Article by Ray Wilson
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This Circuit Has Been Updated
These = circuit has been=20 updated to reflect the changes that were considered mods in the original circuit. The PC board has been = changed to=20 accomodate the components that needed to be kludged in the previous = board. If=20 you made the mods suggested in the original page then your circuit is = the same=20 as this one.=20
Introduction
This phase shifter is designed around the phase = shift cell=20 found in several 1970s to 1980s units. It is not an exact copy of any of = them=20 but is modeled after them. It uses a transconductance amplifer to = produce a=20 voltage controlled variable phase shift. Mixing the phase delayed signal = with=20 the original produces notches in the resultant output.=20Alison Hughes' MFOS Phase Shifter
I just wanted to thank you for your awesome phase shifter circuit = design - it=20 sounds great! Everyone I have showed it to agrees that it's one of the = best=20 phase shifters they've ever heard. We especially like how extremely the=20 parameters can be adjusted to make dramatic, watery sounding effects. I = had this=20 vision of putting it into a hot pink clear acrylic enclosure with an = 80's punk=20 rock aesthetic - large aluminum knobs and those old embossed labels. = With help=20 from a friend, I was able to create this crazy enclosure. We added some = blue=20 LEDs and it looks really cool in low light. My terrible rats nest of a = wiring=20 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=20 the time in his studio!=20
See more photos at Alison's Flickr Site
Eight Stage Phase Shifter Schematic Page 1 PDF
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Increasing=20
Output Levels =20
A diyer who had bought the Phase Shifter board was concerned = that the=20 output levels were too low and that increasing the input level was = causing=20 distortion. It is important not to over-drive the input (or = things sound=20 bad) so I came up with a fix. I increased the output level and = kept the=20 level going into the phase shift array the same as it was by = splitting the=20 feedback resistor around U1-A into a 10K (R19) and a 33K (R65) = and by=20 feeding the phase shifter array from the junction of R19 and R65 = where the=20 level is the same as it was at the output when the feedback = resistor was=20 just 10K. Since I went to a gain of 4 in U1-A I added a = compensating cap=20 (C30 22pF) to insure that no high frequency oscillation occurs at = the=20 output of U1-A. Op-Amps are usually only compensated internally = for a=20 gain of 1. I increased the gain of U1-B by changing it's feedback = resistor=20 to 43K and I also added the 22pF cap for insurance against = oscillation.=20 This change is now incorporated into the circuit schematic and = PC=20 board. |
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U1-A (and associated components) acts as both the input buffer = and the=20 mixer that combines the original signal with the phase shifted = sigal via=20 S2 (Four or Eight Stage Selector) and R15 (Feedback adjust pot). = U1-B=20 (and associated components) buffers the OUT-4 circuit point which = is the=20 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=20 the inputs of a stereo mixing channel. If you change the gain of = the input=20 mixer BEWARE of too much feedback from the phase shifted signal. = I have=20 found that anything higher than a gain of 1 applied to the = fed-back signal=20 and you have an excellent candidate for an alarm system siren. So = for=20 example if you choose to change the op-amp gain setting resistor = (R19) to=20 50K you should also change both R15 and R9 to 50K. =20 U2-A and U2-B (and associated components) combine to form a = simple but=20 effective low frequency oscillator. U2-B is an integrator that = ramps up=20 when U2-A (acting as a comparator)'s output is low and ramps down = when=20 U2-A's output is high. U2-A's output goes high when the voltage at = its=20 non-inverting input goes above the threshold voltage (ground) and=20 overcomes the current flowing through hysteresis resistor R7. At = that=20 time the integrator begins to ramp low until the voltage at U2-A's = non-inverting input goes below the threshold voltage (ground) and=20 overcomes the current flowing through hysteresis resistor R7. At = that=20 point U2-A goes low and the cycle repeats. R16 controls the rate = of the=20 integrator by changing the voltage appearing at R17 and thus the = current=20 into (and out of) the integrator. R63 keeps the integrator from = seeing=20 ground (which would stop the LFO). If your LFO stops at the lowest = setting=20 you could increase R63 a bit (try 150 ohms, 200 ohms etc.). R63 = is=20 mounted on the panel. When closed, switch S1 (Tri Ramp switch) = bypasses=20 the rate set pot with a diode and a 1K resistor so that during the = high=20 cycle of U2-A the integrator ramps down very quickly. During the = low=20 cycle of U2-A (as the integrator ramps up) the rate control pot is = in the=20 circuit since the diode is reversed biased. You will see a rate = shift when=20 you switch from one wave to the other. The ramp will always be = about=20 twice the rate of the triangle wave since the integrator discharge = is=20 almost immediate in ramp mode. =20 U3-A and U3-B are used in the venerable voltage to exponential = current=20 generator. External voltage may be applied to control the phase = shift of=20 the unit. The LM13700s are all fed bias current via Q2 (2N3906 = PNP) and=20 the 20K resistors in series with their bias pin. The 20K resistors = at the=20 bias inputs help to equalize the current into the bias inputs so = all of=20 the sections track together pretty well. The phase shift sections = are all=20 the same as U4-A (and associated components). The transconductance = amp in=20 conjunction with C21 acts to shift the phase of the signal = applied to the=20 input (junction of R23 and C21). The angle of shift is dependent = on the=20 current flowing through U4-A (which is determined by the control = current=20 at it's pin 1 bias input). Each section takes as input the output = of the=20 previous stage and adds more phase shift. When this phase shifted = signal=20 is combined with the original signal in U1-A, cancellation of = frequencies=20 that are at (or close to) 180 degrees out of phase takes place = and the=20 characteristic phasing sound is heard. The more feedback applied = (via R15)=20 the more pronounced the effect. Several very noticable effects = can be=20 heard by experimenting with the settings of R10 and R15 = interactively. =20 Although I used all .0047uF caps in the phase shift cells you = can=20 change this value (I tried from .001uF to .01uF) or even only = change some=20 of them to achieve different sounding phaser effects. Experiment = have=20 fun! =20 One last word... If you don't want to use all four LM13700s you = can=20 just use the first two (U4 and U5) and just take OUT4 back to = feedback to=20 the input mixer/buffer. However using all eight gives you more = full cycle=20 phase shifts per modulation cycle and is really... well stunning. = =20 The Stop The Clicks Kludge is incorporated into the circuit = and PC=20 Board. NO KLUDGE NECESSARY ANYMORE See the original circuit for the history of = this=20 change. =20 The Increase Output Levels Kludge is incorporated into the = circuit=20 and PC Board. NO KLUDGE NECESSARY ANYMORE See the original circuit for the history of = this=20 change. |
| Approx. Current Consumption | |
| +12V | 20mA |
| -12V | 20mA |
Eight Stage Phase Shifter PCB Parts Layout (Parts Side Shown) PDF=20
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Eight Stage Phase Shifter PCB Part Values Layout (Parts Side = Shown)=20
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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=20
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A Cool panel design contributed by Steve Carter of the UK
Click = it for a=20 PDF verion.=20=20
Eight Stage Phase Shifter Front Panel PDF
Eight Stage Phase Shifter Panel Wiring Rear View PDF=20
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Eight Stage Phase Shifter Project Parts List
- Using 1% metal film resistors everywhere will reduce temperature = related=20 drift. =20
- Where 1% metal film is specified 5% carbon comp will work but with = more=20 temperature drift. =20
- Usually biFET amps (quads, duals, singles) can be replaced with an = equivalent from another manufacturer. =20
- Capacitors can be film, ceramic, or silver mica. =20
- LM13700 subs (if applicable) (LM13600, NE5517, AU5517, NTE870). =
Eight Stage Phase Shifter Project Parts List
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| 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,=20 R19 |
| 11 | Resistor 1/4 Watt 5%(s) | 1K | R31, R34, R37, R40, R51, R52, R53, R54, R8, R6,=20 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,=20 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,=20 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,=20 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=20
- 1/16" Thick aluminum plate for mounting the pots and switches. =20
- Unit is typically mounted in a synth case with other synth = modules. =20
- Assorted hardware 1" 6-32 nuts and bolts, 1/2" #8 wood screws, etc = =20
- Knobs for potentiometers, wire and solder. =20
- Digital Volt Meter and a Signal Tracer or oscilloscope for = testing.=20
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