The first commercially successful guitar synthesizer, originally offered by Roland in 1977. The GR-500 consisted of four "sections", or separate types of synth circuits, referred to by Roland as "bass", "polyensemble", "solo", and "external". Unlike its main competitor, the ARP Avatar, the GR-500 could not be used with the user's choice of guitar; the user was required to purchase from Roland a guitar made specifically to work with the synth. The synth itself was contained in a wedge-shaped box with controls mounted on the top, which could be placed on the floor or on an included stand. There was no patch memory.
The polyensemble section relied on waveshaping to generate synth-like waveforms directly from the outputs of the guitar's hex pickup; it was the only section in the synth that was polyphonic. A mixer provided four types of waveform outputs. The section contained a VCA and an attack-decay-sustain envelope generator, which was gated by the output from the strings. The polyensemble section mostly generated organ-like and reed tones, but it could be surprisingly effective when combined with the outputs of the bass and solo sections. The solo section was monophonic, and consisted of a VCO, VCF, VCA, LFO, and its own ADS envelope generator. (Later versions had a second VCO which could be detuned.) Unlike the polyensemble section, this section used a pitch converter to produce a control voltage for the VCO and VCF. The pitch follower followed only one string at a time, since it was monophonic; it used last-note priority. The polyensemble section could be routed through the VCF, along with the solo VCO, for paraphonic filtering, the combination of which could be surprisingly effective and make it sound as if the solo section was polyphonic, which it was not.
The bass section was a simplified version of the solo section. It provided three filtered waveforms, which could be mixed as desired. It was also monophonic and its pitch follower also used last-note priority. A string selector made it possible for only the guitar's bass strings to drive the bass section, so that the bass could be made to follow the root note of a chord, with the proper playing technique. The "external" section contained a CV/Gate interface to drive an external synth. It provide the control voltage and gate from the solo section to the external synth, as well as a second control voltage and gate from controls on the guitar. Additionally, the section contained a VCA with an input for the external synth. The external synth's output could be connected to this, and it would be mixed into the GR-500's mix output, with the VCA level being controlled by controls on the guitar. The external section also had a special 6-pin DIN jack made to interface with the keyboard input jack on a System 700 synth, allowing the GR-500 to be used in place of the System 700's keyboard.
The guitar was called the GS-500. It had a Les Paul-style body, with a bevy of synth controls mounted on its top: an enable switch and volume control for each synth section, several function switches, and an EQ section for the guitar pickup's output. Where a Les Paul normally has a pickup selector, the GS-500 had a switch allowing for selection of the guitar sound, the synth sound, or a mix of the two. The guitar contained an "infinite sustain" circuit, which worked by feeding back an amplified version of each string signal back into the string, where it reacted to a powerful magnet (mounted where the neck pickup would normally be) and produced self-reinforcing vibrations via the Fleming effect. It was controlled by the master volume knob on the guitar; turning it up drove more signal into the sustain mechanism, producing infinite sustain if the volume was turned up high enough. The guitar had one conventional pickup, plus the hex pickup for the synth, and it required a bridge and tailpiece with plastic parts to keep the strings electrically isolated from each other. It had no conventional output jack; it connected to the synth via a 24-pin connector and cable.
The guitar was not ideal. The amount of wood that had to be removed from the body to make room for all the controls and circuitry upset the guitar's balance. (Later Roland guitar synth models would have fewer controls on the guitar, replaced by foot-operated controls in a floor box.) The infinite sustain circuit sometimes created pops in the guitar output whenever the performer touched a string, and performers disliked having to drag around the heavy and stiff 24-conductor cable. Roland had originally intended to offer several choices of guitar, but the GS-500 was the only one produced for this model. Roland's next two guitar synth models, the GR-300 and GR-700, were offered with several guitar models which also used the 24-pin connector and cable; however, the signal assignments were different and these guitars cannot be used with the GR-500, nor can the GS-500 be used with the other synths, without modification.
Despite the limitations, the GR-500 was the first guitar synthesizer to work reasonably well (by the standards of the day), with fairly good tracking. For many guitarists of the era, it provided them with their first exposure to synthesis.