Perfect Switching (part 2)

      Design, Electronics

This is an installment to last week’s Perfect Switching article. Again, let me warn you that this page assumes you know a little bit of electrical wiring and reading basic schematic diagrams. If you don’t know what “parallel” or “series” wiring means, this page will provide you with some basics: http://www.1728.com/guitar.htm.

Our goal is to devise a way to optimally utilize the 3 pickup configuration in order to have the widest sonic range possible. The basic Fender Strat switching gives you 5 possibilities. Brian May’s Red Special offers 7 possibilities (not including phase switching). The Strat pickups are wired in parallel, while the Red Special pickups are connected in series.

Best of Both Worlds

It is possible to have more pickup configurations by allowing both series and parallel wiring. We are already accustomed to parallel wiring since most electric guitars are wired that way. Series wiring, on the other hand, gives you a more powerful (“hotter”) signal that easily overdrives the amp. This is good for lead playing. The tonal quality of series wired pickups compared to parallel is radically different due to various factors, most essentially, shifts in the combined pickups’ resonant frequencies when the total pickup inductance is shifted up (series) or down (parallel). To give you an idea how different the total inductances are, given 3 identical pickups each with 10mH inductance:

  • The total inductance when wired in series is 30mH: (10mH + 10mH + 10mH).
  • The combined inductance when wired in parallel is 3mH:  1 / ((1 / 10mH) + (1 / 10mH) + (1 / 10mH))

With series wiring, the resonant frequency is shifted down while the power is increased.

Basic Building Block

The schematics presented in these pages are 100% original and has been prototyped and fully tested. You are free to copy the ideas presented here subject to the MIT license.

Let’s start with a 2 pickup configuration. This will be our basic building block that can be extended to 3, 4 or even more pickups. With 2 pickups, you have these possible permutations:

  1. B
  2. A
  3. A | B
  4. A + B

(See my previous article to understand the notation I am using)

To get these combinations, you need two DPDT switches. Here’s the basic schematic:

There’s really no on or off settings here. All switch settings will produce an output. Instead of on-or-off, let’s think in Boolean: 0 or 1. 0 (zero) means that the switch is in its “left” position and 1 (one) means that the switch is in its “right” position.

Here’s the switching table:

Sw2 Sw1 Combination
0 0 B
0 1 A
1 0 A | B
1 1 A + B

There are different ways to achieve the same set of combinations. A previous version of this article employed a simpler wiring scheme. What’s unique about this updated schematic is that you have one switch (SW2) to control the choice of using one pickup (A or B) or two pickups (A and B). SW1 chooses between A or B when SW2 is in the one-pickup setting. The same switch (SW1) doubles as the series-parallel switch when SW2 is in the two-pickup setting.

The following illustrations show the signal flow for each of the 4 switch settings.

  1. Pickup B only

  2. Pickup A only

  3. A and B in parallel

  4. A and B in series

3 Pickup Configuration

With 3 pickups, you can simply use the same schematic as above and insert that into either the left box (A) or right box (B). It’s easier seen than said. Here’s the schematic:

With this setup, we have 13 possible combinations. The 3 pickups can be combined in series, parallel, series-parallel or parallel-series. Here’s the list of usable permutations:

  1. C
  2. B
  3. B | C
  4. B + C
  5. A
  6. A | C
  7. A | B
  8. A | B | C
  9. A | (B + C)
  10. A + C
  11. A + B
  12. A + (B | C)
  13. A + B + C

Here’s the switching table. Note that we’ve introduced a new “don’t care” symbol: ‘x’. Where there’s an ‘x’, the switch’s position does not matter.

Sw2 Sw1 Sw4 Sw3 Combination
0 0 0 0 C
0 0 0 1 B
0 0 1 0 B | C
0 0 1 1 B + C
0 1 x x A
1 0 0 0 A | C
1 0 0 1 A | B
1 0 1 0 A | B | C
1 0 1 1 A | (B + C)
1 1 0 0 A + C
1 1 0 1 A + B
1 1 1 0 A + (B | C)
1 1 1 1 A + B + C

Ok, well, there you go. We now have a whole set of pickup combinations to choose from. With 4 switches, we are able to choose from all possible series/parallel pickup combinations. And we haven’t even added phase-switching yet, which, as mentioned in the first part of this article series, gives you a thin, squawky sound that is only occasionally useful.

Stay tuned for the third part that will conclude this series…

Next: Perfect Switching (part 3) next
Chris Soyars says:

Joel,

I know that this is an old post, but it’s relevant to certain recent interests. You say in this post that you have all possible combinations here, but isn’t that untrue? It seems that four are missing:

(A|B)+C
(A+B)|C
B|(A+C)
B+(A|C)

Would these not be distinct from A|(B+C) and A+(B|C)?

You are right, Chris. And that is a very good observation. Yes, these extra combinations are distinct. The limitation of this scheme I presented is that the B and C pickups are hard-wired as a group and actually forms the basic scheme (for two pickups), and that group [ B C ] is again grouped, in a modular manner with A again using the basic scheme. The grouping is as follows:

A [ B C ]

Thus, you cannot have the 4 combinations you mentioned. It is not possible to combine A and B nor A and C as a group. I’ll update the blog and make that correction. Thanks for noting.