MI Audio Boost 'n' Buff

Various

MI Audio (2006-2008)

Introduction

The Boost 'n' Buff is not a reference to some form of plastic surgery. It's in fact the distillation of an idea I've been working on for a few years now. It's a design which is a bit of a departure stylistically for me. It's simple. It only has one knob. No internal trimmers or tweaks. Believe it or not, it actually started out life as a 6 knob pedal, but through a process of consolidation, I've managed to squeeze most of these functions into this neat little package.

Why was the original design so complex? Well go grab a coffee, pull up a seat and let me tell you,...

The complexity arose from the fact that I had isolated a few different functions for booster pedals, and I wanted to create a pedal which could be set up to fulfill all of these. These functions were:

  1. Buffer: With the increasing complexity of today's pedal boards, and with the proliferation of true bypass pedals, guitarists are starting to discover that your average guitar pickup simply doesn't have enough oomph to drive 50 feet of cable. A typical guitar cable has 30pF of capacitance per foot, so if all your effects are true bypass, then in bypass mode you're effectively adding a 1500pF capacitor across your guitar output for the aforementioned 50 foot scenario! Add to this the inductive component of your pickup's impedance and you've got some serious and strange resonances going on.
  2. Flat Booster: Sometimes all that's required is a simple booster with a flat frequency response. This is generally used for small volume boosts during solos. For this application, any alteration to the frequency response is not a good thing. Also, for these applications, a huge amount of boost is generally not necessary. Up to 15db of boost is more than adequate.
  3. Treble Booster: In the case where you're driving a valve amp into overdrive, a flat frequency response is generally the last thing that you want. It will result in a very loose and fuzzy tone, which is very difficult to control. By altering the frequency response, namely by boosting the top end more than the low end, you can keep everything nice and tight. Also, when a guitarist uses a treble booster to push their amp, they're generally not very 'subtle' about it. They don't use 1 or 2 db. They use 15db or more.

In the past, these 3 distinct applications required 3 distinct products to perform all three functions mentioned above. Also, due to the ambiguity of naming, some guitarists would end up buying a treble booster, when what they really wanted was a flat booster and vice versa. "A booster's a booster right?!" Wrong!

I wanted to create one pedal that would perform all of these functions. But this then led to a whole bunch of questions.

  • What should the input impedance of the buffer be?
  • Should it be variable?
  • Should there be input and output sensitivity controls?
  • Should the control for switching from flat to treble booster change the gain as well?
  • Instead of a switch, should there be some kind of control to morph from flat to treble?
  • What about dedicated low and mid controls?
  • Should the low and high controls be roll-off controls or shelving EQs?
  • Active or passive?
  • If we add all this electronics in there, will it get too noisy?

My initial prototypes were very complex (as you can gather from this rather disturbing insight into my thought processes). But despite all the flexibility of the original prototypes, there was something missing. It seemed as if in adding all these configurable parameters, I ended up ruining the original idea of the pedal. There was so much circuitry in the pedal that noise was starting to become significant. There were also so many EQ controls that it was actually difficult to dial in a flat response. At unity gain, you could hear the tone change and it was difficult to 'dial out.'

I had to come up with a different approach. Unlike my other designs, adding an extra knob, switch or trimmer was not going to make things better. It would only make things worse. And then one day, a design of stunning simplicity and utilitarian elegance came to me.

The Idea

The Boost 'n' Buff is designed to fulfill three distinct applications.

  1. In bypass mode, it's an ultra high input impedance buffer.
  2. In the lower volume range, the frequency response is flat for use as a simple volume booster for solos.
  3. In the upper volume range, the Boost 'n' Buff becomes more like a treble booster for driving a valve amp into overdrive without getting loose and fuzzy.

The Circuit

The circuit is a rather elegant design based around one ultra low noise transistor. By not utilizing a large number of active devices, I was able to keep the noise level of the booster down to an almost imperceptible level. This is a very important factor in applications where a booster is going to be used before a gain device, like a dirt pedal, or an overdrive channel of an amp.

The way that the volume control on the Boost 'n' Buff works is also quite interesting. Most transistor based booster designs have a fixed gain, and the volume control simply limits the input or output volume.

Designs that limit the input volume have the advantage of avoiding distortion. However, these designs are also the worst for noise, since the circuit runs at full gain all the time. On the other hand designs that limit the output volume are quieter in terms of noise, but often distort with higher output pickups. For both of these topologies, gain is also limited in order to limit distortion or noise.

The Boost 'n' Buff works by actively varying the gain of the transistor itself. This has the advantage of both avoiding distortion and reducing the noise generated in the circuit. This design also frees up the normal gain limitations of a booster design. The Boost 'n' Buff has an amazing 35db of gain on tap. The design of the volume control is the key to this amazing gain range. One thing to note is that in varying the gain of the circuit with the volume control, some DC operating points also shift. This will be heard as 'pot noise'. But don't worry. That's the way it's meant to be.

This rather elegant and frugal circuit design has also had an interesting side effect. There's actually enough room in the heavy duty diecast aluminium enclosure to fit 2 * 9V batteries. By running the Boost 'n' Buff at 18V, you're guaranteed more headroom than you're ever likely to need! There's obviously also the DC jack for external power if you prefer. But despite the higher voltages, the current draw of the circuit is extremely low. At 18V, the current draw of the circuit is just 3.3mA, 3.0mA of which is used to power the LED!

Bypass Mode

As mentioned above, in bypass mode, the Boost 'n' Buff is NOT true bypass. Now I'll be honest with you here, and admit that making the decision to build the pedal this way was in fact a difficult one. You see, true bypass is the concept 'de jour'. It's become a way for effects builders to demonstrate that their products are serious. Also, it's not technically difficult to do, and is often just the easiest way to build a pedal, especially in small quantities.

Don't get me wrong. I have nothing against true bypass. In fact, all my other pedals use it, and I have no intention of changing that. But with the Boost 'n' Buff, I wanted to do something a bit different. It would have been easier for me to play it safe, and just make the pedal true bypass and be done with it, so as not to incur the wrath of 'experts'. However, after listening time and time again to the audio quality degradation introduced by all true-bypass boards, it was difficult for me to ignore this. By the same token, the sound of a guitar coming out the other end of a regular pedal board with no true bypass effects is similarly displeasing, having passed though a dozen or more low quality buffers. The ideal solution, in my opinion, is a board which is predominantly true bypass, and which also incorporates a high quality buffer somewhere in there to 'resuscitate' the guitar signal as it tries to push its way through a maze of cabling.

So there you have it. I've got it off my chest. The Boost 'n' Buff is NOT true bypass! But trust me on this one,... this is better than true bypass.

In bypass mode, the Boost 'n' Buff acts as an ultra high input impedance/low noise buffer. In fact, the input impedance is approximately 5Meg. With such a high input impedance, your guitar pickups don't need to deliver a lot of current in order to reproduce their full natural bandwidth. You can drive cable lengths as long as you like without sacrificing tone, and due to the ultra low noise design, you can use the Boost 'n' Buff out the front of dirt pedals or high gain amps without worrying about introducing any significant noise.

So what's different about this buffer and the buffer you'd find on mass produced pedals? Well, the primary role of a buffer on a mass produced pedal is not to 'rescue' your guitar tone from degradation, but rather to drive the low quality transistor based switching system. The input impedance of these buffer circuits are thus designed not to keep all the top end sparkle of your guitar tone. In some cases, the input impedance of this buffer is as low as 200kOhms, which is 25 times lower than the input impedance of the Boost 'n' Buff circuit.

By adding the Boost 'n' Buff in your signal chain, especially near the front of your effects, you'll notice that in bypass mode your guitar signal will have a bit more sparkle and top end detail. It's not that there are any special EQing 'tricks', but rather that the Boost 'n' Buff is faithfully reproducing all of the frequencies that are coming out of your guitar. Your tone will feel a bit more powerful, with more punch and almost as if there's more dynamic range and 'sustain' (although these are not the right words to describe it). You've got to try it out to feel what I'm talking about.

Flat Booster

In the first half of the volume sweep, the frequency response of the Boost 'n' Buff is flat. This provides you with about 15db of flat boost before the frequency response of the pedal starts to become more treble booster-like. This is perfect for solo volume boosts. The one thing to note is that if you're going to use digital effects in your signal path, I'd suggest putting the Boost 'n' Buff after the digital effects to avoid overloading the A/D converters.

Another interesting application is using the Boost 'n' Buff in the FX loop of an amp to act as a secondary volume control. Since it has ample headroom, line level signals are not a problem. So even if your FX loop is line level, you can still get quite a bit of boost using the Boost 'n' Buff. This way, your FX loops can also double as a solo volume booster as well as a standard FX loop. By putting the Boost 'n' Buff in the FX loop, you can use it to boost the volume of an overdrive channel. If you're using the overdrive channel of your amp, then putting any booster between the guitar and amp will just increase the amount of distortion as opposed to increasing volume. By putting it AFTER the preamp (in the FX loop), you can achieve volume boosting without affecting preamp tone.

Treble Booster

Once past the half way point, the mid and high frequencies start to climb faster than the low frequencies. At maximum gain, not only do you have an amazing amount of boost (approximately 35db), but the frequency response is perfectly tailored to driving a valve amp. It's literally like hot-rodding your amp with an extra tube stage. Also, with the 18V headroom, your sound will be 'cleaner' going into the amp, so that there's less colouring from the transistor.

Other Features

  • LED status indicator
  • Mechanical switching for buffer/booster mode selection
  • Heavy duty die-cast aluminium enclosure, professionally powder coated and silk-screened
  • Large pointer knob for easy manipulation of the volume control

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