Eight Stage Phase Shifter (+/-12V to +/-15V)

Article by Ray Wilson

This is an intermediate to advanced project and I do
not recommend it as a first project if you are just getting started in synths or
electronics. Only the circuit and some explanation are shown here. A lot of
project building, troubleshooting and electronics experience is assumed.
Additionally, electronic equipment ownership (scope, meters, etc.) is taken for
granted. If you are interested in building this project please read the entire
page before ordering PC boards to ensure that the information provided is
thorough enough for you to complete the project successfully.

Features

Switchable 4 or 8 stage phase shifter.

Variable feedback ranges from mild swooshing to raucous mind
bending.

Two external control voltage inputs.

Pseudo stereo output when 4 stage and 8 stage outputs are
used.

Transconductance amp based phase shift cells.

On-Board LFO produces Triangle and Ramp waves.

Phase angle is modulated in a logarithmic fashion.

8 Stages of phase shift result in some sweet barber pole
effect.

Quirks

With high feedback settings you get a bit of oscillation,
depending on osc. rate.

With ramp wave setting you get some control voltage
feedthrough (and the above).

Not a noise generator but not a piece of L.A. studio
equipment.

Makes a pop when switching between 4 and 8 stages.

You be the judge... Samples
Guitar
With 1/2 Depth 1/2 Feedback

Guitar
With 3/4 Depth 1/2 Feedback

Guitar
With 3/4 Depth 3/4 Feedback

Guitar
With 3/4 Depth 3/4 Feedback (Higher Rate)

Slow
8 Stage Guitar

Slow
8 Stage Guitar (Pseudo Stereo)

Korg
N1 Choir and Strings Slow Rate (Pseudo Stereo)

Knob
Twiddling And 4 To 8 Stage Switching (Pseudo Stereo)

Korg
N1 Choir Slow Rate (Pseudo Stereo)

Korg
N1 Strings Slow Rate (Pseudo Stereo)

This Circuit Has Been Updated
These circuit has been
updated to reflect the changes that were considered mods in the original circuit. The PC board has been changed to
accomodate the components that needed to be kludged in the previous board. If
you made the mods suggested in the original page then your circuit is the same
as this one.

Introduction
This phase shifter is designed around the phase shift cell
found in several 1970s to 1980s units. It is not an exact copy of any of them
but is modeled after them. It uses a transconductance amplifer to produce a
voltage controlled variable phase shift. Mixing the phase delayed signal with
the original produces notches in the resultant output.

Alison Hughes' MFOS Phase Shifter

Alison Hughes used the
MFOS phase shifter board in her excellent phase shifter project.

I just wanted to thank you for your awesome phase shifter circuit design - it
sounds great! Everyone I have showed it to agrees that it's one of the best
phase shifters they've ever heard. We especially like how extremely the
parameters can be adjusted to make dramatic, watery sounding effects. I had this
vision of putting it into a hot pink clear acrylic enclosure with an 80's punk
rock aesthetic - large aluminum knobs and those old embossed labels. With help
from a friend, I was able to create this crazy enclosure. We added some blue
LEDs and it looks really cool in low light. My terrible rats nest of a wiring
job makes it look more frankenstein- like with the LEDs lighting it from
underneath. I gave it to my boyfriend as a birthday present and he uses it all
the time in his studio!

See more photos at Alison's Flickr Site

Eight Stage Phase Shifter Schematic Page 1 PDF

Increasing
Output Levels

A diyer who had bought the Phase Shifter board was concerned that the
output levels were too low and that increasing the input level was causing
distortion. It is important not to over-drive the input (or things sound
bad) so I came up with a fix. I increased the output level and kept the
level going into the phase shift array the same as it was by splitting the
feedback resistor around U1-A into a 10K (R19) and a 33K (R65) and by
feeding the phase shifter array from the junction of R19 and R65 where the
level is the same as it was at the output when the feedback resistor was
just 10K. Since I went to a gain of 4 in U1-A I added a compensating cap
(C30 22pF) to insure that no high frequency oscillation occurs at the
output of U1-A. Op-Amps are usually only compensated internally for a
gain of 1. I increased the gain of U1-B by changing it's feedback resistor
to 43K and I also added the 22pF cap for insurance against oscillation.
This change is now incorporated into the circuit schematic and PC
board.

U1-A (and associated components) acts as both the input buffer and the
mixer that combines the original signal with the phase shifted sigal via
S2 (Four or Eight Stage Selector) and R15 (Feedback adjust pot). U1-B
(and associated components) buffers the OUT-4 circuit point which is the
output of the fourth phase shift stage. When using 8 stage phasing
shifting you get a pseudo-stereo effect by connecting OUT1 and OUT2 to
the inputs of a stereo mixing channel. If you change the gain of the input
mixer BEWARE of too much feedback from the phase shifted signal. I have
found that anything higher than a gain of 1 applied to the fed-back signal
and you have an excellent candidate for an alarm system siren. So for
example if you choose to change the op-amp gain setting resistor (R19) to
50K you should also change both R15 and R9 to 50K.

U2-A and U2-B (and associated components) combine to form a simple but
effective low frequency oscillator. U2-B is an integrator that ramps up
when U2-A (acting as a comparator)'s output is low and ramps down when
U2-A's output is high. U2-A's output goes high when the voltage at its
non-inverting input goes above the threshold voltage (ground) and
overcomes the current flowing through hysteresis resistor R7. At that
time the integrator begins to ramp low until the voltage at U2-A's
non-inverting input goes below the threshold voltage (ground) and
overcomes the current flowing through hysteresis resistor R7. At that
point U2-A goes low and the cycle repeats. R16 controls the rate of the
integrator by changing the voltage appearing at R17 and thus the current
into (and out of) the integrator. R63 keeps the integrator from seeing
ground (which would stop the LFO). If your LFO stops at the lowest setting
you could increase R63 a bit (try 150 ohms, 200 ohms etc.). R63 is
mounted on the panel. When closed, switch S1 (Tri Ramp switch) bypasses
the rate set pot with a diode and a 1K resistor so that during the high
cycle of U2-A the integrator ramps down very quickly. During the low
cycle of U2-A (as the integrator ramps up) the rate control pot is in the
circuit since the diode is reversed biased. You will see a rate shift when
you switch from one wave to the other. The ramp will always be about
twice the rate of the triangle wave since the integrator discharge is
almost immediate in ramp mode.

U3-A and U3-B are used in the venerable voltage to exponential current
generator. External voltage may be applied to control the phase shift of
the unit. The LM13700s are all fed bias current via Q2 (2N3906 PNP) and
the 20K resistors in series with their bias pin. The 20K resistors at the
bias inputs help to equalize the current into the bias inputs so all of
the sections track together pretty well. The phase shift sections are all
the same as U4-A (and associated components). The transconductance amp in
conjunction with C21 acts to shift the phase of the signal applied to the
input (junction of R23 and C21). The angle of shift is dependent on the
current flowing through U4-A (which is determined by the control current
at it's pin 1 bias input). Each section takes as input the output of the
previous stage and adds more phase shift. When this phase shifted signal
is combined with the original signal in U1-A, cancellation of frequencies
that are at (or close to) 180 degrees out of phase takes place and the
characteristic phasing sound is heard. The more feedback applied (via R15)
the more pronounced the effect. Several very noticable effects can be
heard by experimenting with the settings of R10 and R15 interactively.

Although I used all .0047uF caps in the phase shift cells you can
change this value (I tried from .001uF to .01uF) or even only change some
of them to achieve different sounding phaser effects. Experiment have
fun!

One last word... If you don't want to use all four LM13700s you can
just use the first two (U4 and U5) and just take OUT4 back to feedback to
the input mixer/buffer. However using all eight gives you more full cycle
phase shifts per modulation cycle and is really... well stunning.

The Stop The Clicks Kludge is incorporated into the circuit and PC
Board. NO KLUDGE NECESSARY ANYMORE See the original circuit for the history of this
change.

The Increase Output Levels Kludge is incorporated into the circuit
and PC Board. NO KLUDGE NECESSARY ANYMORE See the original circuit for the history of this
change.

Approx. Current Consumption

+12V

20mA

-12V

20mA

Eight Stage Phase Shifter PCB Parts Layout (Parts Side Shown) PDF

Eight Stage Phase Shifter PCB Part Values Layout (Parts Side Shown)

I find this view useful when I'm populating the
board. I don't have to go back and forth from the designator to the value. It
speeds up construction. Click the "Larger GIF" link and print the image as
landscape.

Eight Stage Phase Shifter PCB Bottom Copper (Parts Side Shown)

Eight Stage Phase Shifter PCB Top Copper(Parts Side Shown)

Eight Stage Phase Shifter PCB Top Silk Screen

Eight Stage Phase Shifter PCB Populated Larger Image

A Cool panel design contributed by Steve Carter of the UK
Click it for a
PDF verion.

Eight Stage Phase Shifter Front Panel PDF

Eight Stage Phase Shifter Panel Wiring Rear View PDF

If your mounting panel material is conductive then
you don't need to run ground wires between all jacks since they make physical
and thus electrical contact with the conductive front plate. You must run the
ground wire to at least one jack and any other component leads that require a
ground.

Eight Stage Phase Shifter Project Parts List

Using 1% metal film resistors everywhere will reduce temperature related
drift.

Where 1% metal film is specified 5% carbon comp will work but with more
temperature drift.

Usually biFET amps (quads, duals, singles) can be replaced with an
equivalent from another manufacturer.

Capacitors can be film, ceramic, or silver mica.

LM13700 subs (if applicable) (LM13600, NE5517, AU5517, NTE870).

Eight Stage Phase Shifter Project Parts List

Qty.

Description

Value

Designators

4

LM13700 Dual gm Op Amp(s)

LM13700

U4, U5, U6, U7

3

TL072 Dual Op Amp(s)

TL072

U1, U2, U3

1

1N914 or 1N4148 Sw. Diode

1N914 or 1N4148

D1

2

2N3904(s)

2N3904

Q1, Q3

1

2N3906

2N3906

Q2

1

Potentiometer

100K Audio Taper

R4

2

Potentiometer(s)

100K Linear Taper

R16, R10

1

Potentiometer

10K Linear Taper

R15

5

Resistor 1/4 Watt 5%(s)

100K

R18, R21, R20, R12, R17

1

Resistor 1/4 Watt 5%(s)

43K

R3

13

Resistor 1/4 Watt 5%(s)

10K

R14, R5, R2, R9, R33, R36, R39, R41, R61, R60, R58, R55,
R19

11

Resistor 1/4 Watt 5%(s)

1K

R31, R34, R37, R40, R51, R52, R53, R54, R8, R6,
R1

1

Resistor 1/4 Watt 5%

1M

R13

9

Resistor 1/4 Watt 5%(s)

20K

R24, R26, R28, R30, R44, R46, R48, R50,
R64

1

Resistor 1/4 Watt 5%(s)

270K

R7

1

Resistor 1/4 Watt 5%

2K

R22

16

Resistor 1/4 Watt 5%(s)

30K

R23, R25, R27, R29, R43, R45, R47, R49, R32, R35, R38,
R42, R62, R59, R57, R56

1

Resistor 1/4 Watt 5%

430K

R11

1

Resistor 1/4 Watt 5%

100 ohm

R63 Mounted on panel

1

Resistor 1/4 Watt 5%

33K

R65

1

SPDT Switch

SPDT

S2

1

SPST Switch

SPST

S1

8

Capacitor Ceramic(s)

.0047uF

C21, C22, C23, C24, C25, C26, C27, C28

10

Capacitor Ceramic(s)

.1uF

C2, C4, C3, C6, C8, C10, C9, C13, C11,
C15

1

Ceramic Capacitor

.22uF

C17

1

Ceramic Capacitor

100pF

C19

2

Capacitor Ceramic(s)

22pF

C30, C31

3

Electrolytic Capacitor (s)

10uF

C20, C7, C16

1

Electrolytic Capacitor (s)

2.2uF

C29

1

Electrolytic Capacitor

1uF

C18

Miscellaneous

1/16" Thick aluminum plate for mounting the pots and switches.

Unit is typically mounted in a synth case with other synth modules.

Assorted hardware 1" 6-32 nuts and bolts, 1/2" #8 wood screws, etc

Knobs for potentiometers, wire and solder.

Digital Volt Meter and a Signal Tracer or oscilloscope for testing.