Below is a schematic of a German phaser instrument effects unit from the late 60s/early 70s called “Rotor Sound” and manufactured by Schaller. “Rotor Sound” alludes to the sound produced by a Leslie speaker cabinet. A demonstration of the effect produced by the unit can be heard here.
Similar to the vibrato units Hammond made for its organs, the Rotor Sound uses a saturable reactor as part of the effects filter (as seen at the top of the schematic). The effects filter itself is comprised of a 18-rung ladder network of low-pass filters, the series elements of which are the DCR and the inductance of each reactor’s “power” winding and a capacitor to ground as the sole shunt element.
Driving the reactor’s control winding is a vactrol-based oscillator (lower middle of schematic).
The saturable reactors in the device are two-winding P14/8 pot core transformers:
Someone else measured these saturable reactors; the power winding (the “in circuit” winding) is 2.2mH with a DCR of 65 ohms. The schematic itself does not denote the inductance; all it states is that the total resistance of the power windings in series is 800 ohms (800/18 =~ 44 ohms each).
What I need help with is understanding how to look at the effects filter. My understanding so far is that when the current through the control winding is low, each rung of the ladder network is a second-order LCR filter. When the current through the control winding is high, the core of the reactors saturate and the inductance in the LCR drops significantly, changing the filters topology into that of a first-order RC filter. However, I feel like this is the wrong way of looking at it as daisy-chaining 18 low pass filters is not going to produce an easily calculated response at a given frequency. Still, I feel like this shifting of topology twice per cycle of the LFO is key to understanding the function of the effect.
I’m asking for insight because I have managed to salvage a couple saturable reactors from some Hammond vibrato units and would like to repurpose them for something closer to the phaser schematic above.
Thank you in advance.
Similar to the vibrato units Hammond made for its organs, the Rotor Sound uses a saturable reactor as part of the effects filter (as seen at the top of the schematic). The effects filter itself is comprised of a 18-rung ladder network of low-pass filters, the series elements of which are the DCR and the inductance of each reactor’s “power” winding and a capacitor to ground as the sole shunt element.
Driving the reactor’s control winding is a vactrol-based oscillator (lower middle of schematic).
The saturable reactors in the device are two-winding P14/8 pot core transformers:
Someone else measured these saturable reactors; the power winding (the “in circuit” winding) is 2.2mH with a DCR of 65 ohms. The schematic itself does not denote the inductance; all it states is that the total resistance of the power windings in series is 800 ohms (800/18 =~ 44 ohms each).
What I need help with is understanding how to look at the effects filter. My understanding so far is that when the current through the control winding is low, each rung of the ladder network is a second-order LCR filter. When the current through the control winding is high, the core of the reactors saturate and the inductance in the LCR drops significantly, changing the filters topology into that of a first-order RC filter. However, I feel like this is the wrong way of looking at it as daisy-chaining 18 low pass filters is not going to produce an easily calculated response at a given frequency. Still, I feel like this shifting of topology twice per cycle of the LFO is key to understanding the function of the effect.
I’m asking for insight because I have managed to salvage a couple saturable reactors from some Hammond vibrato units and would like to repurpose them for something closer to the phaser schematic above.
Thank you in advance.
Attachments
For contrast, here is the schematic to one of the Hammond units from which I have salvaged a pair of saturable reactors:
This is the above unit, modified for use as a standalone effect for guitar:
This is the above unit, modified for use as a standalone effect for guitar:
My take is the filters are a "T" section all pass filter, (lattice phase equaliser) with up to 180 degrees phase shift per filter, 3240 degrees total.
The saturable reactor changes the coils inductance, so varies the frequency of the phase shift.
When combined with the direct signal, the phase shifted comb filter response sweeps up and down in frequency which varies with the rate of the oscillator.
