The Headbone's internal switching comprises a sequence of relays, photocells, and load resistors, all of which are controlled via a digitally programmed chip. Hit the footswitch and the Headbone toggles the guitar signal from one amp to the other, disconnects the speaker and diverts the signal to a load resister, reconnects the speaker to the second head and turns on the guitar, all in quick succession.
To ensure safe and reliable performance, the Headbone features SafeMode whereby even when power is disconnected, the Headbone will automatically default to a "head-1 to cabinet" status, ensuring a constant and safe load on the amplifier.
For large stages, the Headbone is equipped with Slingshot, an easy to use remote control system that employs a standard foot switch or contact closure that enables the guitarist to toggle the Headbone's status from his pedal board. This eliminates running guitar and speaker cables to-and-from the pedal board and the amplifier.
Why make a Headbone?
Ask any guitarist for the ultimate stage setup and they will tell you that having 2 amps on stage is tonal bliss. Using a low-power head for crunchy rhythm and a high power 2-channel amp for clean passages and soloing lets you handle just about any situation. The only problem is that hauling around another cabinet is cumbersome at best, and using 2 cabinets on stage requires 2 microphones and 2 mixing console channels. You'll also need more space on stage for that extra cabinet and mic stand, more truck space to transport the equipment and added effort during set-up and tear down. All of these factors led to the development of the Headbone, a device that takes a guitar signal, sends it to 1 of 2 (selectable) amplifier heads, sends the amplfied signal back to the Headbone which connects the selected amp to the speaker cabinet.
Although this sounds like a simple task, in reality it is not. Switching amplifiers requires a complex series of actions to be performed (described below) set into action in such a way as to ensure the amplifiers are run safely and 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. Furthermore, any clicks or pops can be devastating, especially at concert touring levels... and on concert-size stages, running 50 ft. cables from 2 guitar amplifier heads back to a pedal board and then all the way back to the speaker cabinet can be impractical to set-up -- not to mention greatly reducing the amps ability to control speaker excursion, resulting in less punch and definition. Remote cabinet switching would definitely be an asset if not downright essential to keep cable lengths down and to consolidate switching onto (eg.) a pedalboard.
The Headbone was designed to address all of this and still be easy to use. What follows is some insight on how Radial 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. However simple this concept may be, in practice there are many hidden processes that explode into action the moment the foot switch is activated. All of the functions are controlled by a micro-controller chip.
There are 3 different circuits in the Headbone: The Blue circuit routes the guitar level signals; the Red circuit routes the speaker level signals; and the Green shows the micro-controller and Slingshot remote control circuits.
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 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.
At Radial, they do not employ signal degrading op-amps or chips in the audio signal path; the Headbone is no exception; you get their highly acclaimed 100% discreet, 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 directly. This eliminates driving multiple buffers which, no matter how good they are, can increase noise and distortion.
Routing the Guitar Signal to the Heads
Once the guitar signal is in the Headbone, you must route it to the two heads. Guitar signal routing can be accomplished using several different methods such as using VCA's -- voltage controlled amplifiers, 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 using devices such as VCA's that 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 but is very noticeable when manipulating "pre" guitar amplifier signals.
Following the lead set by Leo Fender's 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 Guitar Signal to the Heads
While the inputs are being switched to the 2 amps, the Headbone must also toggle the guitar amplifier outputs going to the speaker cabinet. As anyone that has played around with tube amps knows, amplifiers 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.
The Art of Silent Switching
Now that you 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 micro-controller chip. 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.
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 ever the Headbone's power be accidentally disconnected or should the digital control chip ever fail. In SafeMode, the Headbone will automatically revert to a "default status" whereby head-1 is connected to the speaker and head-2 input is muted and its output connected to the load-resistor.
A Headbone for Tube and a Headbone for Solid-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 Radial had to develop 2 completely different circuits to support each type of amplifier: The Headbone VT for valve-tube amps and the Headbone SS for solid-state amps. Both Headbones employ 100% discreet class-A circuitry for the utmost in tonal fidelity. Class-A circuits, although less efficient and more challenging to develop than simple op-amp designs, sound so much better that they are "the choice" when it comes to retaining the natural tone of the instrument.
The Slingshot Remote
Long input cables can induce electro-magnetic noise while long speaker cables dissipate energy which reduces the amp's ability to control speaker excursion. This led to the development of the Slingshot remote control switching solution. Slingshot allows switching to be controlled remotely using a plain 1/4" guitar cable rather than running long input and speaker cables to and between the switcher, heads and cabinet.
Since most guitarists would like to avoid the complexities of programming a MIDI system for switching, they designed a simple analog interface based on time-tested systems used by most guitar amp manufacturers to toggle amp channels. This approach had the unique benefit of being virtually universal among amp manufacturers as most employ 1/4" jacks for foot switches and since guitarists usually own extra 1/4" guitar cables. Also, Slingshot works using a simple contact closure footswitch; the same type,once again, that comes with most channel switching guitar amplifiers. All this means that just about any foot switch with a 1/4 in. 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 which acts like a master controller on a pedal board. You could set up the Loopbone so that when effects loop-1 is engaged, it sends a 'switch command' to the Headbone to simultaneously switch heads. With one foot-stomp, you can activate your chorus, turn on an overdrive and switch amp heads. Best of all you are making all of this work with simple 1/4" guitar cable.
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