The simplified schematic is pretty simple:
R18 is like a volume control but it bypasses the dummy coil so you can control the % of the dummy coil that is in the circuit. R20 is like a tone control wired up to R18.
With those two controls you can adjust how the dummy coil will work with the sensing pickup (which is wired in series). Too much dummy coil will compress the sound- I would back it off until you got the desired amount of hum reduction. At that point you can dial back R20 to balance the brightness and fullness. Perhaps the Suhr System has something else in there but I think with the two trimmers it should work just fine.
Here is a link to the Suhr patent application, while I will also try attaching to this post:
I am not a patent lawyer- nor do I impersonate one on the internet anymore- but I don't believe that you will run into problems if you use the design as an inspiration for your own non-commercial use. I doubt that you will have a problem if you sell these guitars on a onesy-twosy basis. But if you plan to market more than that you might want to keep quiet about the hum reduction...
P.S. Fry's Electronics sells packages of 200' of 30-32GA magnet wire (I forget which). I made a coil out of one of them wrapping it around a CD spindle cover, which was slightly tapered so it slips off easily. I then wrapped it securely with Scotch 35 Gray electrical tape and that was that. Here it is 8 years later and I never did try putting that into a guitar. My bad!
Actually I am getting ready to test it out on various single coil lap steels just taping it to the back with that blue masking tape. I just need to make up an adaptor that plugs into the guitar and allows the dummy coil to be connected in series.
Does anyone remember the old MEK-based conductive paint used 30+ years ago? It was my 1975 Burgundy LP Standard and I thought I better shield the control cavities not realizing that the metal plate and cover offered plenty of shielding. The instructions said to put masking tape on the pot holes so that the paint would not damage the finish. I didn't have any masking tape but I had some Scotch Magic Tape and used that instead. I thought everything was just fine until I removed the tape and saw that the MEK had mottled the finish around the pots. Ouch! Of course I had to add insult to injury and I later chiseled out the top to mount a Kahler tremelo... That guitar sits on my wall to remind me that some of my ideas really aren't that good.
In my experience, the MG Super Shield bonds very well to existing paint, as well as to bare wood. I think the base of Super Shield is an acrylic, similar to Krylon paints. Joe's right that you do need to make sure that you get good coverage to get good conductivity throughout. I've found that two coats is plenty. You MUST swirl the can for every 5 seconds of spraying to make sure the nickel is coming out.
On my basses, I spray the clear base coats of the paint on the body first, including all throughout the cavities. This makes sure that the wood inside is all sealed up against moisture. Next, I put on a masking board and spray two coats of Super Shield in the cavities. Then I mask off the cavities and continue on with the color coats and top coats.
I used to fight with the Stew-Mac conductive paint some years back. It was a mess to paint it well, and some would always come off, leaving conductive carbon powder floating around inside the electronics! I used copper tape for a few years too. It was effective as shielding, but was a lot of labor to put in, and messy looking. Also, the tape has a habit of peeling up in some corner down in the cavity, and then shorting against some component. That can be a nightmare to troubleshoot! The trick to avoid that is to put down the copper tape first, then cover it with a layer of clear packaging tape, which sticks very well and is non-conductive.
The Super Shield is expensive (about $40 per can), but it's so much faster and neater and more foolproof.
Last edited by Bruce Johnson; 05-24-2010 at 10:16 AM.
Any difference that might have been created by the star ground is lost as soon as it connects to the output jack.
The whole premise of that shielding tutorial is flawed. It assumes that the buzz you hear from electric fields is caused by a ground loop, and that the star grounding will some how magically solve that. It wont. Also you cant have a ground loop with a single ground path... your cable to the amp.
Proper grounds and shielding are important though, but they can be standard daisy chain connection to the pots, and the pots can touch the shielding. That's just like everything being in a metal enclosure.
Going back to dummy coils... I recently had a customer bring me a Music Man Axis Sport to install a new neck pickup. The guy also told me the output jack wasn't working right. I noticed the guitar had an empty battery box, and asked if it had a preamp. He just bought it used, and said it worked without the battery. Someone had already changed the single coil neck pickup for a Duncan Little '59, and he was having me install an Air Norton S. After I opened the guitar up I could see why he didn't like the neck pickup...
What I found was a small active circuit wired to a very unusual 5 way switch. It had a patent number (5569872) on the side, so I looked it up and it's an active dummy coil, just for the neck pickup. I can't imagine what it did with no battery installed.
I used to have the exact schematic for this, as they had them posted on their web site. Now they just show the basic switch wiring block diagram. But the idea is the dummy coil goes to an op amp, and the output of the op amp goes to the pickup coil.
I don't know how well it works, since by the time the guitar came to me it had a new pickup installed. Since I had to rewire the guitar with a new 5 way switch, I kept the dummy coil unit. When I get some time I'm going to play with it.
David, that looks like the idea I have presented here a couple of times, although I might be interpreting that patent schematic wrong. I believe the coil on the left is the dummy. It is amplified and the resulting signal is inserted in series with the ground connection of the pickup (the coil on the right). I did not realize that anyone had patented it. I like my way better; it is simpler, just one FET, but the dummy has to be wound just right with somwwhat more sensitivity than the pickup.
Yes, I thought of your circuit when I saw this. Actually I saw this first, so I thought of this when I saw yours, but didn't realize they were so similar until now.
I had the actual schematic as it applied to one of their guitars, and it seemed much simpler. I lost it a couple of years ago when I erased a hard drive I thought I had backed up. Now they don't show the circuit in their guitar diagrams, just a flow chart.
They have a transistor version in the patent, but I put the op amp version since that's what I saw in the guitar schematic.
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