viva Analog “Precision Shifter” // jcm(c)2018
Bagging a new Phasor circuit …
To recap, all monaural time-based effects – which includes Delays, Phasors, Chorus circuits and Flangers – all tend to follow a similar overall structure comprising of three main parts: (i) a signal path that produces the mixing of dry and delayed signal, (ii) some form of controllable variable-delay technology, and (iii) an interface circuitry acting on the variable-delay technology on behalf of some time cycling modulation source.
If we look at Phasor circuit designs only in terms of two of these three basic system components, ie., (i) the signal path style and (ii) the variable-resistance (or transconductance) technology used as intermediary for creating delay, we can characterize the majority of audio Phasor designs in a simple way:
One way, the old school approach, is made by using (i) Single-Ended (SE) gain stages in conjunction with opto-elements (eg., SHIN-EI Univibe), … another is (ii) Op-Amp based in conjunction with jFET‘s (eg., EHX, MXR) or Opto-elements (eg., Musitronics) or Diodes (eg., EMS Hi-Fli) or PWM‘d switches (eg., MXR M-161), … and a third which is (iii) OTA based (eg., EHX Small-Stone), with built-in variable transconductance replacing the controlled variable-resistor technology part, a two-in-one phasor topology combining both aspects.
In each “practical” case, some deviation from pure mathematical modeling of the Phasor (sweep-able all-pass filter) system invariably occurs. Figuring out how and where the deviations occur can provide hints to possibly designing a better shifter … and then, there’s noise // which is where high headroom comes into play (more about that some other time …), the other side of things
If we want to talk about bettering the “practical” Phasor system these are some of the things we can look at
In principle, the rest of the signal path circuitry won’t “care” how time-varying resistance (or transconductance) gets produced in the cycling process – it should be a secondary matter regardless. The real challenge usually comes not so much from dealing with the particular caprices of a controlled-resistance technology (eg., need for characterizing and matching, etc.) but often more so from the design needs of the interfacing circuitry that links the technology to the signal path or to the oscillator, or both. Here’s an example of a Phasor circuit that re-explores all three basic parts in a brand new low-noise design concept.