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. Furthermore, any clicks or pops can be devastating, especially at concert touring levels... and of course 'concert levels' means performing on concert-size stages. Running 50-foot cables from the two guitar amplifier heads back 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.

The Headbone accomplishes all of this and is easy to use. What follows is how we did it...

The Headbone signal path:
From the outside, the Radial Headbone appears to be a simple foot activated switch that toggles two heads on a single speaker cabinet. Although the concept is simple, in practice behind this seemingly simple function, are many hidden processes that explode into action the moment the footswitch is activated. All of the functions are controlled by a device called a PIC or Programmable Interrupt Controller. To fully understand what is going on, take a look the Headbone block diagram on the 'Using the Headbone' page.

Maintaining a perfect guitar signal:
The Headbone is equipped with both buffered and direct (non-buffered) inputs for the guitar. A buffer is an active gain stage that is used in electronic devices to maintain the level when manipulating a signal to perform various functions such as driving multiple destinations. Most manufacturers employ op-amps or chips to for buffers as one single chip can replace hundreds of discreet components. This makes it easy to route the printed circuit board and reduces costs. Alas, the unfortunate downside to the 'easy school' of engineering, where one employs op-amps, is a sacrifice in tone.

The Radial Headbone does not employ any signal degrading op-amps or chips in the audio signal path. With Headbone, you get our highly acclaimed 100% discrete class-A circuit design for the most natural and transparent tone possible. Furthermore, if you are using a buffering device like the Radial Loopbone, Switchbone, the Radial JD7 or maybe the Radial JDV, you can bypass the input buffer and connect to the Headbone directly. This eliminates driving multiple buffers, which can increase noise and distortion.

Routing the guitar signal to the heads:
Once the guitar signal is in the Headbone, we must route it to the two heads. Guitar signal routing can be accomplished using several different methods such as voltage controlled amplifiers (VCA's), relays , mechanical switches, or photocells (which can be used as electronic switches).

Guitar signals are low-level, high-impedance signals that are easily disrupted. As such, devices such as VCA's, which may work well in line-level mixers, will cause tone aberrations that guitarists find offensive. Relays do not color the tone, but instead introduce a mechanical ‘pop’ when the switch is engaged. Relay switch noise is not audible after the amplification stage, such as when switching speakers, is very noticeable when manipulating ‘pre’ guitar amplifier signals.

Following the lead set by Leo Fender with his early amp designs, the Headbone employs a series of photocells to perform the hi-Z guitar signal switching. By controlling the photocells to smoothly ramp-up and ramp-down the signal, one can enjoy 100% quiet switching. Of course, the downside is cost – photocells are significantly more expensive than both VCA's and relays. This therefore limits the use of photocells to high-end gear and is why most manufacturers do not use them.

Routing the amp outputs to the speakers:
While the inputs are being switched to the two amps, the Headbone must also toggle the amplifier outputs going to the speaker cabinet. As anyone that has played around with a tube amps knows, it must always see a load when being driven or else the transformer will heat up and this could cause the amp to malfunction. To address this, the Headbone employs a large load resistor that couples with the standby amp. For example, when head-1 is active, it is connected to the speaker while head-2’s output is routed to the load resistor.

Actual head-to-speaker and load switching is accomplished using a series of high-cycling silver contact relays. These high performance aerospace industry approved relays are capable of 100W RMS operation and are rated for 10 million operations. Now that’s switch life!

The art of silent switching:
Now that we understand how the various signals are switched, lets look at how the actual switching process works. Here’s what goes on behind the scenes:

1. Guitar signal is muted to head-1
2. Head-1 is disconnected from the speaker
3. Head-1 output is sent to a load resistor
4. Head-2 gets connected to the speaker
5. Guitar gets connected to head-2
6. Guitar signal is un-muted

Of course all of this has to be controlled with absolute precision. This is accomplished using a digital chip called a PIC (Programmable Micro Controller). What this means is that 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 time-proven high performance analog Class-A circuitry, photocells and relays with state-of-the-art digital control.

SafeMode™ operation:
One of the very 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 ever be accidentally disconnected or the digital control chip fail. In SafeMode, the Headbone will automatically revert to a ‘default status’ wherein amp head-1 is connected to the speaker and amp head-2 input is muted and its output connected to the load-resistor.

A Headbone for tubes, a Headbone for solid-state, a headbone for 'tube+state':
Tube amps and solid-state amps work completely differently; a tube amplifier must always see a load while a solid-state amp can be disrupted with 'back emf'. This means that we had to develop completely different circuits to support each type of amplifier. The Headbone VT for valve-tube amps, the Headbone SS for solid-state amps and the Headbone TS for one of each. All three Headbones employ 100% discreet class-A circuitry for the utmost in tonal fidelity.

The Slingshot™ remote:
The Headbone by itself is perfect for small club stages but on a large concert stage, it is both cumbersome and inneficient to run 50-foot guitar and speaker cables to and from tyour pedalboard. Long guitar cables are prone to induced electro-magnetic noise while long speaker cables dissipate energy which reduces the amp’s ability to control speaker excursion. We figured the best solution would be a remote control. This led us to the development of our Slingshot™ remote control switching solution.

Since most guitarists would like to avoid the complexities of programming a MIDI system for switching, we designed a simple analog interface based on the time-tested systems used by most guitar amp manufacturers to toggle between amp channels. This approach had the unique benefit of being virtually universal among amp manufacturers as most employ ¼” jacks for foot switches. Since guitarists usually own extra ¼” guitar cables, this was the perfect solution!

Slingshot works using a simple contact closure footswitch, the same type that comes with most channel-switching guitar amplifiers. This means that just about any foot switch with a ¼” jack can connect to and remotely toggle the Headbone’s switching. And there’s more...

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. Simply mind-boggling!