course, try the usual IM pop resistor at all pertinent locations before going after 'better' DIs.

[Paul Marossy]

Thanks for the advice guys, I'll try the 1M resistor and see how that works. Gives me a place to start anyway. 

[wavley]

one of these?



Or maybe the red one?



I'm gonna go with this:

My gut says "Because the inputs to the mixer are predicated on the assumption that the connection to that input will NEVER be interrupted, so they have unterminated input caps with 'free' ends that pop for the same reason that a pedal with similar input caps pops".

Make sense?

I'm gonna suggest that you stick a 1M cap on the output jack of the last pedal in line.  Where the resistive termination of the effect circuit board prevents audible popping within the pedal, that additional resistor across the output jack will drain off the input cap on the board, whether the effect is engaged or not.

But amend that statement to be true of the input coupling cap of the active DI.

I'm pretty fond of those Behringer DIs, they're not as nice as my ADL Tube DI (that I only own because I got it cheap), but they're pretty decent DIs for the money.

[Paul Marossy]

I'm pretty sure they are not a Behringer product. IIRC, they are a Whirlwind product, but I won't bet the farm on that. I won't be able to check on what they actually are until Thursday.

[wavley]

I'm pretty sure they are not a Behringer product. IIRC, they are a Whirlwind product, but I won't bet the farm on that. I won't be able to check on what they actually are until Thursday.

The Whirlwind stuff is even nicer, but for all intents and purposes most of them are just opamps, fets, or tubes driving a transformer, I bet most of them lack the pulldown resistor, I think you guys are on the right track with that.

[Paul Marossy]

I'm pretty sure they are not a Behringer product. IIRC, they are a Whirlwind product, but I won't bet the farm on that. I won't be able to check on what they actually are until Thursday.

The Whirlwind stuff is even nicer, but for all intents and purposes most of them are just opamps, fets, or tubes driving a transformer, I bet most of them lack the pulldown resistor, I think you guys are on the right track with that.

Yeah, I think so too. In a couple of days I will find out!

[Paul Marossy]

Well son of a gun, the pulldown resistor works to get rid of the popping noise. Now I just have to figure out a different minor problem.  

BTW, they are a passive Whirlwind DI box.

EDIT: my other problem appears to stem from the fact that the Bulldog Speaker Cabinet Simulator didn't have a cap on the output so I added one this morning. I think that should finally do the trick...

[Mark Hammer]

I guess the lesson is that some things are built in anticipation of no change in the connection status, and other things are predicated on the assumption that input and output will be regularly changed.

[Paul Marossy]

Yeah that's something that is easy to overlook if you don't deal with it on a regular basis.

[Paul Marossy]

Just to follow up and put a 100% conclusion to the matter...

The 1M pulldown resistor worked great. But then I found that I had a problem that sounded like DC on the signal when I adjusted the pot on the cabinet simulator. So I looked at the cab simulator schematic and realized that it didn't have a DC blocking cap on the output, so added that in and now there is no more noise problems to do with the pedalboard. Interesting learning experience.  

[joegagan]

thanks for the followup. each application presents unique problems, every time they get solved here adds to our collective knowledge.

[Paul Marossy]

each application presents unique problems

Yeah, true for me lately