Phantom Powered Bass Buffer Box Part 3

We're already up to part 3 for such a simple device.  That's engineering though.  A thousand little interconnected details and considerations so that a design can work all the time, every time.  Without even realizing it, we have very high expectations of the products we use.  As the complexity of a device increases, so does the work necessary to make sure all the pieces work together over a broad range of conditions.  Moving on....

Click to enlarge.

The new and improved buffer box requires no external power but the sound sucks.  And hardly unexpected.  That 100nF output cap just doesn't cut it.  We have a first order high pass RC filter when plugged into the mixer.

For those familiar with RC circuits, the cutoff frequency is determined by Fc = 1/(2*PI*R*C).  Just by observation we can see that as R or C get smaller, the cutoff frequency goes up.  Plugging R_Out, R_In, and C_Out (ignore C_In for now) into the equation gives us 578Hz.  The low E on a bass is about 41Hz.  The series combination of C_In and C_Out will yield an overall smaller capacitance meaning that the actual cutoff frequency will be higher.  So I did some empirical tests using different output capacitor values:

The solid lines are of most interest as they show the frequency response thru the mixer.  It took a 100uF cap to get that flat green line.  With both the 100nF and 4.7uF caps, the cutoff was above 1000Hz.  That explains the bright sound.  It is kind of interesting that the no load response wasn't flat like the green line, considering the large input resistance of the scope probes,  Perhaps the AC coupling cap inside the oscilloscope in series with the output cap had an effect?  Not entirely sure, nor is it as critical as the loaded response.

So why did this circuit work prior to the phantom power modification?  As it turns out, the original design still had to go thru a 1/4" to XLR adapter.  And the particular adapter being used was more than just a couple of point to point wires, it had a transformer inside.  The transformer presented a higher impedance (something like 40K ohm) which brought that cutoff frequency down quite a bit.  Not enough impedance for a bass or guitar to drive directly, but a keyboard, effect pedal, or even buffer box would have no problem.  By adding the XLR connector directly to my buffer box, I removed the adapter and exposed a design flaw.  With a 100uF cap in there, things are sounding good with very low noise.

Measuring sin waves

I would have done a little more tweaking, but they don't have a lot of down time (the bass is used twice a week).  This was a scenario where the details mattered in a very obvious way.  Engineers have to learn to recognize the causes for certain outcomes, and sometimes it's only one component that makes for all the problems.

Added a piece of an iron spike to give it a little weight.  Otherwise
it felt like an empty box.  I wanted people to take it seriously....

Phantom Powered Bass Buffer Box Part 2

The buffer amp I built up from the last post served well for awhile, but I never liked having a power supply involved.  After the addition of a few parts I was able to take advantage of the phantom power supplied by the mixing console.

A couple of things to note in this design.  First, there is that middle contact on J1 that I connected to ground.  This keeps things nice and quiet when nothing is hooked up.  When a cable is inserted, the plug physically breaks the connection between the grounded contact and the signal contact.

The next item to look at is the signal output.  The audio is being carried over pin 2 of the XLR connector, but what's is with the stuff connected on pin 3?  If we look at the mixer side, we see that whatever signals are carried on pins 2 & 3 will be subtracted from each other via an op amp.  When a microphone is used, it will actually present a differential signal to the mixer input.  Any signals present on both pins will cancel out, and the difference between the signals becomes the amp output.  But I only have one signal, right?  Ideally yes.  However, the cable carrying the two signal lines will be subject to all sorts of noise.  So pin 2 will carry the desired audio signal plus some noise, while pin 3 will only carry the same noise as pin 2.  When they get subtracted, all that is left is the audio.  C4 & R7 are chose to match the frequency response of the JFET buffer output, which essentially makes a high pass filter (this ensures that the noise on both lines is filtered similarly).  C4 can just be made equal to C2, but R7 has to be calculated.  I did this by running a 1KHz tone through the buffer with no load and with a dummy load to determine the voltage drop internal to the buffer.  I took the measurement prior to C2 so that I would not get any reactive component to my values.

Simplified circuit showing only power related parts.

Lastly, there is the phantom power circuit.  It's pretty simple and ingenious.  The mixer applies a DC bias to both audio carrying conductors, which the device just has to tap off thru a pair of equal resistors.  Capacitors at the inputs and outputs block that DC from the signal drivers and receivers, making the rest of the circuit none the wiser.  You do have to be careful though, as the more current you draw, the less voltage you will have to bias your circuitry.  That's where the zener diode comes in.  It will draw whatever current is needed to get the voltage drop across the 6.8K and 10K resistors to leave us with 12V.  As long as the current thru the zener is greater than any current your device needs, we'll have a steady enough voltage rail.

So after the solder smoke cleared I plugged the buffer amp into the mixer and happily watched the LED light up.  I hooked up an electric guitar and..... well..... it was punchy enough, but piercingly bright as well.  This wasn't a huge surprise, but I had gotten away with the circuit up until now so wasn't going to change it at first.  C2 is the problem.  It's too small.  It was creating a high pass filter with the mixer input that didn't include any of the bass.  Or mid tones.  But it had worked well for years right?  Yes, and I found out why, and I'll tell you all about it in the next post.

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.