Wiring a "virtual ground" circuit to cut off battery power when input unplugged?

[soma89]

Just built an ESR Graphic Fuzz. Op amp uses a +4.5v/-4.5v power configuration. Therefore, I cannot use the -v on battery to the input tsr jack to cut the battery off the circuit. Both battery terminals are soldered to the pcb. How else can i wite it so the pedal doesnt eat up batteries when unplugged from guitar? Thanks in advance

[drdn0]

Why not? How is the split rail created?

[PRR]

I can't find an authoritative complete schematic. Can you link to one, please?

The obvious hack is a DP switch and the discipline to use it every time.

Another could be switcher-module 10V to +/-5V.

[Rob Strand]

The schematic is here, based on a verified retracing of the schematic by Jerms,

https://www.diystompboxes.com/smfforum/index.php?topic=125921.msg1204786#msg1204786

In that thread it is noted the circuit can be oscillate and the solution was to put a 470 ohm
resistor in series with the 100nF cap which connects across the opamp input.

As far as switching the power:   One way it can be done is to use jacks which have separate switch contacts.  Then you can either switch the battery positive or negative because the switch contacts are isolated.  These sockets tend to be less robust than the common jacks - nonetheless MusicMan and some Ibanez basses used them.

As far as I remember, converting the circuit to a negative ground set-up might need some care since the "Vref" circuit is part of the sound.

Another way around it would be to have the socket contacts (or a common stereo jack) control a BJT or MOSFET then the BJT or MOSFET would switch the battery.

[amptramp]

For single supplies, there is a standard way to connect the input jack so when it is disconnected, the power is no longer applied.  If you do that with the input jack and the output jack, you could shut off both supplies.  Just make sure your circuit will survive with one supply connected and the other not connected.

[soma89]

thanks for the responses thus far.

Here are the schematics and layout i worked off of..

Veroboard layout
https://blogger.googleusercontent.com/img/a/AVvXsEhSNCt4i62Fnh-z7VQ5vu7dE5lnlajb89mKVRdWVCXyevUXpdOx4mGnZ_BnfW_1ezrqM-7YWg3LhoN2rKoA6zB7kj0S7e2_-gnFf8GLmnD1OYQZLMPmcl40qv9I5mfyREMHVsblP3lMjuMbLqsjzePPNhiXsP-xVi8L_Us4_go8IFiZzEJKzGTbBiYF=s1299


Schematic
https://postimg.cc/1fP7JcrR

I originally wired the footswitch so it cuts off the battery when switched off but I dont trust myself to turn it off all the time. I've seen clones that dont look like theres anything special going on with the way the jacks are wired but id have to keep looking

[soma89]

 Here's a gutshot of a Jerms copy. I cant really tell how its wired but maybe somebody can chime in.

Is he using a jack with a switch perhaps?

[ElectricDruid]

 Here's a gutshot of a Jerms copy. I cant really tell how its wired but maybe somebody can chime in.
Is he using a jack with a switch perhaps?

Yes. If you look at the jack on the left, you can see there's an insulated tab which will be pushed by a plug being inserted and will then contact the piece underneath it. That's the switch that's going to power the thing up only when something is plugged in.

[merlinb]

Have you built it on a proper PCB? If not, it might be easier just to add a couple of caps and convert the circuit into the more ordinary configuration where you can switch power with the jack.

[soma89]

 Here's a gutshot of a Jerms copy. I cant really tell how its wired but maybe somebody can chime in.
Is he using a jack with a switch perhaps?

Yes. If you look at the jack on the left, you can see there's an insulated tab which will be pushed by a plug being inserted and will then contact the piece underneath it. That's the switch that's going to power the thing up only when something is plugged in.

Ive found a jack like this.
Can it be wired the same way? Thanks

[merlinb]

Ive found a jack like this.
Can it be wired the same way? Thanks

No, that will only work with a standard-style 9V circuit. You need a Switchcraft 13E type jack.

[Rob Strand]

If you look follow the link to the earlier thread it has gut shots of the original pedal.  You can see one of the jacks is plastic.
The wiring looks like it switches the negative terminal.

https://www.diystompboxes.com/smfforum/index.php?topic=125921.msg1204786#msg1204786

The jack looks just like this one, where there is a contact on the chamfered corner of the connector,

SWITCHCRAFT 113X JACK ISOLATED 6.35MM 1/4 INCH MAKE CIRCUIT SWITCH
https://www.ebay.com.au/itm/276487300015?var=0&mkevt=1&mkcid=1&mkrid=705-53470-19255-0&campid=5338590836&toolid=10044&customid=c8de1ae96a821ff86a4f72e96e7ad4f6&gclid=c8de1ae96a821ff86a4f72e96e7ad4f6

This link contains a link to the datasheet, (the actual part being sold doesn't have the switch).
https://au.mouser.com/ProductDetail/Switchcraft/113X?qs=iwvYsH9bSFbrUaj7QgyZ6A%3D%3D

You can see from the datasheet and the gut shots:
- battery negative connects to the RING SPRING (R) contact
- the circuit negative connects to the RING SHUNT (RS) contact

[ElectricDruid]

Ive found a jack like this.
Can it be wired the same way? Thanks

I don't think that jack can even be used with the usual connect-the-ground wiring, because it looks like it has a normally-closed contact that gets opened when you push the jack in. A mono jack plug into a stereo jack socket acts the other way around - to short together the ring and sleeve when a jack is inserted. And the jack that's in that picture you posted works the same way around - it connects when something is inserted - but it has the added benefit of being isolated from the jack connections.

[Rob Strand]

If you want to use a common Stereo Jack then you need to add a circuit.

Here's the basic idea: (a real circuit will need some ESD protection)

[Rob Strand]

So one thing about that MOSFET circuit is it won't work, or could misbehave, when the battery voltage is low.  A BJT version, or different MOSFET, might operate down to lower voltages.   The BJT version would load down the pedals 10k divider a bit.

[R.G.]

You do have to pick your MOSFETs to get the threshold voltage appropriately low.

As you say, Rob, you get half the battery voltage to enhance the MOSFET. A 2N7000 has a Vgson of about 0.8 to 3V, typically 2.1V (OnSemi). If you get the typical Vgs, the switch works down to a hair over 4.2V of battery. If you get the worst case, it works down to 6V of battery. Probably workable.

The TN0702 has a maximum Vgson of 1V, so it would start to switch with a 2V-ish battery - well and truly dead for a 9V battery.