Using PNP transistors to switch power to FX circuits

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Using PNP transistors to switch power to FX circuits

Postby jalmonsalmon » 17 Nov 2014, 22:53

Hey all...
Quick question about this one...
Looking at the schematic here http://www.geofex.com/FX_images/PNP_power_switching.pdf
Red wire from battery Power goes into the FX Circuit board from +V and the 0V would be ground? ( connect ground wire to RBase along with the battery black wire and then to the sleeve of the jack)
That is how I am understanding the schematic anyway...
I want to try this out with a pedal I have that is high gain that oscillates and hopefully this trick will solve that issue, or at least help out.

Any help much appreciated!
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Re: Using PNP transistors to switch power to FX circuits

Postby ggedamed » 18 Nov 2014, 15:01

It's like this:
2014-11-18_154317.png

The red (positive) wire from the power supply (battery) goes to the junction of Q1's emitter with R-jtbs. The collector of Q1 (where it says +V on the drawing) then connects to the positive supply rail of the effect.
The negative (black) wire from the power supply (battery) goes to the PCB where it connects to the negative supply rail (usually named GND) of the effect.
Rbase goes directly to the ring of the jack - it is NOT connected to the PCB or the negative side of the battery. Wherever you see that kind of loop in a schematic, it's the old style of saying "these two wires are not connected".

All this arrangement is based on the assumption that the effect PCB is well laid out, especially the ground traces. I'm really curious if it'll work against high gain circuit oscillations. My impression is that the current magnitude in a stompbox (not including digital ones) is not high enough to be of concern. It seems far more probable for me that the oscillations are caused by feedback due to the layout and/or wiring.
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Re: Using PNP transistors to switch power to FX circuits

Postby jalmonsalmon » 18 Nov 2014, 17:31

ggedamed wrote:It's like this:
2014-11-18_154317.png

The red (positive) wire from the power supply (battery) goes to the junction of Q1's emitter with R-jtbs. The collector of Q1 (where it says +V on the drawing) then connects to the positive supply rail of the effect.
The negative (black) wire from the power supply (battery) goes to the PCB where it connects to the negative supply rail (usually named GND) of the effect.
Rbase goes directly to the ring of the jack - it is NOT connected to the PCB or the negative side of the battery. Wherever you see that kind of loop in a schematic, it's the old style of saying "these two wires are not connected".

All this arrangement is based on the assumption that the effect PCB is well laid out, especially the ground traces. I'm really curious if it'll work against high gain circuit oscillations. My impression is that the current magnitude in a stompbox (not including digital ones) is not high enough to be of concern. It seems far more probable for me that the oscillations are caused by feedback due to the layout and/or wiring.


Thanks ggedamed!
I have a couple pedals that oscillate and am going to clean up the wiring, use some shielded wire for all the input and outputs and make sure there is no cross-talk anywhere and try out this little trick to see if there is any merit to this. Could be nice for dual effect pedals that contain a high gain circuit / booster or whatnot
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