Strange probleme & incomplete wiring

[SebbyNC]

Hi all,

I'm currently installing a piezo system (GraphTech bridge and buffer preamp) alongside a Noll preamp. Right now, I've wired everything, consisting of 5 pots: 1 magnetic vol/balance/push-pull (active/passive switch), 1 piezo volume, 1 treble (tone control when switched to passive), 1 mid gain/mid frequency, 1 bass.

I followed GrapTech's advice which recommends to plug the magnetic output to the GraphTech "magnetic input" slot and the magnetic preamp power wire to the GraphTech auxillary power pin ("pwr"). I also used a 9-lug jack as recommended by GraphTech for using an active magnetic electronics alongside the piezo. As of today, I still haven't installed the pickups I got from SGD Lutherie, so the balance pot pickup lugs are not wired.

When I first tested the piezo (mag preamp and controls not wired) it worked, I had normal piezo signal controled by the piezo volume pot. But since I've added the magnetic electronics wiring, now I hear the piezo sound only when the magnetic volume pot is in middle position (no signal when magnetic volume is turned all the way down or up). Is there a problem with my wiring, or is it just because the magnetic pickups are soldered yet to the balancer lugs?

[SebbyNC]

Note that I didn't touch the wiring of the EQ pots which were prewired and just had to wor with the vol/bal/push pull pot. It's an MEC pot, and I resented the ground lugs on the side of the pot in the following diagram:



How does this look to you? I'm sure I have some bizarre ground connections (only the bridge and the tone pot in passive mode are connected to the cavity's copper shield), but otherwise, anything suspect?

[SGD Lutherie]

I think the issue may be that the Graphtech unit is expecting passive magnetic pickups to be attached, and not the output of a preamp. That may work fine, but you need to get the magnetic pickups/Noll working first, and then try incorporating the piezo into that setup.

Also some Noll preamps require a 9 pin jack like that because of the way the power goes to the preamp. Double check the Noll wiring. It wont work correctly if the jack is wired up wrong. I know this because I had to rewire one of these not long ago.

[SebbyNC]

Thanks for your response David. Ideed my Noll preamp requires a 9-lug jack, and I already have Noll's recommended diagram.
The GraphTech buffer doesn't absolutely require passive mag pickups, and I found on their website a diagram showing how they advise to hook active pickups to their buffer, and using a 9-lug jack.
However, Noll and GraphTech don't use the jack the same way, so I'm a little confused here, it looks like I don't have the easiest hardware combination! I guess I'll try to send an email inquiry to GraphTech's tech service, maybe they can help me out.

[SGD Lutherie]

You have to wire the Noll up to the jack the right way, or it's not going to work correctly. That's because it doesn't ground the one end of the battery to turn on the power.

I had someone bring me a bass with the jack wired wrong and there was almost no output at all.

Noll says their preamp can work with piezo pickups on the blend pot, so you might be able to just wire the piezo up as one pickup to the blend, and then wire up an additional blend control and send that to the other pickup connection on the Noll blend. So the Noll blend would fade between the piezo and mags, and the other blend would fade between pickups. Alternately you can wire the magnetic pickups to a selector switch and forgo the blend, and use the Noll's blend for mag/piezo.

[SebbyNC]

Isn't that less optimal to have 2 blend pots, than separate volume pots? I got high end pickups and electronics, I don't want to sacrifice their quality... but if it's the only solution, I guess I'll need to try this out.

[SGD Lutherie]

What I would do is get the Noll working right at the jack, and then see what you can do about the piezo buffer.

[SebbyNC]

I'm a noob in electronics, so this may be a stupid question: is it possible to solder the outputs of the Noll and of the Piezo preamps right to the jack?

[SGD Lutherie]

Yes, that would probably work. I think the problem is the Noll is not powered up the right way. You have to get the Noll wired to the jack the right way. The Noll does not have one side of the battery going to ground, so that's the reason for the 9 pin jack. He must be using a virtual ground on his power rails in the preamp.

When I get a chance I'll look over your layout and see what's going on. Maybe they can be combined.

[SebbyNC]

Thanks David, I hope I can install your pickups before the weekend and telle you how it works.