Wiring an active preamp to output jack?

[Steven D. Wilson]

I am working on an acoustic guitar with an under saddle piezo pickup running to a side-mounted active Shadow P3-Pro Preamp/EQ box. The shielded wire from the piezo is there - but the wiring that runs from the box to the output jack has gone missing.

The missing wire should connect to the preamp via a 1/4" plug. I know there are usually three wires running to the active output jack, with one disabling the battery when a plug is not in the output jack. 

Does anyone have any idea what I need here? I tried a standard stereo headphone plug and wire, but that did not seem to work. Would a mono plug work? Where does the third wire come from?  I cannot seem to find any info on this. I would appreciate any input anyone has.

Steve Wilson

[Steven D. Wilson]

Hi Victor,

Turns out the guy whose unit I was working on upgraded and gave me the old shadow unit to keep. Your pictures are a little blurry, but everything I can see looks just like mine with all the right color wires going to the right points on the board. Hard to see where your wires go into the jacks themselves, but referencing your top picture, the red and black wires on the left both go to the smaller jack - the red one goes to the bottom lug and the black one goes to the top. The red wire on the same side as the black and green runs directly to the positive battery clip. The green wire goes the the middle lug on the larger jack and the black wire goes to the lug of the larger plug closest to the input opening (the front). The black wire attached to the top left lug of the larger jack goes to the battery negative.

One other thing, I remember this unit did not work when the mini plug from the pickup was pushed all the way in the jack. When I backed it out slightly it worked fine? Hope this helps.

[Victor Harwood]

Fantastic!  Thanks for the quick reply!  Sorry for the blurry photos, my camera is an old POS - these photos were the 'good' ones.

Just to be extra certain I've got this right, and for anyone else who comes along later, I'd just like to clarify things a little regarding the jacks...

So, for the smaller [3/32"] jack: the red wire from board goes to the bottom lug - this would be the tip connection - and the black goes to the top lug , which would be the outer sleeve connection.  Clear enoiugh.

Since I have replaced the 1/8" stereo mini-jack, my lugs have a slightly diferent physical arrangement from stock. 

In John's reply, he says, "The 1/4" output jack is 3 conductor or tip ring sleeve. The tip is the signal from the preamp the ring is the battery ground and the sleeve is the ground for the preamp circuit."  The 1/8" mini-stero plug and 1/8" mini-stereo jack would therefore have the same configuration.

You're saying that the black from board goes to the lug closest to the front of the jack, which is always  the outer sleeve connection.  Therefore, the black from the board woud have to be the ground from the preamp, connecting to the outer sleeve.

Then we have the black from the battery going to the top left lug on your stock unit, which would therefore have to be the center or ring connection of the 1/8" mini-jack from John's description.

That leaves us with that the green wire on your stock unit going to "the middle lug on the larger jack" [the 1/8" stereo mini-jack], and therefore it would have to be the signal wire from the preamp and would have to be the tip connection.

Does this sound correct?

 

PS - The most important connection to get right is the signal out  from the board, since the black from the battery and the ground from the board are shorted together by the insertion of a 1/4" mono plug on the outside of the instrument to connect the battery ground to the preamp ground, which then powers up the preamp.  Therefore, those connections could be interchanged without any consequence.

[Steven D. Wilson]

Hey Victor,

So, for the smaller [3/32"] jack: the red wire from board goes to the bottom lug - this would be the tip connection - and the black goes to the top lug , which would be the outer sleeve connection.  Clear enough.

This is all correct! The red and black wires from the side of the board WITHOUT the green wire attached.

And for the larger jack...These three wires are all coming from the side of the board WITH the green wire attached.

You're saying that the black from board goes to the lug closest to the front of the jack, which is always  the outer sleeve connection.  

YES!

Therefore, the black from the board would have to be the ground from the preamp, connecting to the outer sleeve.

It would seem so

Then we have the black from the battery going to the top left lug on your stock unit, which would therefore have to be the center or ring connection of the 1/8" mini-jack from John's description.

I cannot see inside the jack unit, but I would think this black wire coming directly from the battery would be the tip as it is the attached to the jug the furthest away from the opening of the jack sleeve.

That leaves us with that the green wire on your stock unit going to "the middle lug on the larger jack" [the 1/8" stereo mini-jack], and therefore it would have to be the signal wire from the preamp and would have to be the tip connection.

The green wire does go to the middle lug, but since it is in the middle of the jack it doesn't seem like this would be going to the tip. Again, I cannot see inside the jack, but if logic dictates.

Hope this helps!

Steve

[Victor Harwood]

Hmmmm...

Ok.  You have posted all of the proper physical connections for the stock jacks and to the circuit board.  Excellent!

We also have determined with certainty the proper geometry of electrical connections on the smaller [3/32"] jack, and also that the black wire from the board is electrically connected to the outer-sleeve of the 1/8" stereo jack.  Excellent!

The only uncertainty now is regarding the internal geometry of the 1/8" jack and which of the two remaining terminals electrically connects to the end-tip vs. the middle-ring.

I went thru a box of parts and located a 1/4" mono plug and a 1/4" stereo jack to test what happens when a mono plug is inserted into a stereo jack.  In John's reply, he says,

"The tip is the signal from the preamp the ring is the battery ground and the sleeve is the ground for the preamp circuit. When you use a mono 1/4" plug the ring and the sleeve are connected allowing the battery to go to ground."  Testing revealed that the middle-ring and outer-sleeve do in fact connect, leaving the end-tip as the only remaing location for the signal.

Since we have certainty that the black from the board electrically connects to the outer-sleeve (and is therefore the ground from the preamp), the black from the battery must be electrically connected to the middle-ring.  And therefore, the green from the board must be the signal and electrically connected to the tip.

I am going to wait a few days to allow for any more replies and then wire my unit up and test it.  I wil report back with the results.

Thank you so much, Steven...  I couldn't have done this without your help!

[Victor Harwood]

...IT"S ALIVE...!

Many thanks go out to all who have posted!  My unit is alive and fully functional again.

It took me a long while to get around to it, but I did successfully re-wire the larger (1/8") jack as described above (posted on April 4, 2013 at 8:07am) and the wiring to the circuit board is correct as photographed in this thread.

Thanks again,

Victor

[Steven D. Wilson]

Great Victor!

I love it when it works out that way.

Glad I could be of some help.

Steve