Behavior of stompboxes with no instrument cable connected (DC power vs. battery)

...and how to do so...
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BlitBit
KVRist
258 posts since 28 Nov, 2013 from Germany

Post Sun Jun 10, 2018 2:08 am

Hi all,

how do most stompboxes behave when there is DC power connected, e.g. via a wall plug, but no instrument cable is plugged in? Are they active or not?

I know that most stompboxes are not active if they are run using the battery and if there's no instrument cable plugged in. This can be implemented by using a stereo jack for the instrument input so that the battery can be disconnected from ground when no cable is plugged in.

I'd also like to completely disconnect the battery when the DC plug is connected. For that I use a DC power jack with an additional contact switch that can be used to disconnect the battery if the DC plug is plugged in.

I now wonder how most stompboxes behave if the DC plug is plugged in but if there is no instrument cable in the input. Put into a table my situation is as follows:

Code: Select all

------------------------------------------------------------------------------------------
| DC Jack Connected | Instrument Connected |                   Result                    |
==========================================================================================
|         -         |          -           | No power is drawn from the Battery / FX off |
------------------------------------------------------------------------------------------
|         -         |          X           | Power drawn from Battery / FX on            |
------------------------------------------------------------------------------------------
|         X         |          -           | ? ? ?                                       |
------------------------------------------------------------------------------------------
|         X         |          X           | Power drawn from DC Jack / FX on            |
------------------------------------------------------------------------------------------
One might argue that if the stompbox draws power from the wall plug we do not have to care that much whether an instrument is plugged in or not and that the stompbox should be active in that situation. However, it seems that this behavior is harder to implement than the behavior that the stompbox is simply off if no instrument cable is connected.

Thanks for any help!

User avatar
BlitBit
KVRist
258 posts since 28 Nov, 2013 from Germany

Re: Behavior of stompboxes with no instrument cable connected (DC power vs. battery)

Post Sun Jun 10, 2018 10:55 am

I have just checked the behavior of my old and trusty Marshall Shredmaster which is the only stompbox that I currently own (which hopefully will change soon with the builds of some DIY boxes).

When no instrument cable is plugged in
  • the LED will light up when it is used with DC power
  • the LED won't light up when it is used with a battery.
So somehow it should be possible to get the behavior that the instrument input only affects the battery operation. Guess I will check some schematics of that box now and try to find out how it's done. However, if someone wants to post a simple schematic in this thread that would be greatly appreciated as well. :)

subterfuge
KVRist
229 posts since 7 Sep, 2012

Re: Behavior of stompboxes with no instrument cable connected (DC power vs. battery)

Post Fri Feb 15, 2019 1:41 pm

BlitBit wrote:
Sun Jun 10, 2018 2:08 am
how do most stompboxes behave when there is DC power connected, e.g. via a wall plug, but no instrument cable is plugged in? Are they active or not?
I don't know about the most, but for example in some of the old Boss pedals the DC jack works like with the battery; no plug in the input jack > pedal not active. The DC jack negative is wired to the same stereo input lug as the battery negative.
I'd also like to completely disconnect the battery when the DC plug is connected. For that I use a DC power jack with an additional contact switch that can be used to disconnect the battery if the DC plug is plugged in.
That's also how the old Boss pedals had it wired. The battery negative still goes to the stereo input lug, so it gets grounded when the plug is inserted. But the battery positive goes to the lug in the DC jack and when the DC cable is inserted, that lug gets disconnected.
One might argue that if the stompbox draws power from the wall plug we do not have to care that much whether an instrument is plugged in or not and that the stompbox should be active in that situation. However, it seems that this behavior is harder to implement than the behavior that the stompbox is simply off if no instrument cable is connected.
Depends on the build how hard it would be. At simplest it would be hardwiring the DC negative straight to the ground, and not to eg. stereo input lug. I don't see why that would be needed though. Why would there be need for an active circuit without any input?

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