Again thanks to electrosmash for many of the schematics and values presented here, and a special thanks to everyone at the Audio Electronics DIY discord server who helped me develop a lot of this.
Most guitarists will have heard of active pickups before, usually made by either EMG or Fishman with proprietary active systems are becoming more widespread, but will struggle to tell you in objective language what makes them different/better. Words like "Natural compression" "hotter" "tighter" and "Toan" get thrown around a lot and don't make much sense when you consider active instruments as a whole. When we take a look at what's actually going on under the pickguard we can get a much better of what active pickups do for your sound, and the advantages and disadvantages that come with them.
What actually are active pickups?
The differentiation between active and passive instruments is not in fact based on the coil construction or properties as many believe. Alumtones are a passive pickup but have the lowest winding count of any pickup, the Eric Clapton mid boost works with most standard passive pickups, and yet Alumtones are a passive pickup and the mid boost is an active setup. Rather than being a difference in pickups, the active/passive split is more easily defined based on the circuitry around the pickups. Some forms of pickup construction necessitate active circuitry to function as expected, and some do not. However any pickup can be used in an active guitar with the right circuit design
As Discussed in part 1, most guitar pickups consist of a coil of wire, some magnets and some slugs. A simple single single coil pickup guitar with one volume control and one tone control might look something like this internally.
We can tell by looking at the schematic that this is a passive setup. You simply Connect the pickup to the volume pot (or in some cases directly to the output jack) and strum to get a signal out, with no DC power required.
Active Instruments on the other hand are much more complicated. Many contain pickups with lower windings (and therefore a lower output level) and increase the level with an onboard preamp like the one shown below, which is drawn based on a schematic posted by electrosmash. A more detailed explanation will follow in a later article.
The following is the internal schematic of an active guitar I recently built, based on the Hacksaw by Dylan 159, which uses a set of regular pickups. The pickup selector has been simplified to a strat style 3 way blade for clarity, but this setup will work with most conventional guitar wiring methods.
Extreme examples of active instruments include products such as the Kaoss pad, popularized by Muse. With so many possibilities out there the easiest definition for an active instrument is simply an instrument that requires a DC power source to function.
That is pretty interesting but why would I want to use an active guitar?
What are the advantages?
There are several actually.
Impedance - This is probably the most universal of the benefits, as most seasoned guitar players will know that long cable runs = loss of treble without a buffer. The reason for this is impedance, and with an active pickup you can ensure that you have whatever output impedance you need to keep your guitar pright and sparkly. Think of it like having a buffer built into your guitar (because that's exactly what it's like).
Active EQ (and other controls) - Ever wondered why your strat, tele, les paul, and even your uncles cousins brother-in-laws vintage soviet guitar have pretty much the same tone control? The answer is that with passive components only, the number of useful ways in which you can manipulate an audio signal are very limited. Other than simple high and low pass filters there is very little available to the aspiring modder. With active components this is not the case, and your options for tonal tweaking are limited only by your imagination.
Ability to change coil resonance - As mentioned in the previous article, the coils particular resonance is largely responsible for it's tonality. With active preamps, it is almost trivial to manipulate this peak in any way desired, simply by changing a few parts values. This gives the user control not only over EQ but over the fundamental sound of the pickup itself, and allows for many different pickup "models" to exist within the same pickup. You want something hot and mid focused, but also really like low output vintage sounding things? No problem, an active guitar can do both with the flick of a switch
Wow that does sound cool, why isn't everyone using these?
Advantagesn't
Or you can call them disadvantages
Batteries
Batteries
Batteries
Yeah that's about it. Active guitars need batteries and there are a number of issues presented by this. They are listed here in order of severity.
They run out - Picture the scene, you're on stage in front of 100,000 people about to lay down an immense solo, the best ever, when suddenly your guitar shuts down because the battery died. Most active designs will not pass audio without DC power, so when the batteries are flat, so are the live shows. This can especially be a pain for long shows or rehearsals, or with poorly designed systems using more power than they ought to.
They aren't usually environmentally friendly - Most pp3 batteries are non rechargeable, meaning that once they're flat you have to throw them away and buy a new one. In the current age of ecological disaster, we shouldn't have to ask ourselves if the supreme tones we are getting is worth adding to the ever growing mountain of E waste we produce.
They're chonky - The most commonly used battery for active systems is a 9V battery, which takes up a reasonable amount of space. This is obviously not ideal in guitars where the routing is barely big enough for pickups + wiring, and while routing out a little extra slot under the pickguard is always an option this a) means you have to take the pickguard off every time the battery runs out and b) devalues your guitar if you ever want to sell it on
Oh damn I guess they really do suck. Back to passives for me
But wait I have some solutions
Several solutions have been proposed for the problems presented by active guitars, and each has it's own benefits and drawbacks. I will outline some of the most common solutions below, as well as a novel solution that to my knowledge has not been written about in depth before.
Bro just use active pickups they aren't that bad - see above
The "governmental solution" - There are a number of battery pack based available that claim to be the be all and end all solution to active guitar use, for example this spring cover from fishman. These do present an attractive solution to two of the issues mentioned above, needing little to no extra routing and drastically reducing the amount of E waste generated over the lifetime of the guitar. However it shares it's name with the fact that it solves the comparatively minor issues, leaving the most severe of the drawbacks untouched, all the while claiming victory nonetheless. No matter how rechargeable your battery, or how neatly it fits into the various holes present on a strat, the battery can still run out at crucial moments, and so this can not be used as a reliable solution for many cases. It also does not appear possible to charge the guitar while in use, so I hope you remember to plug it in after every session
The "bedroom solution" - By using a TRS cable with 9V across the ring and sleeve, a number of hobbyist designers have had success using an external power supply, perhaps one mounted to a pedalboard to power onboard electronics. This appears at first glance to solve all of the major issues with active guitars, and it does, provided you're always using your pedalboard. As a guitarist who frequently travels with his guitars, and attends the occasional bar jam, having my guitar tied to a specific external piece of kit that is liable to be forgotten or lost, is entirely unacceptable to me. This solution is therefore also unfortunately no good
So what are we left with? Taking the better points of the two meaningful solutions, we would want a system that requires no external sources of DC, but also does not run out. Such a system sounds impossible. Such a system probably is impossible. As a compromise I propose the following
This combines the best aspects of both options. When used normally, the relay remains closed, and any active circuitry connected to VCC draws power from the 9V cell. When the special external power cable (with 9V on the ring) is inserted, the relay flips and VCC is now supplied by the external power, allowing for use both with and without an external supply. It is also theoretically possible to use the second pole of this relay to charge the internal battery when an external supply is connected, although I am waiting for confirmation on this from someone far more knowledgeable than myself, as rechargeable batteries can be quite explosive, and are high up on the list of things I don't want anywhere near my crotch without thinking things through.
This does not completely remove the problems associated with a flat battery, however it is assumed that for any critical moments like big shows, an external pedalboard based supply is practical, and in those where it isn't a fully charged battery should be sufficient to get you through a song at a jam. Pay careful attention to the wiring of the relay coil, and notice that it is NEVER connected to VCC when the battery is in use. Relays drain current even when off, and so having the relay connected in this way extends the battery life significantly.
This is a bit of an abrupt ending but that's about all I have to say on the basics of active guitars. I hope to have some interesting preamps on the way in the next few weeks, as well as some measurement of my own pickups and a discussion on designing my own coils.
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