HEADBONE DEVELOPMENT

Ask any guitarist about the ultimate stage setup and they will tell you that having two amps on stage is tonal bliss. Using a low-power head for crunchy rhythm and a high power two-channel amp for clean passages and soloing lets you handle just about any situation. The problem: using two amps on stage is both large and cumbersome - especially if you are lugging around 4x12 cabinets! This also means that you need two mics, two channels on the mixer and of course a bigger truck. We thought to ourselves... what if we could share a single speaker cabinet with two heads? This would save space, make setups easier and finally give the guitarist the range of tone he needs.

The Headbone signal flow

The Radial Headbone is a seemingly simple device that takes a guitar signal and sends it to two amplifier heads plus a speaker cabinet. Although this sounds simple, to perform this task you need to combine a bunch of cables with a series of sequenced switch actions done in such a way as to ensure the two amplifiers run safely and that switching is both smooth and transparent.

Then of course, one must manage the guitar signal without affecting the tone in any way. When switching heads, you are not just diverting speaker signals; you are also redirecting the original guitar signal. This means retaining the natural tone of the instrument without introducing distortion or coloration. With guitarists like Joe Perry and Mark Tremonti using the Headbone, you can be sure that they will not accept anything messing with their guitar tone. Furthermore, any clicks or pops can be devastating, especially with high gain amps at concert touring levels. And of course 'concert levels' not only means loud, but also means performing on concert-size stages. Running 50-foot speaker cables from the amplifier heads to a pedalboard and then all the way back to the speaker cabinet is impractical. This not only makes setup more difficult, but long speaker cables reduce the amplifier's ability to control the speaker excursion which results in less punch and definition.

When we set about deciding on how to build the Headbone and what features were needed to get the job done, we had to take all of this into account. This is how we did it:

Maintaining a perfect guitar signal

The Headbone is equipped with both a buffered and a non-buffered input. A buffer is an active gain stage that is used to maintain the signal level when manipulating it to perform various functions such as driving multiple amplifiers. Most manufacturers employ op-amps or integrated chips for buffers as one single chip can replace hundreds of discreet components. This makes it easy to route the printed circuit board and significantly reduces costs. The downside to this approach is sacrificing tone. The Radial Headbone does not employ any signal degrading op-amps or chips in the audio signal path. Instead, you get Radial's award winning 100% discrete class-A buffering circuit for the most natural and transparent tone possible. If you already have a buffer in your signal chain, the direct (un-buffered) input lets you bypass the Headbone's internal buffer. Less buffers in the signal chain generally results in less noise and distortion.

Routing the guitar signal to the heads

Once the guitar signal is in the Headbone, we want to toggle the signal between the two heads. Guitar signal routing can be accomplished using several different methods such as voltage controlled amplifiers (VCA's), relays, mechanical switches or opto-couplers. Guitar signals are low-level, high-impedance signals that are highly susceptible to noise. As such, devices such as VCA's, which may work well in line-level mixers, often cause tone aberrations that guitarists find offensive. Relays do not color the tone, but introduce a mechanical 'pop' when the switch is engaged. This is particularly noticeable with high gain amps. Instead, the Headbone employs a series of opto-couplers to perform the hi-Z guitar signal switching. An opto-coupler is a combination light emitting diode and photocell that can be used as a switch to smoothly ramp-up and ramp-down the signal. Because this eliminates the contact 'spark', you can enjoy 100% quiet switching. The downside is cost: opto-couplers are significantly more expensive than either VCA's or relays. This therefore limits the use of opto-couplers to high-end gear and is why most manufacturers do not use them.

Routing the amp outputs to the speakers

While the guitar input signal is being switched between to the two amps, the Headbone must also toggle the amplifier outputs going to the speaker cabinet and standby load resisters. Before we discuss switching, it is important to understand that tube amps and solid-state amps work differently. For instance, tube amps must always see a speaker load. Without a load, the tube amplifier's power output transformer can heat up which could cause it to malfunction. To address this concern, the Headbone employs a load resistor that couples to the standby amp. This way, when head-1 is active and connected to the speaker, head-2's output is safely routed to the Headbone's internal load resistor. Actual head-to-speaker and load switching is accomplished using a series of high-performance silver contact relays. These mil-spec relays are able to handle over 100 watts and are rated at over 1,000,000 operations!

The art of silent switching:

Now that we understand how the various signals are switched, let's look at how the actual switching process works. When you stomp on the Headbone footswitch, this is what goes on behind the scene:

  1. Mute the guitar signal going to head-1
  2. Disconnect head-1 from the speaker
  3. Connect head-1 to a load resistor
  4. Disconnect head-2 from the load resistor
  5. Connect head-2 to the speaker cabinet
  6. Toggle the guitar from head-1 to head-2
  7. Turn the guitar back on

Of course all of this has to be controlled with absolute precision. This is accomplished using a digital chip called a PIC (Programmable Interrupt Controller). When you hit the Headbone's footswitch, you are actually sending a status change command to the micro-controller which then goes into action by muting the guitar, turning off the speaker, applying the resistive load and then turning everything back on again. The Headbone is a true marvel of hybrid technology that marries 100% discrete class-A analog circuitry, opto-couplers and relays with state-of-the-art digital control.

Your amps are protected with SafeMode™

One of the clever features built into the Headbone is a function called SafeMode™. This was designed into the Headbone as a fail-safe method to prevent disaster should the Headbone's power supply ever be accidentally disconnected. In SafeMode, the Headbone automatically shuts off the guitar input going to the heads and sends the tube amplifier outputs to load resistors and the solid-state amplifier outputs to an open circuit. You merely reconnect the power supply and the Headbone will automatically reset itself for normal operation.

Three Headbones to choose from

Tube amps and solid-state amps work differently: a tube amplifier must always see a load while a solid-state amp can be presented with an open circuit. We developed two completely different circuits to support each type of amplifier so that they can be safely switched. It is important to note that we are talking about the power-stage output of the guitar amplifier, not the preamp out. Some solid-state amps have a tube preamp stage and solid -state output stage. These are considered to be solid-state.

Headbone VT

The VT is the Headbone you should use if you have two tube or valve amplifiers such as those made my Marshall, Mesa-Boogie, Fender and others. These are easily recognized by the larger tubes that are almost always visible.

Headbone SS

The SS is the Headbone you would use if you have two solid state guitar amplifiers or are switching PA type power amps. These do not have any tubes in the power section.

Headbone TS

The TS is the Headbone you would select if you have one solid state amp like an old Roland JC120 and wanted to switch this with a tube-powered Fender Twin.

Connecting the Headbone

To connect the Headbone, you will need three guitar cables and three speaker cables. It is essential that you employ good quality high-impedance coaxial guitar cables and heavy duty 12 gauge speaker cables in order to ensure proper signal flow. This will also help keep noise down in the high power environment. It is also a good practice to keep all cables as short as possible. This is particularly important with speaker cables as you can lose a lot of power in the cable which will affect the tone and punch.

The Slingshot™ remote control

The best way to set up the Headbone is to locate it behind your amp where you can hide all of the cables. To remotely switch heads, the Headbone is equipped with a function called Slingshot that lets you employ any contact closure to perform the task. This is basically an on-off footswitch with a mechanical latching function. If you are equipped with a MIDI controller, these are normally equipped with a contact closure output that performs the same task.

Slingshot is also implemented on other Radial Tonebone products including the Loopbone™ and the Cabbone™. The Loopbone, for example, is equipped with a Slingshot output – this can act like a master controller on your pedalboard. You could set up the Loopbone so that when The Loopbone's effects loop-1 is engaged, it sends a 'switch command' to the Headbone so that it simultaneously switches heads. With one foot-stomp, you could activate your chorus, turn on an overdrive and switch amp heads! Best of all you are making all of this switching work with simple ¼" guitar cables.

With a Headbone... the possibilities are mind-boggling!