Tonefiend Forum

The phase shifter is one of my favorite effects sounds. We are talking rubber-band chewy here. Just seems to have more life to it than chorus (pleasant) and flange (occasionally nausea inducing).

As I began to study effects circuits, I found an interesting variety of methods used for accomplishing this. I'll add schematics as I find them.

The basic phase shift stage uses a single gain element (like a tube or transistor) with equal resistors in the load and for lack of a better term, "ground return" path. When a signal comes in, the "ground return" resistor voltage follows the input, and the load resistor reproduces it 180 degrees out of phase. The signal from the ground return point is passed through a variable phase shift network, then summed back together with the other signal to give the frequency-dependent cancellations and reinforcements we know and love so well. This is a bit of an oversimplification!

Some of the things I've seen used for the variable phase shift elements (which are controlled by the LFO):

Lighte dependent resistors (LDRs) combined with a fluctuating light source (as in a Fender optical tremolo "bug"). The Uni-Vibe was one such beast.

FETs. Witness the Oberheim Phase Shifter.

LM3080A or other "transconductance op-amps". These are used as voltage controlled amplifiers in the phase shift circuit. However, to avoid distortion of the controlled signal, the level has to be brought way down, and afterwards, boosted way back up again. For this reason, audio circuits using transconductance op-amps tend to be quite noisy.

In a Hammond organ amp, a long chain of capacitors and inductors forming a delay line with taps at each stage going to a mixer. The phase variation is accomplished mechanically by a rotating capacitive scanner. This one's the coolest in my book - not necessarily for the sound but just what lengths they went to! Check it out!

Another Hammond implementation uses the phase splitter gain stages as well, but the variable phase shifting is accomplished by using half of a transformer (which is just an inductor) along with a capacitor. Now that in itself does not make variable phase. But, the low frequency oscillator goes into the secondary winding of the transformer. I have never read a theory of operation, but I can only imagine that this works by putting enough current into the secondary to partially saturate the core, which makes the inductance of the primary fluctuate. Freakin' brilliant! The schematic and project I found for this replaces whatever was originally used for the LFO in the Hammond organ with an Arduino microprocessor board! Talk about your old school meets new school.

Schematic here: https://www.solorb.com/elect/musiccirc/liquidator1/

Last but not least, Magnatone amps use some currently-impossible-to-get-or-replace-with-an-exact-modern-equivalent high voltage doodad that varies its resistance as you modulate a fairly high voltage across it.

The level of ingenuity that went into these designs really impresses me.

A very thorough article on all of this is here.

And here's another one.