When you play guitar with bass expansion, you need a full spectrum sound, and so a keyboard amp is a better choice than a guitar amp.  Of the many I have used, the Roland K series, keyboard amps, sound best for my guitar style..

Guitar strings: 
Acoustic guitars:  D'Addario "Flat Tops" phosphor bronze semi flat wound  X-Light Gauge EFT15, a sweet, quiet and easy string to fly on. (I change the .010 to .011 for the first string gauge).  
Archtop guitars:  D'Addario Chromes light gauge.

Bass Expansion
  refers to the use of an extra pickup signal from 5th and 6th strings sent to octave-lowering electronics (EBS Octabass) used in conjunction with the regular pickup system blender signal, the normal guitar output signal.  There are now a variety of creative choices and types of pickups to install on your particular axe to give you that extra signal off the two low strings.   This new bass signal is blended into the performance on a separate channel, or through a separate amplifier, not too loud, but to just the right volume for the feel of a double bass accompanying the guitar.  Play in the Kingster's style for a simultaneous walking bass line to accompany the solo fingerstyle  guitar.  This system is used on most of King's solo guitar CDs, and sounds like there is a walking upright bass player accompanying the solo guitar. 

Bass Expansion update as of June 2012:

Many of you are aware of my "bass expansion" technique of adding some kind of pickup or pickups to my guitars that read and produce a signal from the low 5th and 6th strings.  I have now decades of experience into this working with various repair technicians having adapted many existing pickup styles to mount on various guitars in non-conventional ways, so as to only get those two low strings, while the guitars' existing pickup systems work as usual pumping out all six strings' signals.   Then this new simultaneous signal I have off the two low bass strings is run either to a new 1/4 inch female jack (as is most common on my Taylor acoustics), or I switch the guitar's existing jack to a TRS running the new bass strings signal to the "ring", and the stock guitar output to the "tip".

Finally I have found a pickup builder,
Mathew Bullock (Matt), email , who for no more cost than what you would spend on another commercial guitar pickup, builds you pickups to order, even to suit my current order as follows:

I bought a new Epiphone ES175, as I have always felt so comfortable on my Gibson ES175.  (And by the way, what a sweet solid guitar this is, well worth $500 new current price).  Matt will make me two individual heavily coiled pickups (for good output) BOTH HOUSED IN ONE PICKUP BODY that will fit between the two existing humbuckers, so it looks rather stock, like it's a third humbucker!  He suggested that this middle pickup not have screw poles, I like that idea. His pickups will work right through the metal pickup cover.   He will run each of the two individual outputs to one pot, a "blender" pot, where even volume is at middle, then I can balance volume as needed turning slightly one way or another (lest the low e string overpower the a string sound, as we need an even sound volume here).   I will have that blended output run to the "ring" of the TRS jack I will replace the stock with.
Now I simply take the hardware to my local guitar tech to install it (a world-class guy here in Spokane named
Rick Rubin, if you ever are looking and need to send a guitar to him for work... at a place called "Dutch's, 415 W Main Ave, Spokane, WA 99201  (509) 747-5284  Rick's email is  I am estimating $100-150 for his work.  Not bad.

So my total cost for this new pickup and hardware and shipping:  under $200 (plus add Rick's installation fee)   I have spent more than that for Baggs Tune-O-Matic bridge pickups installed on this type of guitar and all three times was disappointed in the signal strength and the tinny tone.  I knew I needed a fat humbucker tone this time and I knew it needed not to be mounted close to the bridge.

Now, dig this,  we explored something else he can do with this setup, which I think I will not do, but I am officially sleeping on it tonight... which is to add a third pickup for the D string hidden under that new middle pickup housing... then I would not need a blender pot, but I believe 3 volume pots mounted somewhere to blend to a single volume-matched signal.   If I decide to go with this setup, I would often keep the D string turned off, as I have learned that it is more valuable as a chord and melody string for most songs than as another bass string.   But it sure would be nice on "Billie Jean" and "Day Tripper" if all I needed to do is turn up a volume pot on the fly.

A Fingerstyle College guitarist in Chicago who does bass expansion is finding, as I have, that to do the bass off a guitar synthesizer may be too slow a response, and too flat & unchanging a sound from patch to patch.   Here's where my analog approach shines in bass expansion, because you feed the blended signal to a preamp pedal (or two in line, as I do, for extra smoothness) then to a bass octave dropping pedal, such as the EBS Octabass.   This analog approach gives all kinds of control to your fingers to control expression, tone, volume, etc.

Sorry I have not yet a magical answer for nylon sting guitar bass expansion.

Bass Expansion older information from mid 1990' thru 2011, and currently still in use on my acoustics:
A fairly easy way to get that extra signal from the low A and E strings is to adapt a Fishman Neo-D Magnetic non-battery Soundhole Pickup (in combination with the L.R. Baggs GigPro mini battery preamp as described further down in this article)  and mount it exactly centered under the A string. This way the E string signal won't overpower the A string, and the crosstalk of the D string usually won't matter when using the Octabass, which is designed to process only the lowest note it hears. So, understand that this pickup is mounted parallel to the strings, not perpendicular as is usual for such a pickup to read all six strings.  I used to recommend the Fishman Rare Earth pickup with its built in preamp and sleeker profile, but there have been too many times when the Rare Earth signal has not been powerful enough to run that octabass properly, not to mention the hassle of changing its batteries once you get it mounted. The Neo-D with the Baggs GigPro really does the job well.

     Other systems of bringing that extra low A and E string signal can include a signal from individual bridge or saddle pickups, or from a specially adapted short length embedded piezo pickup.  These options necessitate an inline preamp of some kind, either hidden in the guitar or outboard.  

     An approach that I use on my Gibson archtops is trading the Gibson Tune-O-Matic for a Baggs T-Bridge, which can give you a separate hex output from string 5 and 6, routed to stereo preamp or to two preamps that are blended somehow to a mono output (perhaps with a tiny mixer), that can go into your bass octaver.   This works great and looks invisible.   But for acoustics I have to say my best working model is the simple cheap Neo D model of the Rare Earth tonehole pickup set parallel to the strings, centered under the A string.  It gives a mono output that you run to a Baggs GigPro preamp then to the Octabass.  It's positioning is adjustable in the tonehole to get the volume matched pretty well between the A and E strings.  And I don't worry about bleed through from picking up any of the D string, as the Octabass processes only the lowest note it hears.  (It's not perfect, it's analog technology, but it works well enough to sound sweet).

You certainly can use single coil pickups. You can use almost any pickup you can make fit.  But you nearly always need a little preamp to boost the signal enough to get a smooth sustaining sound out of the Octabass.  Lately I have been using the Seymour Duncan SA1 Acoustic Tube Acoustic Guitar Pickup in one of my Taylors and my Breedlove acoustics, with very good results.  I am able to mount one of the end clips on the tone hole top wood (for where the mounting was designed) under the A string toward the bridge side of the guitar, but the other end clip may need to mount under the tone hole top wood inside the guitar, under the end of the fretboard somewhere, because at the point on the tone hole where we want the pickup to be, which is directly under the A string, the smaller diameter length of the hole there is too small for the length of the clips on the pickup which was designed to fit across the real diameter of the hole perpendicular to the strings, as we know. So we find a crevice that lets the spring tension of the clip grab and hold the pickup at an angle as it goes under the tone hole on that end.  Not to worry, enough of the pickup is in close enough proximity to the strings to read the bass strings as we want.  Then direct the pickup's attached cord along the end wood of the fretboard and finally down around where the neck strap button is.  I actually use a bit of super glue to hold that cord in place as it winds around the edge of the fretboard wood, but purists would rightly take issue with that. Perhaps a bit of tape would be less offensive.  Anyway, using this Seymour Duncan pickup allows for no altering of the guitar top, no drilling, etc.

As I mentioned, the L.R. Baggs GigPro mini battery preamp is a workable preamp solution.  Sometimes other brands of preamps will work, sometimes they won't if they don't give enough boost.  Then the signal goes into the EBS Octabass.  I have not tried any other bass lowering electronics lately, so I don't know if there is anything better out there.

As for the octave-lowering electronics:  I use the
EBS Octabass, a rich analog octave converter.  For the "EBS Octabass" do a Google  web search to find dealers or web order stores that sell them.  As of this writing I have not yet tried the latest generation of Boss brown bass pedal, so I am curious as to how it stacks up against the EBS Octabass.   Please email me if you have any experience with it.

If you don't use a seperate pickup signal from your 2 bass strings for Bass Expansion  you can't get a clean bass sound, as all the strings will be trying to feed the octabass.

One way to get Bass Expansion on your acoustic is to get a Fishman Neo-D Magnetic non-battery Soundhole Pickup;  super-glue or epoxy 1/4" to 1/2" extended wood or plastic pieces to the end clamps, then mount it parallel to the strings, exactly centered below the A string.  If you like, you can drill and mount a 1/4" female plug on the guitar for it, or have your luthier do it.   Then a guitar cord goes from your new pickup's guitar out plug into a Baggs GigPro preamp (you absolutely need this premp to make this work, it doesn't seem to respond to other preamps) and then from the GigPro to an EBS Octabass (and to get the best analog octaver for the best sounding bass, you really need the EBS, not a Boss or other brand).  Calibrate equal volume between the E and A strings by gently moving the pickup up or down as needed, then tighten into place.  You keep most of the acoustic qualities with this pickup in the hole. 

I accomplished adding bass expansion to an old Gibson ES175D by replacing the bridge with a new Fishman bridge that contains one pickup for each string (this bridge, which looks just like a Gibson classic rock type bridge, has the stainless steel saddles which I like to replace the wood bridge on the ES175 - I think a wood bridge is very nice on a jazz guitar, but I use the 175 with its double pickups as a full-service variety working gig guitar, and I need the fuller spectrum of tone and sustain from stainless steel saddles with this guitar).  Of the built-in pickup wires coming out of the Fishman bridge,  I used only the wire leads from strings 5 and 6 (and ground lead) and soldered them up to a female TRS plug, drilled a new mounting hole for that plug near the existing plug, and mounted it. (To find which leads they are, put the bridge on the guitar with the lead wires sticking out; have a test amp with a guitar cord plugged in and while holding the ground lead of the Fishman to the guitar cord ground, hold each colored wire lead to the tip, one by one, and pluck the low e and a strings to determine which wire amplifies the string you want; cut off the lead wires for strings 1 through 4).  I use a stereo male guitar cord plugged into my newly-installed TRS plug I put on the guitar, and use a a  Radio Shack adapter cord to break the output into two seperate 1/4" guitar plugs, one for the low e and one for the low a string. Run those signals  into a Rolls MX4 mini mixer ( a terrific mixer and perfect preamp for the right bit of signal gain;  can be found for $45 plus shipping (this is an old blog, don't count on these prices today) using Google/Froogle internet search on "Rolls MX4"), so you have individual volume control over each of the bass strings to best equalize their volume (which is the most important thing you need to make this work effectively), then run the output of the Rolls to the Octabass, and octabass to your amplifier for your new Bass Expansion signal..  I keep the Octabass set to its deepest low setting for deep bass tone, but relatively low volume.

Q.   Why do you recommend the Fishman Neo D, in particular? How would it work (track/sound) compared to, say, a Baggs Hex system, running the bottom two pickups to the octave pedal?
Why the GigPro, as opposed to, say, the Para DI?
Have you ever tried the bass expansion with a nylon string guitar?
I've tried the bass expansion with several different guitar/pickup combinations and more often than not I've run into tracking problems. I sure would appreciate any advice you can offer.

A.   I recommended the NeoD with the GigPro preamp as a combination that works, as I use them.  My experience trying the NeoD with a ParaDI was that it did not work.  The net result needed, no matter what pickup system and/or preamp combination is to produce a signal that is powerful enough to drive the EBS Octabass.  If the signal is not strong enough the Octabass produces bad tones, weak signals, and apparent "tracking" problems.   I have used a few Fishman Rare Earth pickups (they have a preamp built in), and some of them power the Octabass just fine, while others do not, until you run them through some kind of preamp first.  Odd, but that's how it is.  Advantage of the Rare Earth over the NeoD:  it has a sleeker profile, takes up less of the sound hole. Disadvantages of the Rare Earth over the Neo D: it is priced 3 to 4 times higher and it has two expensive batteries that require you to remove the pickup from the soundhole for changing... and you still won't be guaranteed that it will not need a preamp to power the Octabass (even though it has its own preamp).

if you can get a strong enough signal off those two bass strings, you will power your Octabass with no tracking problems.

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Steven King's Instruments