Series vs. Parallel Wiring
What it Means, Why it Matters

Series vs. Parallel Wiring:
When components are wired "end to end" where current must flow through one component to get to the next, they are said to be wired in series. When they are wired such that they "share ends" where current can divide so that part of the current flows through one component and part flows through another, they are said to be wired in parallel. Sometimes parallel is also called "shunt."

In the right-hand circuit in the image above R1 is in series with the parallel combination of R2/R3.

Series vs. Parallel Wiring as Applied to Guitar Pickup Selection:
On a stock Strat, pickups are wired in parallel at the "2" (neck/middle) and "4" (middle/bridge) positions. One of the modifications detailed on these pages not only allows you to select pickups in any combination, but to select whether they will be wired in series or parallel. I consider this one of the most useful modifications possible (after shielding). Without going too deep into the theory we can say that series vs. parallel pickup wiring affects the final tone in a couple of ways:

  1. Pickups typically have an impedance of about 4k to 10k ohms. When the pickups are in parallel, each is a fairly low impedance inductive load on the other. In short, each pickup is effectively a primitive high-pass filter between the other pickup and the amplifier. When the pickups are in series, they do not load each other and the filtering effect is much less noticeable because the variable impedance component is now in series with the (typically quite high) input impedance of the amplifier.
  2. The above mechanism also has a noticeable effect on the output level of the pickups. In every case that I've tried the guitar is noticeably "hotter" when the pickups are wired in series. Again, this is because the pickups are not acting as a load on each other.
  3. The effective inductance of two inductors (coils) is radically different when those inductors are wired in series than it is when they are wired in parallel. Inductance is one of the factors that modifies frequency response.
  4. Depending on how sensitive the first gain stage (stomp box, amplifier, etc.) is to impedance and inductance, the actual difference in tone between series and parallel wiring may be subtle or very noticeable.

Inductance is similar to resistance in that the total inductance is simply added when inductors (pickups) are wired in series -- but you take the product over the sum for parallel inductors. Let us say simply for the sake of argument that each of the pickups has an inductance of 10mh (inductance is measured in Henries, or more commonly, in millihenries):

  • In series, our total inductance will be 10mh + 10mh -- or 20mh.
  • In parallel, our total inductance will be (10mh X 10mh) / (10mh + 10mh) = 100mh / 20mh -- or 5mh.
Remember, inductors work just the opposite of capacitors -- they tend to impede high frequencies while passing low frequencies. Of course, it's not quite this simple because the pickups aren't simply inductors -- each is also generating a signal. However, I'm sure you can see that tonal response will vary quite a bit depending on whether the pickups are in series or parallel. It's difficult to reliably quantify this difference but I'll go out on a limb and describe pickups wired in series as having a bit more midrange punch than the same pickups wired in parallel. Usually. No guarantees.

So, which is better?
Unfortunately, that's one of those questions that has no answer. The sound of the two methods is just different. Keep in mind, though, that most pickups were designed with the idea that they would be combined in parallel by the stock wiring and that is the "sound" the pickup designer was striving for. I would recommend that you wire Strat pickups in parallel if you aren't going to provide switching between series and parallel. On the other hand, I regard the ability to switch between series and parallel wiring as one of the most tonally useful modifications you can perform -- much more useful than pickup phasing, in most cases (though combining series and out-of-phase wiring produces very interesting results in some cases).

Wiring the Humbucker:
Most humbuckers have the two coils of the pickup wired in series. With four-wire humbuckers you can install a switch to change between series/parallel wiring for the humbucker coils. If you aren't going to use a switch, series wiring of the two coils will provide the "intended" sound and usually a "hotter" output.

Hey! What Happened to the Sound?
When coils (or pickups) are wired in series you have to short across one coil (or pickup) to turn it off -- you still have to provide a complete circuit for the other coil or pickup. With coils or pickups wired in parallel you have to open the circuit to a coil (pickup) to turn it off -- shorting the coil will short all of the coils when they are in parallel.

One mistake I've seen people make is wiring a four-wire humbucker with two or three switches -- one to select series/parallel wiring for the coils and the other(s) to short one coil or the other to use the pickup as a single coil. The only problem with this scenario is that if you have the series/parallel switch set to parallel, and then try to select a single coil, you kill the pickup completely. This is okay if you're expecting it and know why it happens (and know to avoid that combination of switch positions) -- but I've seen it put people in a panic, too! It is better to use a multi-pole, multi-position rotary switch or an on-on-on mini-toggle if you want this kind of capability.