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MI AUDIO BOOST N BUFF.
The Boost 'n' Buff is not a reference to
some form of plastic surgery. It's in fact the distillation of an idea
MI Audio has been working on the Boost n Buff 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, they've managed to squeeze most of these
functions into this neat little package.
Why was the original
design so complex? The complexity arose from the fact that they had
isolated a few different functions for booster pedals, and 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.
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:
o LED status indicator
o Mechanical switching for buffer/booster mode selection
o Heavy duty die-cast aluminium enclosure, professionally powder coated and silk-screened
o Large pointer knob for easy manipulation of the volume control |
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