Help with strange noise after EMG SPC and PA-2 install

[defdwight]

Hello all, I have just finished installing an EMG SPC and a PA-2 in my Fender Strat. I'm running 3 Lace Sensors, a 250K volume pot, a 250K tone pot (with a .01uf cap) and the SPC in the 2nd tone pot position. I have the setup wired as per the instructions in the PA-2 install guide under "PA-2 on the output with an EQ circuit." The only difference is my electronics are star grounded.

So plugged in, everything works fine. My problem is that at certain points of the volume and tone knob positioning I get this bizarre oscillation/feedback noise. It's very similar to parasitic oscillation commonly heard in DIY high gain amp builds. With the tone knob at 0 or 10, it occurs at all volumes. With the tone knob in between those points somewhere (like 5), it occurs only when the volume knob is between 7 and 8. The rest of the time, there is no noise. The guitar is completely hum free. The oscillation/feedback noise is a constant tone. It changes pitch with slight adjustments of the knobs as long as they are still in a position that causes the tone to be present. The tone occurs regardless of whether the PA-2 is switched on or off and regardless of how much the SPC is dialed in. I'll add that the noise is only present when the battery is plugged in. When the battery is unplugged, I cannot get the guitar to make the noise anymore.

I've searched the net tirelessly trying to find an answer and I can't find anything. EMG has little to no troubleshooting info on their site and the forum is pretty lifeless. I'm really stuck here, any help is GREATLY appreciated!!

[Sir Cuitous]

Welcome to the forum defdwight.

Since the oscillation occurs seemingly independently from the PA-2, I would first remove the PA-2 and see if I could get the SPC to work by itself.

I'm not sure what you mean by start grounding in a guitar. The EMG diagrams all ground to the pots, though I'm not sure if grounding has anything to do with your oscillation.

Both the SPC and the PA-2 connect between the volume control and the jack. Did you wire them in series or parallel? I believe EMG would recommend "series".

According to EMG the SPC is a mid boost. The PA-2 is a booster pre-amp. How much boost do you need?

[Sir Cuitous]

defdwight, can you also post the wiring diagram you used? All I can find on the EMG site is this:

http://www.emginc.com/content/wiring...%20%20PA-2.pdf

It doesn't show "PA-2 on the output with an EQ circuit".

[defdwight]

Welcome to the forum defdwight.

Thank you.

Since the oscillation occurs seemingly independently from the PA-2, I would first remove the PA-2 and see if I could get the SPC to work by itself.

The gain boost on the PA-2 is switchable, but regardless of the switch position the PA-2 converts the output to a low impedance. So in other words, it is still "working" in a way even when it's not introducing a gain boost.

I'm not sure what you mean by start grounding in a guitar. The EMG diagrams all ground to the pots, though I'm not sure if grounding has anything to do with your oscillation.

Star grounding is connecting all grounds to a single point. Grounding from pot to pot creates ground loops. Star grounding is a common mod in Strats to reduce/eliminate hum from ground loops that the factory design is responsible for.

Both the SPC and the PA-2 connect between the volume control and the jack. Did you wire them in series or parallel? I believe EMG would recommend "series".

They are indeed wired in series.

According to EMG the SPC is a mid boost. The PA-2 is a booster pre-amp. How much boost do you need?

The SPC does more to shape the tone rather than boost the signal. It's not a preamp, but rather an active EQ. The PA-2 is basically just a preamp. The two actually complement each other well.

Two answer your other question, I just noticed that the EMG site has an updated wiring schematic posted. My PA-2 is an older model. Same circuit design, but older layout (not quick connect compatible). They do not include the schematic I used in this newer rev. So here's a quick description of the wiring:

Output of the volume pot connects to white lead of the SPC. Green lead of the SPC connects to the white lead of PA-2. Green lead of PA-2 connects to tip of output jack. Both SPC and PA-2 grounds are grounded at the star point of the entire circuit (the point where all grounds are grounded). Both the SPC and PA-2 red leads are connected to the 9v battery clip. Neg side of the battery clip is connected to the ring of the output jack. Sleeve of the output jack is connected to star point.

Thanks so much for your input!

[Sir Cuitous]

The SPC does more to shape the tone rather than boost the signal.

EMG says the SPC is a mid boost right in the PDF, hmmm. I read your wiring description and looked at the diagrams that are offered online by EMG. It sounds like you got it right. The only other thing I could add would be to setup the pickguard next to the guitar or with a tester jack connected and push some wires around while the noise is on. Listening for changes could give you clue about where the noise is coming from. It may have something to do with the power in the circuit sympathetically driving the oscillation since it needs the battery to happen. Maybe the layout of the red wires allows the circuit to feedback under certain conditions.

[defdwight]

The only other thing I could add would be to setup the pickguard next to the guitar or with a tester jack connected and push some wires around while the noise is on. Listening for changes could give you clue about where the noise is coming from. It may have something to do with the power in the circuit sympathetically driving the oscillation since it needs the battery to happen. Maybe the layout of the red wires allows the circuit to feedback under certain conditions.

Funny you should mention that as I did do that. Today I had the pickguard unscrewed and inverted to do just that. I have built amps before and run into this problem so having troubleshot that way with amps, I tried it with the guitar. I did not notice any difference in moving wires around which has me very perplexed.

I really appreciate your input, thanks so much for chiming in. If you have any other ideas, I'd love to hear them. I'm gonna try to get on the horn with EMG tech support tomorrow but don't know what kind of luck I'll have.

[Sir Cuitous]

I've wired a few guitars in my day.... not many actives though. It's an odd problem that you have. There are many good "techs" on this forum. Maybe someone else will have some input. There could be some kind of impedance matching issue, since the oscillation is dependent on the settings of your pots. It may just require a resistor inbetween the SPC and the PA2 or somewhere, but now I'm just fishing for ideas. Good Luck.

[David Schwab]

Star grounding is connecting all grounds to a single point. Grounding from pot to pot creates ground loops. Star grounding is a common mod in Strats to reduce/eliminate hum from ground loops that the factory design is responsible for.

You can't have a ground loop in a guitar since it only has one ground point, at the jack. It's a myth. If you look at EMG's prewired assemblies, they have a big bus wire connecting all the backs of the pots together, so they obviously don't agree about the risk of a ground loop.

Oscillations are often caused by input and output wires being too close together and not shielded.

It could also be a missing ground because of the star setup.