Cancelling electromagnetic interference on a single coil guitar.
Started: May 2012.
Edited: October 2014
Edited: May 2016
Here’s a quick video of the test coil after it was fixed to my Godin.
A device can be created that replaces the backplate of a strat style guitar. This consists of a large noise picking coil that is placed in series between the guitar’s pickups and signal ground. The electrical properties of this coil are such that it does not affect the sound of the instrument.
- Create a device that cancels out or mitigates hum interference from audio frequency EM sources
- Device does not change the tone of the instrument.
- Device is a reversible modification to an instrument.
- Device is significantly cheaper than the commercially available offering.
Bill Of Materials
- Enamelled wire 36AWG ~ £12 inc VAT
- SPST Switch
- Hookup wire
Build Instructions – May 2016
- Create a loop coil former of the desired size and shape. I used small picture hanging nails hammered into a bit of softwood and angled them outwards to trap the wires against the wood.
- Secure the starting end of the wire to a spare nail and solder a stranded insulated wire to the end (7/0.2 is fine).
- Wind the loop wire around the former. 200 turns recommended. If using the recommended wire you will have to join wires reels together at more than one point. Use heatshrink.
- Secure the finishing end end of the wire to a spare nail and solder a stranded insulated wire to the end (7/0.2 is fine).
- Use small pieces of insulation tape to wrap the coil.
- Affix to the back of the guitar using a method of your choosing
In April 2012 I played guitar in the pit for a production of The Full Monty at the Palace Theatre in Mansfield. The show being largely funk based, I took along a guitar with single coils (SCs). SC hum has never been the biggest issue for me, but like many theatres the palace theatre is fitted with an induction loop system. The SCs picked up the loop signal (which of course contained my guitar sound from a PA feed) and horrible feedback ensued. I finished the week’s run of shows using the pickups only in RWRP mode to cancel the interference.
Let me be clear: I’m not against loop systems(I admit to bias in their favour!). It’s down to the musician to sort the problem so here we go…
- Humbucker. A four conductor humbucker can be wired in parallel. Certainly cancels hum, gives a sound something like a single coil, but not entirely!
- Stacked Humbucker as offered by Seymour Duncan and DiMarzio. These are reported to work well and give a better single coil approximation, but by necessity, they reduce the size of the ‘wanted signal’ magnets and thus change the characteristics of the pickup.
- Backplate cancellation loop (marketed as Suhr BPSCC or Ilitch BPNCS). Ideal, but the commercial options are not wallet friendly. Ideal for a pro who can afford the £300+ Suhr price tag to complement a £2000 Suhr guitar, but not an option for me or other semi-pros and ‘weekend warriors’. Also, these products aren’t compatible with strats with reverse polarity middle pickups (such as mine). Edit September 2014: Lindy Fralin prices now down to $200.
A cost effective DIY implementation of the backplate cancellation loop will be the discussion of this page along with a proposal for a tweak to allow usage with RP middle pickups.
Note: This experiment makes use of information from US patent 7259318B2 filed by Ilitch S Chiliachki on Aug 27th 2007, although the idea of hum cancellation in guitar dates back to Seth Lover in the 50’s and magnetic field cancellation loops are regularly used in other fields. I don’t claim it as my own work. To the best of my knowledge I do not infringe any of the inventor’s rights by disseminating the information below. It is not my intention to make these for profit. Suhr and Lindy Fralin offer systems licensed from the inventor.
- If desired, you could wax pot the large noise picking coil to eliminate the risk of microphonics. Discussion of this procedure can be found elsewhere. CAUTION: Do not attempt to wax pot the large coil unless you are sure you can do it safely.
The large noise picking coil is wound with thicker wire than the signal picking coil. This reduces the resistance of the coil. It also has fewer turns and no core, this reduces the coil’s inductance. The bridge pickup of my strat has a DC resistance of just over 6K. Say we use 200 turns on an example 20×20 cm loop using the 36AWG wire listed above. The wire needed would be 160m long. Since it has a resistivity of 0.987ohms/metre the total resistance would be 157.92 ohms.I haven’t measured the inductance of my pickups, I’d guess at about 1-3Henries, where the noise pickup coil will be more in the milliHenries range. Because of the large difference in magnitude, it is assumed that RDC(signal coil) ~= RDC(signal coil + noise coil) and ZL(signal coil) ~= ZL(signal coil + noise coil).
Could the coil be implemented as a thin track on a PCB? First instincts say that to lower the resistivity enough of the track it would have to be made thicker which might end up with an unmanageable size of backplate.