positive ground switching idea / question

[fuzzy645]

I know that a common approach to pedal switching is such that when you insert a mono plug into the stereo input jack of the stompbox, it makes a connection from the black battery lead to the circuit ground, therefore causing the circuit to be powered on.  This way when you disconnect the input jack, you break this connection, and you are not draining the battery.

Would it be possible to accomplish the same type of thing via the foot switch, so that when the pedal is switched off, it does not drain the battery (even if a plug is inserted into the input).

I'm thinking if a 4PDT switch were used, 2 of the poles can be used for the true bypass, 1 pole for the LED, and the final pole to disconnect (and reconnect) the battery's black wire from the circuit ground.  My only fear is this will cause a loud, audible pop.

[R.G.]

My only fear is this will cause a loud, audible pop.

It most probably will. When the circuit is powered on, the input and output caps have to charge up to their operating voltage. That probably causes a pop.

There are some circuit tricks to get around this, but it's very difficult with a single supply.

[fuzzy645]

Thanks RG.  That makes sense.

What about using an independent mini switch to disengage the battery.  I'm thinking if this independent switch does not get engaged/disengaged at the same time as the foot switch, it might work, but not sure.  So at the beginning of a gig I would switch on the mini switch to connect my battery, and then use the footswitch to turn my effect on/off as needed, and then disengage the battery when done for the day.  Would that work?

[R.G.]

What about using an independent mini switch to disengage the battery.  I'm thinking if this independent switch does not get engaged/disengaged at the same time as the foot switch, it might work, but not sure.  So at the beginning of a gig I would switch on the mini switch to connect my battery, and then use the footswitch to turn my effect on/off as needed, and then disengage the battery when done for the day.  Would that work?

It sure would. That's how some of the first-ever stompboxes worked, up through about 1970 or so. The stereo-jack-switching was invented to not have to use a switch, possibly to save cost. It was the case at the time that guitarists might have one or maybe two pedals.

In today's world of pedalboards with 10-40 pedals, unplugging the input jack is far worse than switching on the batteries, which is where I suspect you may be coming from.

The AC-powered 9V adapter is the next step in the evolution, though. It (can at least) power many pedals, and they all go off when you unplug the one adapter, restoring the power-off to being one action at the start of a gig.

[LucifersTrip]

What about using an independent mini switch to disengage the battery.

remember, they make pots with switches on them
http://www.luciferstrip.com/fuzz/demonmojo1/demonmojo-bottom-before2.jpg

the pot on top right is the volume and on/off (simple SPST switch)...full left is off...turn right to click on, then adjust volume. note: only mono in/out jacks used

[fuzzy645]

Ahhh, a switched pot, that will work well, thanks!!

The reason for my inquiry is that I just finished up the circuit board for my Tonebender mKII clone and it really sounds great so I'm now ready to start thinking about putting it into a box and adding switching/LED, etc..  I'm reluctant to add a DC jack since this is a positive ground pedal.  If I do add a DC jack though, what kind of DC power supply can I use? Do they sell positive ground wall adapters?   Also, do you have a good diagram on how to wire a DC jack for positive ground circuit?

I'm a bit torn as to which way to go: 9V only, vs. DC jack.  Thoughts?

[Mark Hammer]

I made a PNP Fuzzface for a guy (who takes delivery tomorrow), and had a 1k pot with a SPST switch, so I used that for gain/fuzz, and turning off the battery/power.  Given the requirement to use a separate supply for this pedal, because of the flipped-around polarity, I thought this would prove useful.  This way he can leave it installed in a pedalboard, run it on a battery, and not have to worry about running it down between uses.

Kind of a triple-stage powering system.  Input jack enables the grounding of the power supply, 2.1mm power jack enables switchover from battery to external power, and on/off on the fuzz knob also enables power supplied to board.

[fuzzy645]

Kind of a triple-stage powering system.  Input jack enables the grounding of the power supply, 2.1mm power jack enables switchover from battery to external power, and on/off on the fuzz knob also enables power supplied to board.

That sounds like a great approach.  I think I will try that, as it will give me the flexibility to go either way.

2 follow up questions:

1.  Does anyone have any recommendations on a good positive ground wall adapter I can use with it in my pedal board?  I know I won't be able to share with my other effects since they are all negative ground.

2. Which of these 2 schemes will work best for General Guitar Gadgets (positive tip vs. negative tip):
    a)  For a PNP Positive Ground Circuit With a Positive-tip Power supply:
          http://www.generalguitargadgets.com/pdf/ggg_sw_3pdt_tb_pnp_dcj_gi_pt.pdf

    b). For a PNP Positive Ground Circuit with a Negative-tip Power supply:
          http://www.generalguitargadgets.com/pdf/ggg_sw_3pdt_tb_pnp_dcj_gi.pdf

Thanks!

[R.G.]

1.  Does anyone have any recommendations on a good positive ground wall adapter I can use with it in my pedal board?  I know I won't be able to share with my other effects since they are all negative ground.

Many people have constructed a charge-pump converter to make -9V out of +9V for powering a positive-ground pedal. In its simplest form, this needs only one 8-pin IC and several 10uF capacitors. The LT1054 is probably best for this, but there are a couple of other similar chips.

If you put something like this inside your positive-ground pedal, it will now take in negative ground power and make that into the -9V to power it.

[fuzzy645]

1.  Does anyone have any recommendations on a good positive ground wall adapter I can use with it in my pedal board?  I know I won't be able to share with my other effects since they are all negative ground.

Many people have constructed a charge-pump converter to make -9V out of +9V for powering a positive-ground pedal. In its simplest form, this needs only one 8-pin IC and several 10uF capacitors. The LT1054 is probably best for this, but there are a couple of other similar chips.

If you put something like this inside your positive-ground pedal, it will now take in negative ground power and make that into the -9V to power it.

Wow, that is awesome, thank you.  To clarify, would it be configured so that when running the pedal off of a standard 9V battery it would bypass this charge-pump converter, but then when inserting an adapter into the DC jack, the charge pump-converter would get activated (for lack of a better term) so the pedal could then be powered by a standard negative ground adapter?

Do you have a schematic posted somewhere up on Geofex?

[Mark Hammer]

You can use any supply, as log as the leads from the adaptor are flipped around.  There are a few issues to contend with, all of which are manageable:

1) Inserting a power plug needs to disable the battery, and switch over to external power.  It will not be enough to simply use a standard adaptor and connect the contacts on the jack to different places than usual.  That means that your "hot" still needs to be the outside of the plug, because it is the outside that does the contact switching.  Here, your "hot" is going to be the negative, and your ground will be the positive.

2) Even though the barrel jack will, in all likelihood be a plastic body that insulates the contacts from the chassis, grounding is still somethng to pay attention to.  The critical thing is that ground can't be both positive AND negative at the same time, so you need to use separate supplies for positive and negative ground pedals. Remember, the power jack may be insulated from the chassis, but you WILL be running a ground wire from the circuit board to the in/out jacks, which likely WILL be in contact with the chassis.  If a single wallwart supply gets daisy chained to pos ground and neg ground pedals, the pos and neg will be shorted out by their individual gropund connections in pedals.  

In many respects, you need to think of using separate supplies as if you were powering your pedalboard with a bipolar supply.  The +9v has a ground that is negative, relative to +9v, and the -9v side has a ground that is positive, relative to -9V.  Ignore the fact that the pos and neg supples are sitting in separate moulded black blobs, and ignore the fact that some of your pedals only make use of the positive side of your bipolar supply, while others only make use of the negative side.  You are running a +/-9v-powered system.

[pinkjimiphoton]

IME, you don't need a positive center wall wart. if you're gonna have to use a separate wall wart for it anyways, just use the standard center negative, and use that wart for that one pedal only. just remember to wire the pedal accordingly, ie ground is actually +.

my very first fuzzface, i thought i'd be slick and wire it up with the switching on the jack backwards. not a good idea. i also had the pcb wired as a positive ground, but the led wired like an npn. i don't even know how the dang thing worked in the first place, it was so wrong.