# HELP - Morley ABY Pops LOUD when switching inputs



## studiodog (Sep 10, 2010)

Hi folks,

I'd like to use my old Morley active ABY switch pedal to switch between an acoustic guitar (active pickup) and a mandolin (passive pickup). My problem here, though, is that when I hook it all up and start switching, the amp receives a VERY loud POP noise. Ouch! 

I suspect the problem is the impedance difference between the active and passive pickups in the two instruments. The pedal has been around a long time, so I'm guessing it was built with passive magnetic pickups in mind? Or maybe simply time to put the Morley out to pasture? 

Can anyone shed some light on this one?


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## mhammer (Nov 30, 2007)

Ever hold a whizz in for a LOOOOONNNG time, and when you finally get to a washroom, the sudden voiding of one's bladder is both painful and consoling?

Audible pops when switching are somewhat analogous to what happens when you just GOTTA let it out. The problem is capacitors.

Capacitors play a role in accumulating and storing charge in an assortment of strategically useful ways. Virtually every audio device will have a capacitor on the input and output to a) prevent unwanted DC voltage from entering and screwing things up, and b) prevent unwanted DC from leaving and screwing up something else that might not have a DC-blocking capacitor on the input.

When you switch off a pedal or amp or any other audio device, what happens to the charge stored in those caps? In some instances, it drains off, but very often the drain-off can take a long time because no path has been provided for it to do so. The result is that when you switch the circuit back into your signal path, you may be providing that stored charge with a means to FINALLY drain off. And what you hear as a "pop" is the sound of a sudden drain/bleed of stored current.

Is there a cure? Yes. Ideally, every input and output of a pedal should have some sort of drain resistor there, so that while you've bypassed the pedal, whatever is accumulated in the cap can bleed off to ground, through that resistor. Something on the order of 1meg or higher is usually fine, though it depends partly on the value of the cap: the greater the capacitance, the more stored charge there is to drain off, and the more efficient a drain-off path is recommended to make sure you've gotten rid of stored charge in time.

Many pedals that use solid-state switching will have a "hanging" capacitor at their input, because bypassing is done internal to the pedal, and not by disengaging the jack from the circuit. Look at the schematic for the Boss CE-2 chorus here - Boss CE-2 chorus pedal schematic diagram - and you can see that the input jack goes directly to a cap (C1) that absolutely no connection to anything when nothing is plugged into it.

Why am I telling you this? Because a completely passive A/B/Y unit like the Morley _does the bypassing at the jack_. What that means is that when you reconnect your signal path to the input jack of something with a similar sort of input like the CE-2 shown here, a path for sudden bleed-off will be provided and you will hear a pop.

The solution is basically to add a bleedoff resistor on the inside of the pedal, such that when your guitar is disconnected from it, the charge stored in that input cap has somewhere to bleed off to. As noted earlier, 1meg is about right.

Does all of this make sense in your context? have I explained it well?


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## studiodog (Sep 10, 2010)

Thanks mhammer. A very concise explanation, although I did have to take a break to whizz partway through! LOL The power of suggestion! 

Any idea where I can find a schematic for the Morley ABY? Ironically, I've got 1M resistors on hand!


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## mhammer (Nov 30, 2007)

studiodog said:


> Any idea where I can find a schematic for the Morley ABY? Ironically, I've got 1M resistors on hand!


Morley Pedals ...and more specifically: http://www.morleypedals.com/abyes.pdf

You can see that all the unit does is make and break connections. Although the schematic would seem to suggest that there is always a 100k resistance to ground across the input of whatever you feed the A and B outputs to. If so, this device would be contra-indicated for use with pedals. Amps, yes, pedals no.

My suggestion for de-popping would be to replace the 100k resistors on the output jacks with 1meg, but *add* a 1M resistor across the input jack of the ABY as well. 

The rationale: When the input is not connected to one of the outputs, the output of the device you are feeding into the ABY is "floating". Even if the preceding device has a termnating resistor to bleed off the output cap, connecting that preceding device to a 100k drain resistor creates an even more efficient path for any stored charge in the output cap to suddenly bleed off. So, if there is a 1M resistor across the hot and ground lugs of the input jack, then whatever you plug into it will always have a bleed-off/drain resistor of reasonable value, regardless of whether you've connected the input to the A or B output. 1M (input jack) + 1M (output jack) in parallel is 500k, which is not "high", but likely high enough. Perhaps equally important, it does not represent a huge contrast in bleed-off efficiency when switching, and should be much less prone to audible popping.

Using the 1M resistors in this application may well get you back some top end you were losing.


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## studiodog (Sep 10, 2010)

Thanks for the schematic link and the tech advice. Time to break out the soldering iron! I'll report back when I finish the mod. 

Cheers!


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## mhammer (Nov 30, 2007)

Great! I'm not guaranteeing anything, just following up on my instincts and experience. If this doesn't solve the problem, we can try tackling it a different way. But this is the least invasive or costly for now.

Fingers crossed.


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## studiodog (Sep 10, 2010)

mhammer said:


> My suggestion for de-popping would be to replace the 100k resistors on the output jacks with 1meg, but *add* a 1M resistor across the input jack of the ABY as well.


That did it! A $0.30, 10 minute fix! No more popping. Thanks!


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## mhammer (Nov 30, 2007)

You're most welcome. My reputation is intact!

Best part is, though, that you understand how it all works now, and can recognize when it will be a useful strategy to apply elsewhere. Congrats. It also means you can help others with the same problem. I like those sorts of endings.:food-smiley-004:


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