Buffers and what they’re really doing.

There’s a lot of information around the internet on buffers, their use and placement.  The following is my professional opinion on buffers as they apply to guitar signal.  Just a warning to the reader, I am going to be using the term “Impedance” quite a lot.  Impedance is just the technical term for the measure of the opposition that a circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit.  I took that last bit from Wikipedia.org.  You can think of impedance as the resistance of a circuit to an AC signal.

While buffers really can’t hurt your signal, there is such thing as over use or “less than optimal use”.  Buffers come in many different varieties such as discrete transistor, JFET, MOSFET or OpAmp types.  Different types of buffers have different ideal uses, but all perform the following 2 duties,

  1. Present the Guitar (or input signal) with a fixed input impedance.
  2. Provide the signal with a fixed output impedance.

JHS Prestige, one of many
MOSFET-based Buffer
& Sparkle Boosts

A few things to note.  You may have noticed that I did not write “high” input impedance.  Some buffers are designed to give the guitar a very, very high input impedance, much higher than that of an amplifier.  This has the effect of adding “Sparkle” to the guitar signal.  The thing to understand is that, when you plug your guitar straight into an amplifier, some high frequencies are being rolled off.  This is just how a passive guitar (read: not low output impedance) is affected by the input impedance “seen” at the amp input.  By using a buffer with a very high input impedance, you are giving the guitar a much stiffer load, and so the highest treble frequencies that were previously rolled off at the amp are preserved at by the buffer.

This is the function of many “sparkle boosts” such as the Zvex Super Hard-On, JHS Prestige & discontinued Mr. Magic Boosts.  “Sparkle Boosts” are usually designed around a discrete MOSFET transistor for its very high input impedance.  Actual input impedance is fixed by resistors to be anywhere from 2.2MΩ to 10MΩ.  A typical tube amp has an input impedance of 1MΩ.

Just a note for comparison, standard JFET stages are usually designed for an input impedance of 1MΩ, whereas simple Bipolar Transistors (read: regular transistors) would have slightly lower input impedance, 100kΩ to 500kΩ.  The quick and dirty explanation for this is that traditional transistors are current-controlled devices and need to draw more current from either the signal source (guitar) or the bias network, resulting in smaller biasing resistors and a lower input impedance.  JFET/MOSFET transistors on the other hand are voltage-controlled devices and draw very little current from their inputs.  For a concise but informative look at different buffer styles, I invite you to have a read over at the AMZ/Muzique.com writeup by Jack Orman.

On to the 2nd point.  I also stated “fixed output impedance” and not necessarily low output impedance.  Lower is not always better when it comes to interacting with other impedance.  Some fuzz and dirty pedals really want to see the guitar directly, and thus will work best when connected to a medium output impedance, but will seem less dynamic driven by a low impedance, and just fart out completely if driven by a high impedance.   You may have noticed that the run-of-the-mill Dunlop Crybaby Wah will stop its waka-waka magic when driving a vintage style Fuzz Face or similar.  This is because the output of the Wah is unbuffered and on the high side, and the input to the Fuzz is on the low side.  When combined, the fuzz actually loads the wah enough to change its operation, and that’ just a shame.

The FoxRox Electronics Fuzz Wah Retrofit,
a perfect example of an Isolation Buffer.

So what is the fix?  Add a buffer to the output of the Wah so that it always sees a fixed load.  But if said buffer has too low of an output impedance, the fuzz loses its dynamics.  The solution?  Raise the output impedance a bit.  This may seem counter intuitive, because before adding the buffer, the Wah had a high output impedance but this brings us to the actual function of a buffer:

  • A buffer provides impedance isolation between source signal and load

You can see a wonderful example of this isolation buffer in the Foxrox Electronics Wah-Retrofit, a buffer module designed to drop in at the output of a wah, letting it play nice with vintage style fuzzes.  The output impedance of this circuit is higher than your typical/traditional opamp buffer.

This brings up a quick point I’d like to make.  Manufacturers will sometimes tout their design as JFET or BiFET preamp when in actuality it is an OpAmp.  Technically this is correct because OpAmps are in fact made of many internal transistors, including combinations of JFET and Bipolar Transistor (BiFET).  There are even MOSFET-based OpAmps.  This is just something to keep in mind.  Usually the honest manufacturers will only say “Discrete” when it is actually individual transistors, JFETs or MOSFETs.

So it seems like isolation is a pretty good thing to have when you have multiple effects pedals chained together.  What other places will isolation buffers benefit the performance of your gear?  Take this next example.  Some designers are lazy, and don’t put output buffers last in their circuit.  This could mean that the output impedance is not constant and can actually change with the volume control (usually the last control in the signal path).  Sometimes this can affect their performance with other effects down the line.

The MXR Carbon Copy, often blamed
for the poor design of the Fulltone OCD.

A PERFECT example of this poor design is the Fulltone OCD Overdrive paired with an MXR M169 Carbon Copy Analog Delay.  Marketing hype aside, the OCD has one very big flaw that has never been addressed in its 7 (8?) revisions: the Output Impedance is too high and varies with the volume setting.  Many users have complained that the tone of their OCD changes when they activate their Carbon Copy after it.  This is because the Carbon Copy may not have an input impedance replicating that of an Amp, but also because the output impedance of the OCD can reach as high as a few hundred Ω’s.  Not cool.

Now dig, simply by placing a traditional buffer between the two (1MΩ Input/1KΩ Output) you can eliminate this problem.  What this buffer does is isolate the drive pedals from the modulation/time-based effects.  All drive pedals will see a constan 1MΩ load and all time based effects will see a constant 1kΩ signal source.

This is where I usually encourage users to try out the Coldcraft Micro Buffer, as its specifications were designed specifically for this sort of isolation application.  You can think of this buffer as a sort of “repeater”, taking everything from the dirt pedals and resetting it back to low output impedance, usually mid-pedalboard.

Lastly, buffers can be used to drive isolation transformers.  Often, guitarists will use multiple amps, sometimes driving them with stereo signals.  This can make for some really sweet, thick and rich guitar tones, however it can add many noise headaches by creating a ground loop.  Say you have 2 amps, plug them into the same output, and use the stereo out from your Strymon Timeline to drive them both.  The cables running from the pedal board to the amp are grounded both at the pedal board and the amp.  This creates a ground loop that may or may not add a nasty hum to your rig.  This depends on many things including the quality of power where you are playing, and often times the power at the bar is very noisy.

Suhr Iso-Buffer

Now you could disconnect one of the amps from ground, but this can be very dangerous for the player, even fatal.  So don’t do that.

What you can do is use a stereo buffer/splitter at the end of the pedal board to drive one amp directly, and the other through an isolation transformer.  The transformer eliminates the ground at the pedal board end while leaving the signal grounded at the amp.  Perfect, not only do you have a low output impedance taking care of really long cable runs, keeping your signal low noise, but you’ve eliminated the possibility for nasty ground hum.  This is the basis for the Iso-Buffer by Suhr as well as the Humdinger by theGigRig, among others.  Neither example takes a stereo input unfortunately.

So to summarize, I believe the 4 most important applications and types of buffers are the following,

  • Input Buffer – Personal Preference, Post-Guitar
  • “Wah Buffer” – Slightly higher than low output impedance
  • “Repeater”  – OpAmp to Isolate dirt from mod/time effects
  • “Output Isolation” – Transformer Isolated for driving multiple Amps

And this post would be complete without putting my money where my mouth is so here is exactly what I would do if building the ideal buffer system according to me.  I’ve selected a hybrid input buffer that uses a bootstrapped transistor for low noise and RFI rejection, coupled to a op-amp that provides a little filtering and plenty of drive power.  The signal splits, providing an isolated tuner out, and a main output with a tunable output impedance.  I would recommend this if you’re into the fuzzies that have low input impedance.  Next is a quick and dirty wah-output buffer, again with the tunable output impedance for best performance. This gets tacked onto the output of the wah, and is only active with the wah.

Ultimate Buffer System

The third section outlines the mid-pedalboard “repeater” design, not all that unlike the Micro Buffer sold through this website.  Lastly, the OpAmp has been used yet again to provide transformer isolation driven from either a Mono TS, 2x Stereo TS or Stereo TRS signal send to 2 amplifiers.  For pedal board wiring purposes all this would be in separate boxes.  I usually recommend TL072cn OpAmps for buffers, which are JFET-input.   Ignore the LM4558N, that’s a typo.  The 42TL018 is a standard isolation transformer sold by Mouser and Small Bear Electronics. There are probably many others that will provide similar or better performance.  Dig.

, , , , , , , ,

6 Responses to Buffers and what they’re really doing.

  1. Lloni July 29, 2013 at 10:42 pm #

    Thanks for the info! I was thinking of building a Microamp-based buffer with a fixed gain resistor for unity and just leave it at the begining of my pedalboard. As you said, it has a noticeable “sparkle” wich I really like, due to it’s 22M input resistor, I think.
    Any comments on this are welcomed.
    (and sorry for my awful english)

    • admin August 12, 2013 at 3:31 pm #

      Hi Lloni,

      I’m not famliar with the Microamp circuit, but it will certainly do what you want. I’d recommend making an always on version with the boost knob, and using the footswitch to go from unity gain to boost. This way you would still get the buffering benefits, but have the option of boosting at the front end of your pedalboard.

      Thanks for visiting!

      Austin Z.

  2. Emilio April 6, 2014 at 5:09 pm #

    Awesome article!! Just one question: what “VA” (in schematics) is standing for?

    • admin April 6, 2014 at 5:12 pm #

      Thanks! VA is the 4.5V virtual ground, or reference voltage.

  3. Stephen Michael French April 13, 2015 at 6:44 am #

    I’m looking at the “Wah-Output Buffer” part of the diagram, and have a few Q’s:
    what value is that pot? what value is the R3 attached to it? what does ‘ “Z”1 ‘ mean?
    Also, I have a couple of TL071CP chips here, I assume they will do fine? (assuming I refer to the data sheet for proper pin values)

    • admin April 19, 2015 at 10:36 am #

      TL071s will do just fine. There are quieter opamps, but you could do much worse as well. For the POTs, I recommend starting with 50k Linear, but 100k would be ok as well. “Z” usually stands for impedance, ie input impedance or output impedance. for R3, I would start with 10k.

Leave a Reply

Powered by WordPress. Designed by Woo Themes