Tag Archives | buffers

Introducing Limelight True Power Bypass

Coldcraft Effects is proud to announce our latest update to the exceptionally clever True Power™ Bypass system: Limelight™ Bypass

The Limelight™ system simultaneously allows momentary bypass and standard latching bypass without the need to switch between exclusive modes.  Operation is simple and intuitive.  If the footswitch is held for 1 second or longer when enabling the effect, it responds as a momentary switch, turning off upon release.  Otherwise it is latching.

Other key features include:

  • True Power™ bypass – should the pedal lose power, it reverts to true bypass.  This includes the EchoVerberator, which has a buffered bypass with trails when powered.
  • Quiet, reliable relay bypass –
  • Opto-FET muting – silent
  • π-filtered power – no noise gets in, or out.
  • Sophisticated Reverse and AC Power Protection

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PCBs Now Available

Previously, I sold a few DIY projects for sale through a separate website called the Stompbox Depot.  I’ve since moved them here and added them to the shop.  I’m not sure the international shipping is set up yet, but domestic will be $2.25 regardless of quantity for a while.  If you are outside the US, for now just use the contact form to get in touch. So far the original Micro Buffer project is available.  This was a dual channel opamp buffer based on the Klon Overdrive bypass buffer.  Shortly, I’ll be reintroducing the Micro EQ, a mini-sized Baxandall Shelf EQ as well as the Micro Buffer II, based on the Cornish Buffer.

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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.

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The Case Against True Bypass – Revisited

You’ve seen this argument many, many times.  Hopefully, my peers who have written about this before me have convinced you that the revered 3PDT stomp switch, the “True Bypass with LED” savior is really just a bad idea.  Mechanical parts fail, and they fail way before electronic components.  Things wear out and its just how it works.  When it comes to signal switching in electronics, electronically controlled relays present a much more robust solution than simple mechanical stomps.

So dig this, the more complicated the mechanical switch, the more moving parts it has, and the lower the mechanical life expectancy should be.  The best of the best Single-Pole Carling Switches are rated at 100,000 cycles.  You can bet your ass that a run-of-the mill triple-pole switch comes doesn’t even come close.  Possibly within factor of 5, maybe 10 of that, meaning 10,000 cycles. Total shit.  And that’s a statistical figure, with a BIG spread because in the real world, shit breaks, a lot.

Now dig, the Panasonic TQ2-5V relay used in Coldcraft products, and by many colleagues in the music business is rated at 10^8 to (10^7 minimum) cycles.  That’s 10,000,000 activations, MINIMUM.  The MOMENTARY single-pole switch used to activate the relay will wear out before the relay gives up on you.  Unfortunately, relays can still cause switch POP, or audible clicking when used, but luckily, the same logic controls that switch the relay can be used to silence or quiet the switching action.  This is the basis of our bypass system and others including the Jack Deville-designed “Clickless True Bypass” now available at Mammoth Electronics.

Now the problem of reliability has been solved, dare I say its been obliterated.  However, there’s one more thing to consider, and that is Contact Voltage.

What the F is Contact Voltage?

Well, anytime there is a mechanical connection, there is a resistance present because the connection is imperfect.  Maybe the surfaces are rough, possibly oxidized, corroded or just don’t make great contact.  When a voltage is applied across the contact (AKA your guitar signal), the resistance creates a voltage drop across it, known as the Contact Voltage.  We already know that relays are sealed from the outside dirty world (read: your dirty shoes, beer-soaked bar floors, and worse).  The contacts inside the Panasonic relays are made of Ag and Au (Silver and Copper), two of the most conductive metals in town.  Contact Voltages inside a relay will be very, very small, but what else is involved?

Jacks.

Yes, every pedal has 2 jacks, and requires 2 plugs.  So lets say you have a modest pedalboard of 10 true bypass pedals.  If you were in the know, and purchased effects from VFE Pedals, Dr Scientist Sounds, and the like (that includes Coldcraft…), your pedals have relay-based bypass.  Minimal contact voltages inside the relays.  Ok, so back to the MATH.  2 Jacks, 2 Plugs, 1 DPDT bypass connection (2 contacts), 10 pedals.  That’s 80 contacts using a mechanical stomp switch!

80!

What I am getting at here is that’s an awful lot of places where your signal can degrade.  80!  It really only takes one faulty, dirty or otherwise weak connection to wreak havoc on a passive guitar signal.  So lets do some more MATH.  Suppose a typical mechanical connection is 10 ohms of contact resistance.  10 ohms is tiny, minuscule.  No one even uses a 10 ohm resistor when building pedals because its basically a jumper.

Now dig, 80 mechanical connections X 10 ohms = 800 ohms!  800 ohms is basically 1K ohm, and that can be a big deal on a passive guitar signal.  And this is all assuming there are no problem children, err, dirty, broken, intermittent contacts.  And don’t even get me started if you’re using one of those fancy programmable switchers.  You should just give it another factor of 2-4x.

Now for the punchline.  Say you have a buffer.  Doesn’t really matter if its an Op-Amp or Discrete, you just have one.  A buffer is any preamp/active element that presents the guitar with an ideal input impedance (resistance) and provides a lower, more ideal output impedance for driving heavier loads.  Put that buffer after the guitar, (or some where in the middle of the pedals even).  The output of that buffer will have no problem handling an extra 1k ohm on its output.  Hell, it was designed to do this.

Here’s a nice, tidy example of an Op-Amp based buffer.  This is my Micro Buffer/Splitter design that I have been selling for almost two years as a pre-built mono buffer, or a DIY project.  Personally, I think this buffer sound best AFTER all drive and fuzz pedals, but before any modulation or time-based effects.  That’s just my opinion anyway, your ears may tell you otherwise.  It can be configured as a splitter, mono or stereo buffer depending on how you populate the jumpers (J1, J2, etc). Micro Buffer Schematic

There are many other interesting products out there such as the Cornish LD-2 which is a discrete, bootstrapped design, as well as the Visual Sound Pure Tone, which is another Op Amp buffer.  You can read an argument from Pete Cornish against True Bypass here, and another here from Andrew Barta of Tech 21 NYC.

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