# Loud pop when engaging pedals - but only on one input



## pattste (Dec 30, 2007)

Good evening,

I have a Allen Old Flame amplifier. It has two inputs, one marked Normal and one marked -3db. Connected to the Normal input, when I engage a pedal (they are true-bypass, as far as I know) there is a loud pop sound. This happens even if the volume is turned all the way off or even if my tuner is engaged. I have read online that engaging each pedal several times before playing would reduce or eliminate the pop sound but it doesn't make a difference. However, if I plugin into the -3db input, engaging pedals on and off is completely silent!

This isn't a big deal as I tend to use the -3db input (now that I know it solves the problem) but I'm still curious to know what's causing this and whether the amp needs servicing.

I have googled for this problem and found tons of discussions on it but nothing that would explain why one input does it and the other doesn't. No good solution either.

Anyone?


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## mhammer (Nov 30, 2007)

I don't know anything about Allen amps apart from the ads I see in Vintage Guitar, so I'm waving my hands around in the dark here. However...

Switching-related pop generally occurs when there is a capacitor on the input or output of something that has no opportunity to drain off, because it is unconnected at one end. The pop one hears is essentially the sound of charge/current that has been temporarily stored in a capacitor and _finally_ has a moment to drain off once connected. It's not quite a spike, but it behaves sort of like one.

Normally, input and output caps will have a terminating resistor to ground, that allows the cap to drain off, preventing audible pops. However, that terminating resistor is essentially in parallel with the terminating resistor of whatever comes before the input, or after the output (depending on whether we are talking about input or output capacitors). 

When you stick those terminating resistors in parallel, they behave like a much lower value resistor, and circuits get loaded down, degrading audio performance. Maybe not be all that much, but designers like to provide whatever sonic advantages they can. And if the circuit is one where you expect that there will be no frequent changing in the contact made with a cap (whether input or output), you tend to omit a terminating resistor, just to keep that resistance where it oughta be. 

Indeed, Boss pedals typically have no terminating resistor on the input, just to keep loading-down of the input as minimal as possible. They can do so, because they do their switching electronically, internal to the circuit. Unfortunately, when a person uses a true bypass pedal before a Boss pedal, the mechanical switching momentarily disconnects, then reconnects the TB effect to that input cap, producing a pop.

As you gathered from comments you've read, pressing the stompswitch multiple times CAN allow any stored charge to drain off, such that there may be no audible pop the next time you hit the switch. But that will depend on the value/size of the free-hanging cap (larger values accumulate more charge, yielding more pop unless discharged), and how long it has been since you last hit the switch. That's why this received wisdom about "step on it a few times" is sometimes true, and sometimes not.

I mention all this because it may be the case that what you are experiencing with you amp is analogous to what happens with a TB pedal before a Boss pedal...for the one amp input...andf analogous to what happens when an input is properly terminated for the other input. The good news is that, if this is true, the solution is very very simple. If you can get in your amp, solder a 2.2meg resistor (red-red-green, 1/2W) between the hot lug of that input jack, and ground. That's it.


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## dtsaudio (Apr 15, 2009)

And the reason the -3dB input doesn't pop is because there is a series resistor and a terminating resistor at that input.


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## Church-Audio (Sep 27, 2014)

Thats exactly what I would do you can even go higher if you want like 5m but anything above 1 meg should be fine. And actually if you want to keep the amp stock have someone do it at the live switch end of the stomp switch. Or on the output of the pedal to ground. Its dc that is held over by a charged cap like you said. All you need is a bit of resistance to bleed that charge off before it gets to the input stage of the amp. I assume the allan is probably a 68k then right into the preamp tube on the high channel.


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## mhammer (Nov 30, 2007)

Agreed. Higher value resistor is better (and infinite is best). Larger value input caps need a smaller-value bleed resistor, if one is going to be disconnecting and reconnecting the amp input with a true-bypass switch on a pedal. If we're talking about a .01uf input cap, then 5M or maybe even 10M is fine. If we're talking about a .22 or .47uf cap, then it may be wiser to go with a 1M resistor.


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## pattste (Dec 30, 2007)

mhammer and others: thanks for the detailed replies. The Allen Old Flame is a handwired amp derived from a Fender Super Reverb. They are super rare in Canada so I'm not surprised that you wouldn't be familiar with them. The explanation is way over my head but I've archived it for reference if I ever decide to have it fixed. I'm happy that the -3db input doesn't have the problem and happier still that I understand why (sort of). I'm surprised that David Allen hasn't designed the amp to prevent the normal input problem in the first place.


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## Steadfastly (Nov 14, 2008)

I get the pop too if I turn on my amp first and them turn on my Digitech effects unit. I simply turn in the effects unit first which solves the problem.


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## mhammer (Nov 30, 2007)

pattste said:


> mhammer and others: thanks for the detailed replies. The Allen Old Flame is a handwired amp derived from a Fender Super Reverb. They are super rare in Canada so I'm not surprised that you wouldn't be familiar with them. The explanation is way over my head but I've archived it for reference if I ever decide to have it fixed. I'm happy that the -3db input doesn't have the problem and happier still that I understand why (sort of). I'm surprised that David Allen hasn't designed the amp to prevent the normal input problem in the first place.


As noted, Boss still hasn't changed their own pedals' input to anticipate players using true bypass pedals or loop selectors in conjunction with their pedals. I respond several times a year to queries from folks on this and other forums who have been happily using other pedals employing electronic switching in pop-free manner for years, and then start experiencing switching pops when they add a TB pedal to their arsenal. Of course, one's initial inclination is to think the pop comes from the _new_ pedal, and not think that maybe the pop comes from a pedal that has behaved itself for years and now misbehaves because of _another_ pedal. It's too bloody counter-intuitive! :sSig_Idontgetit:

So, Dave Allen may have designed the amp under the assumption that a person plugs their guitar directly into it, and maybe sticks a commercial pedal or two (all properly terminated and using electronic switching) between the guitar and amp, such that there is no anticipated momentary discontinuity. In fairness. it may be an older amp, produced when the tidal wave of TB boutique pedals hadn't yet hit.


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