Phantom Powered Bass Buffer Box

It may seem like a lot a lot of projects going in parallel, and that is partially cause I do, and partially cause it takes a lot of time to do a write up.  I'm hoping this project doesn't go more than one or two posts as it's already completed and pretty small.

Backstory:
A few years ago I made a little signal buffer for a church as I saw that they were plugging their bass guitar directly into their mixing console (almost directly, but more on that later).  And to make it worse, they were using the XLR mic inputs instead of the line inputs.  To be fair, they didn't have much choice.  A cable snake ran from the sound booth in the back of the room under the floor and to the stage, where several panels of XLR connectors at the stage allowed for the connecting of mic's and instruments.  The problem here is that the bass is a single ended high impedance output, and the mixer mic inputs are quite low impedance.  The result is a weak and unenthusiastic tone, susceptible to noise (the passive pickups are pushing a signal over about 100 feet of cable).

A simplified model.

As a rule of thumb (at least for guitars), the input impedance of a device should be at least 10 times greater than the output impedance of the thing driving it.  This will ensure that the audio signal is preserved with little or no effect on tone.  As an example, a guitar might have around a 10K$\Omega$ output impedance, and a mixer line input is often around 10K$\Omega$ as well.  That's a 1:1 ratio.  The XLR mic inputs go down closer to 1K to 2K$\Omega$.  If we look at this as a voltage divider, we can see that much of the signal seen by the mixer is attenuated.  By half if using the line input, and only around 20% or so of the voltage appears over the mic inputs.  There are also reactive elements in the guitar too; ultimately, the larger the input resistance, the less impact there will be on the guitar circuit.

So what can be done?  Well generally you'd use a direct box.  These passive devices(though there are active ones too) use a transformer to well.... transform the impedance.  These things can range in price and performance.  Also, pretty much any guitar effect pedal or amplifier with a preamp output solves the problem too.  But what fun would that be?  So I built them a JFET based source follower out of parts I had and called it a day.  It has high input impedance, and low output impedance, problem solved.

JFET Q1 as well as R1 & R2 all present high impedance paths for incoming signals.  J3 plugs into a 9V wall transformer.

And despite an inherent design flaw, it worked well.  But recently I was designing phantom power into a headphone amplifier and that got me thinking: why not do the same for this project.  So I did, and I'll continue this in another post.