Chuck D. Bones
Circuit Wizard
A client asked me if I'd ever heard the Roland GP8's phase shifter section. I hadn't, but I found a schematic and breadboarded one. It bares so similarity to the MXR Phase 100 in that it contains both variable and fixed phase shift stages. The Phase 100 uses LDRs and has 10 stages; the GP8 uses OTAs and has 8 stages. The GP8 uses an IR3109 quad OTA to realize the four variable phase shift stages. That part is nearly identical to the Maestro MP-1 Phaser, which begat the EQD Grand Orbiter. With a total of 8 phase shift stages, we get 4 notches that sweep up and down. This is how the overall freq response looks near the bottom of the sweep, with RESONANCE set to 0.
Here's a preliminary schematic. I would have liked to use the AS3109 from CabinTech, but it wants to run on ±15V and the minimum supply voltage is ±10V according to the datasheet. No problem though, we can use a pair of LM13700s. I left the audio path pretty much intact. The GP8 is synth-based and has voltage control for all of the knobs. Here's we have plain old knobs for SPEED, DEPTH, COLOR (center freq) and RESONANCE. I employed the STOMPLFO chip to generate the sweep. It has a waveform (named Sweep) specifically for that purpose. SPEED goes from 0.05Hz (one cycle every 20 sec) up to around 8Hz. U1 is the input buffer, mixer and output buffer. U2 & U3 form the variable phase shift section. U4 makes up the fixed phase shift section. U5B and Q1 form a current source which drives the LM13700's control pins. U5A is a level shifter & buffer between the STOMPLFO's output filter and the current source. Sweep Trim adjusts the minimum control current. We don't want it to go to zero or the sound will *POP*. It sounds great as shown. I'm just making little tweaks here and there, almost done.
It produces a range of phaser tones, from smooth and subtle, to lush, and at extreme setting it sounds like a Star Trek weapon. It can make a UniVibe-type throb, or a Leslie-type shimmer. Plenty of dynamic range; no noise and no distortion.
The LFO is top left, level shifter & current source to the right of that, input and output buffers to the right of that. The 5V reg is top center. Bottom right are the fixed phase shift stages. Bottom left are the two LM13700s that comprise the variable phase shift stages. The trimput next to the current source is the Sweep Trim. That trimpot next to the LFO is the waveform select. There's really only one good waveform for this circuit. so no need for a waveform select on the final version.
Knobs (L-R): SPEED - DEPTH - COLOR - RESONANCE
Here's a preliminary schematic. I would have liked to use the AS3109 from CabinTech, but it wants to run on ±15V and the minimum supply voltage is ±10V according to the datasheet. No problem though, we can use a pair of LM13700s. I left the audio path pretty much intact. The GP8 is synth-based and has voltage control for all of the knobs. Here's we have plain old knobs for SPEED, DEPTH, COLOR (center freq) and RESONANCE. I employed the STOMPLFO chip to generate the sweep. It has a waveform (named Sweep) specifically for that purpose. SPEED goes from 0.05Hz (one cycle every 20 sec) up to around 8Hz. U1 is the input buffer, mixer and output buffer. U2 & U3 form the variable phase shift section. U4 makes up the fixed phase shift section. U5B and Q1 form a current source which drives the LM13700's control pins. U5A is a level shifter & buffer between the STOMPLFO's output filter and the current source. Sweep Trim adjusts the minimum control current. We don't want it to go to zero or the sound will *POP*. It sounds great as shown. I'm just making little tweaks here and there, almost done.
It produces a range of phaser tones, from smooth and subtle, to lush, and at extreme setting it sounds like a Star Trek weapon. It can make a UniVibe-type throb, or a Leslie-type shimmer. Plenty of dynamic range; no noise and no distortion.
The LFO is top left, level shifter & current source to the right of that, input and output buffers to the right of that. The 5V reg is top center. Bottom right are the fixed phase shift stages. Bottom left are the two LM13700s that comprise the variable phase shift stages. The trimput next to the current source is the Sweep Trim. That trimpot next to the LFO is the waveform select. There's really only one good waveform for this circuit. so no need for a waveform select on the final version.
Knobs (L-R): SPEED - DEPTH - COLOR - RESONANCE
