Pseudo Phantom Power and DC stability/protection

[Le québécois]

I have a preamp (Tillman) in my guitar and I would like to power it with +9V using the third lug of a stereo jack (so a 9Volt phantom power that is not really phantom).
The +9V would come from my newly multi-fx build in which the jack input would also be stereo and would be connected directly to the +9v power supply. This way, I'm using the left signal and ground for my guitar has usual and the extra wire to conduct the power to my guitar preamp.

My concern is -

Q1 : The +9volt on the stereo jack will inevitably be in contact (for few millisecond) with the ground part of the input jack in my guitar (when I introduce the stereo jack). This will introduce a positive DC voltage in the pickup and back in my multi fx through the ground wire (even up to the amp). Is this an issue?

*** One would argue that I can simply turn the power on AFTER the stereo jack is introduce or before I introduce it in the multi-fx. But I would prefer not to rely on that.
Can a blocking capacitor correct/protect against this problem? If so what µF value, polarized or not and orientation if polarized?


Two other not related questions if I dare!

My newly build multi-FX unit (TS808/BMP/D+/Rebote/Smallclone/TremulusLune/Dynacomp/Dr.Quack/MXRµAmp) with led and border lighting drain about 300mA. I'm using a 500mA power supply from B*SS that is connected directly to a PCB separated in to two big part (+9V and Ground). This PCB is utilize as a hub for powering all my FX and LED and for current to come back to ground star grounding.

Q2 : Would it be a good idea to implement this PCB with a high µF Cap (between +9V and ground) to serve as a DC fluctuation buffer and if so how to calculate the require µF. Remember that the beast need about 300 mA to work.

Q3 : Polarity protection would also be possible but my understanding of how to do this is limited. Should I place a Diode with the negative end on the +9V and the positive on the ground and this is it?

many thanks

[Le québécois]

Let me rephrase ..

if 9V is applied on the ring of a stereo jack with a power supply, the electricity will inevitably be in contact with the ground part of the stereo input for the few millisecond require to introduce the jack in the input. This mean that +9v would briefly be present on the ground wire of the entire circuit (guitar, pedal, amp). Should I care? (Noise, popping, fx polarized capacitor ....) 

Hope this was short enough not to discourage anyone to read this.

 

[alanlan]

I think there will always be the possibility of a short on the supply so it should be current limited at the very least - imagine putting the jack on a metal surface.  Proper phantom power is current limited by resistors.  There is no inherent problem as long as a circuit (other than the one intended) can be formed.  You can touch a 9V battery terminal to anything and nothing will happen unless there is a return path the the -ve terminal, even if it is via a capacitor.

What is the cost of a new battery? 

[Le québécois]

What is the cost of a new battery? 

When I forgot to disconnect the jack from my guitar, the battery is drain overnight (not to death but still enough). I would prefer to power my guitar preamp with lower environmental footprint than a battery! Plus, I found that the preamp is less noisy and more sparky with brand new battery so a power supply would give my that all the time.
Any alternative would be appreciated but I'm constrain by this : I have a stereo input in my guitar (so spare ring to be use) and my 9V source is my multi-fx pedal (I also have a stereo input there).

[Hides-His-Eyes]

It sounds like a good idea to me. I'd add DC blocking on the tip if there isn't some already, and put a Current limiting resistor inside the MFX box where it supplies power; put it before a large (100u) capacitor. Like, 100R or so, at 1 watt (5 470Rs in parallel would be fine if you only have 1/4W resistors)

[Le québécois]

It sounds like a good idea to me. I'd add DC blocking on the tip if there isn't some already, and put a Current limiting resistor inside the MFX box where it supplies power; put it before a large (100u) capacitor. Like, 100R or so, at 1 watt (5 470Rs in parallel would be fine if you only have 1/4W resistors)

I'm confuse by the DC blocking on the tip. Is it to protect the signal line in the case of a short between ring and tip? I didn't think about that situation! I tough about ring sleeve short but your right, ring tip short could also be an issue.

So two question 1) the DC blocking cap on the tip need to be in the guitar (between guitar pickup and tip of the stereo input). Since there is already an output cap on the Tillman preamp. It will do the job right? The setup is : pickup into preamp to jack tip.

Question 2: the large cap in the MFX (what is it for?). The resistor will limit the current in case of a short so the power supply won't ''explode'' but the Cap.... no clue. 

thank you

[Le québécois]

Can someone check this scheme and confirm that it will be danger free for the PSU, MultiFX and ME!

I would also like to know which capacitor value to choose and if the propose orientation are correct (red wire)

http://www.myphotos.yahoo.com/s/20tkmzkk0adaxqkf5vxn


If you want to explain why these part are require, you are more than welcome.

thanks

[Hides-His-Eyes]

The 100uF should go to ground; capacitors block DC when in 'line' with it and smooth DC when put from a DC line to ground.

I'd use 100n or 470n Film for the blocking caps, or a non-polarised 1u electrolytic.

I wouldn't be uncomfortable putting that in a guitar as drawn. The power supply and circuitry are both protected.

[Le québécois]

Thanks for your help.
I will try it this week end.

[tempus]

I have a similar setup - I use an XLR plug/jack at the multifx end of the cable (and into the multifx unit). I plug guitar in first, then the XLR, avoiding any issues. You could also just use XLRs at both ends, then you're pretty safe from any shorts.

That being said, I do have a current limiting resistor in the multifx end. Another option would be to put the current limiting resistor for the JFET in at the multifx end of the circuit.