Bill and Will's Synth
Jürgen Haible "Krautrock" Phaser


November 2007, May 2009 -

In November of 2007, Jürgen Haible posted a message on the MOTM list server announcing what later he referred to as his "Krautrock" Phaser.  I was very intrigued by the use of LDRs in the circuit.

True to character, Jürgen offered us tireless generosity in answering our questions and for letting us post some of the images he created here on our page.  Our interest in doing so is to document his work and provide an idiot's guide to building this, yet another marvelous gadget.  We hope we succeed.

This module requires E10 7V 100mA lamps - virtually impossible to find Stateside - and the particular LDRs Jurgen got from Reighelt in Germany. We were lucky - he provided the LDRs with the PCB we bought. But the lamps were very hard to get. We finally went in with a friend and bought a whole pile of the Lamps and E10 sockets.

Table of Contents

This page has become really long, so here's a table of contents that we hope will make it easier to traverse:

Background - presents Jurgen's initial description of the effect

Recapitulation of Construction/Feature Options - presents a simple list of the different possible implementations

Option Details - presents all the details of the different possible implementations - you'll need to consider these in deciding how you'll build yours

Parts - presents a Bill of Materials and notes about it

Panel - presents how we, in collaboration with Jürgen and others,  came up with our panels' design - ultimately Scott Deyo at Bridechamber fabricated the MOTM format one

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Power Plugs, MTA headers

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Use notes


Jürgen's site describes the effect:

If you listen to German records from the 1970's, you often hear a rather unique phasing that comes from a device called "Compact A Phaser" from the Berlin company "Gert Schulte Audio Elektronik". It's often referred to as "Schulte Phaser", and I have heard it being called "Krautrock Phaser", "Schulze Phaser" (because it's prominent on many early KS albums), "TD Phaser" (because of Tangerine Dream). In my opinion, this phaser has been important to the sound of these early electronic musicians almost as much as the synthesizers the used.

My Compact Clone from 1999

In 1999 I have built a clone of this on veroboard, and put it into the enclosure you can see in the pictures below.

This has been partially sponsored by the vintage synthesizer shop Touched by Sound back then, who also have taken my Phaser to NAMM to test the waters for commercial production. Well, it didn't come to this for various reasons, but I've been enjoying my Compact Clone ever since, and mostly use it on my Korg Lambda string ensemble.

The new Compact Clone 2007

I've decided to do another redesign of that circuit and make a PCB layout.

All who have followed my DIY projects over the years know that I'm using the term "clone" rather loosely. I'm not actually "cloning" anything, which would be a 1:1 reproduction of the original circuit and/or the appearance. I certainly do strive for a most perfect reproduction of the sound that made a vintage design famous, but I also add features of my own that I find useful, and tailor the circuit to my own needs and standards.

In case of the Compact Clone this means:

Things I keep:

  • The incandescent lamp / LDR combination that is responsible for the unique way of sweeping Big incandescent lamp as sweep indicator on front panel
  • The circuit topology: 8 Stages of Phasing, 2 Stages of Feedback. Negative, lowpass-filtered feedback
  • The 741 opamps
  • The unique "Osc. Period" potentiometer that has maximum LFO rate on ccw end. (optional)
  • Works with original 7V 100mA lamps.
  • The possibility to mount potentiometers direcly on board. (Alps RK11 vertical mount types or similar.) Of course you can connect about every other potentiometer with wires, too.
  • Mains-Powered (optional). For +/-15V powered, see below.
  • Power supply for mains powering on the PCB, including secondary fuses, rectifiers, electrolytic caps, voltage regulators and heatsinks.

Things I change:

  • It's possible to adapt the circuit to slightly different lamps and LDRs.
  • I've added a lot of coupling capacitors to keep offset voltages from the outputs
  • A Hard-Bypass with a relay that is controlled by an momentary switch. (Push: Turn on. Push again: Turn off.) - -
    Several of these switches can be wired in parallel, i.e you can have on on the front panel, and another one connected via jack for remote control.
  • Circuit redesigned for easily available potentiometers: 50k lin and 50k log (47k is the same, actually.) - -
    It's also possible to adapt it for slightly different pot values (feel free to ask), if your favorite form factor of potentiometer only comes in certain values.
  • You don't need that special "staircase" enclosure when using pcb-mounted potentiometers. In my version, the pots are mounted on the solder side of the board. So the component side looks down inside the enclosure, not being in the way of the front (or rather "top") panel.
  • Complete redesign for +/-15V operation. Option for MOTM-style power connector on the PCB.
  • Large heatsinks and stronger Lamp Driver transistors.
  • Mains transformer and primary fuse not on PCB for safety reasons.
  • No DIN jack.
    Switch for LFO / Manual sweep on front panel
    1/4" TRS jacks for remote control of Sweep Rate and Modulation Depth.

Learn more...

Jürgen's PC board diagram - you can click on the image to download a higher resolution image.

But Jürgen's posting doesn't fully describe the details of the different configurations possible. Here are the different options:

Recapitulation of Construction/Feature Options

1. Power Supply Options

Option 1 - On-board power supply.

Option 2 - MOTM-style dual power supply.

2. Connection/Control Options

A. Basic Connection/Controls (common to all options)

Modulation Knob
Feedback Knob

Oscillation Period Knob

Modulation Depth Knob

Bypass Momentary Switch

Mode Switch (Auto / Manual)

Modulation Indicator

Bypass Pedal Jack

Rate Pedal Jack

Depth Pedal Jack

Input Jack

R (Mono) Jack

L (Stereo) Jack

B. Effect Bypass LED

Option Details

1. Power Supply

Option 1

On-board power supply - what Jürgen wrote for the Triple Chorus applies to the Krautrock, "It contains a power supply (less transformer and primary fuse). You only have to connect 18V AC from a transformer."

Option 2

MOTM-style dual power supply - also what Jürgen's Triple Chorus description applies, "I've also included the footprints of MOTM and .COM power connectors, if you want to run the board directly from 15V, without a transformer of its own.  I've placed the footprint for the connectors beneath the (secondary) fuse, because you never need both at the same time.  When connecting to 15V DC, you must omit a lot of the power supply components, too..."

In this diagram, the elements omitted for the MOTM style 15V power supply are shaded red, the MTA header for the 15V power is shaded green

2. Connections and Controls

A. Basic Connection/Controls (common to all options)

Modulation Knob

50k linear Pot - to "MODULATION" on PCB

Feedback Knob

50k linear Pot - to "FEEDBACK" on PCB

Oscillation Period Knob

50k log Pot - to "OSC. PERIOD" on PCB

Modulation Depth Knob

50k linear Pot - to "PHASING / AMPLITUDE" on PCB

Bypass Momentary Switch

We chose an NKK ON-NONE-(ON) switch (Mouser 633-M201502-RO) from EFFECT ACTIVE / BYPASS to "SWITCH" on PCB

Mode Switch

DPDT switch from MODE switch AUTO - MANUAL to "MANUAL" on PCB

Modulation Indicator

Special panel mount E10 Lamp holder to LAMP on PCB

Bypass Pedal Jack

Isolated, nylon shaft 112A type jack (N112AX) to SWITCH on PCB (it's not clear to me that you can't use a regular 112AX jack but we're going to use the nylon one)

Rate Pedal Jack

Isolated, nylon shaft 112B (TRS) type jack (N112BX) to RATE on PCB

Depth Pedal Jack

Isolated, nylon shaft 114B type jack (N114BX) to AMOUNT on PCB

Input Jack

Switchcraft 112A Jack

Output L/Stereo Jack

Switchcraft 112A Jack

Output R/Mono Jack

Switchcraft 112A Jack

B. Effect Bypass LED

Active/Bypass Indicator LED

LED (green) to LED on PCB


Will and I have developed a parts-list / bill-of-materials in the form of an XL spreadsheet. Jürgen has been very patient and helpful answering our many pesky questions.

In the BOM, the left-most column is the "part."  The parts we've ordered have a green background.  These parts we have a high (but not perfect) level of confidence that we've specified correctly - we caught a mistake or two in part numbers / prices as we were ordering.  please double-check us and let us know of mistakes you find.

Click here to see Jürgen's Bill of Materials.

Corrections to BOM:

None yet -


None yet -

Click here to download the spreadsheet (apx. 48K). 

7V 100mA E-10 Lamps

The Krautrock requires three hard-to-find-in-America lamps - 7V 100mA lamps with E-10 bases. These are impossible to find on the American side of the Atlantic (at least as of 7/4/09). Thanks to Terry's efforts, we got ours, with him, from "France Lamps" and we got two types - what they call "tubular" which is a little elongated, and "mini-round" which is short and round.

We'll use the tubular ones in the two PCB-mounted sockets and a mini-round in the panel-mount socket.

Now - in the interest of completeness - Jurgen says that it's possible to construct the Krautrock using 6.3V lamps, more common west of the Atlantic.  But they have to be 100mA lamps - we couldn't find any.  And to do so, you need to install two 1N4002 diode in series with the lamps.  He's provided two places for them on the PCB (where we've put jumpers).

Well - it also may be possible to use 7V lamps on the PCB and a 6.3V lamp for the Panel Mount one - a 1N4002 diode would need to be in series with the lamp.  The advantage being if you could find a 100mA 6.3 lamp, maybe you could get it in a "bayonet" base.  Why would that be handy?  Read on -

Panel-mount Lamp Holder

OK - so per Jurgen's original design, the Krautrock REQUIRES three 7V 100mA lamps.  Two are mounted on the PCB - and one is panel-mounted.  All three are included in the circuit, and for the circuit to operate properly, all three must be in line.

You can buy a panel-mount E10 lamp holder from Reighelt in Germany, but we wanted one with a cool retro faceted lens.

We tried a couple options none to our satisfaction, and for a time our construction effort was stalled.  But then Will, looking at my 60's vintage Fender guitar amp pointed out that it has exactly the kind of lens we wanted.  We began to search the web.

We found replacement Fender parts at Antique Electronic Supply.  There's a holder and a separate lens, but the holder has a "bayonet base" (see - this is why it would be handy to have a lamp with bayonet base):


But Radio Shack has a "E10 Vertical-Mount Bulb Holder" that comes in a 4-pack - part# 272-360:

Here's how we put them together:

1. we disassembled the lamp-holder:

2. we cut off the tip-end of the center, spring-loaded contact by pushing it out with a screw-driver and then clipping it:

3.  We cut off the other contact - it holds on the socket:

4.  We prepared the holder-bracket, after measuring, by bending it thus:

5.  The lamp socket has a hole that just fits a 6-32 screw.  We have a bunch of 1/4" screws handy - put it on with a kepps nut:

6.  And here's how it goes together:

hex nut over socket

lamp into socket

the panel nut-thing into bracket and screw on the hex nut

lens screws in - Ta Da!


With the help of Jürgen and in collaboration with and with input from Mike (mrmike of clickbang) and Scott Deyo (, Will and I developed the Panel design at the top of the page.

The evolution of the design isn't really worth exploring. The design went back and forth between us and Jurgen several times - each time he was clarifying how the features should be interpreted.

The hole for the panel-mount lamp socket has to be drilled out to 11/16 and a 5/16" hole needs to be drilled for the LED.

We determined the location for the LED hole - directly below and centered on the momentary switch and to the left and centered on the panel-mount lamp.  The old drafting machine came in handy.

Center Punch

Taped up

First we drilled a 1/8" hole

Then used a step-drill to increase the size to 5/16"

We used a step-drill to increase the size of the hole to fit the panel-mount E10 socket we made - 11/16"

We had gotten a piece of crap step drill from Harbor Freight... the one on the right of this pic... that cost $7. Maybe it was good for cutting balsa, but couldn't make its way into aluminum - we can't imagine what it would do in - well - oak. We got a $36 one from Lowes (on the left). Cut like butter.

The hole cut, here's how it goes together.

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.



There are lots of places on the back of the PCB for the 100nF SMT caps. Jurgen says their only necessary if the circuit oscillates or if one upgrades to faster Op Amps. So we're just going to put them here:

Semiconductors, IC Sockets

We paid particular attention to their orientation - here's what they look like soldered in.

MTA Headers, Power Transistors

Besides the .156 Power Header, we wanted to include a couple little headers to attach the LED and the Indicator Lamp.  We accidentally soldered one into the switch position too.  No biggie.  In the same stroke, we soldered in the three power transistors paying particular attention to lining them up with where the heat-sinks will go.

here's how they line up

The Big 120 Ohm Resistor

Snack - Corned Beef Hash Breakfast for Supper

Sometimes breakfast makes a great supper - we had Corned Beef left over - here's how we made it:



it's easy - in our Dutch oven, cover the brisket with water and cook.

Now to make the hash

chop potatoes, onions

cut and shred the beef

cook together 'till potatoes are done - then fry

eggs over easy

served with English muffins, OJ


Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

Relay, Trimmers, E10 Sockets, Heat Sinks

By the way - we used a 500K trimmer instead of the 470K one

Connection Wires

We decided to install the connection wires at this point in the construction because some of the remaining PCB elements are fragile. So they'll come after.

We roughly mocked-up the position of the PCB, mounting bracket, and panel:

Here's how we figured the wires should be (oh - an important note - in the case of twisted wires, these are the lengths after twisting.  So we cut lengths of wire longer than needed, twisted them, then cut them to these lengths.):

MODULATION pot (50K lin) - twisted triple wire 5in.

FEEDBACK pot (50K lin) - twisted triple wire 5in.

OSC. PERIOD pot (50K log) - twisted triple wire 7in.

MODULATION pot (50K lin) - twisted triple wire 9in.

EFFECT ACTIVE/BYPASS momentary switch (to PCB) - twisted pair wire 7in.

MODE switch - twisted triple wire 7in.

BYPASS pedal jack to EFFECT ACTIVE/BYPASS momentary switch - twisted pair wire 6in.

RATE pedal jack to RATE (on PCB) - twisted triple 10in.

DEPTH pedal jack to AMMOUNT (on PCB) - twisted triple 9in.

IN jack to IN (on PCB) - coax 8in.

OUT L jack to OUT L (on PCB) - coax 8in.

OUT R jack to OUT R (on PCB) - coax 8in.

the wires cut

the PCB end of the coax

soldered into the PCB


The Input and Output Jacks:

The N112B RATE Jack:

The N114B RATE Jack:

first the jack's switches get soldered together


a jumper goes between the middle contacts


All four are 50K - OSC. PERIOD is log taper; the others are linear:

Installing the CD4007 CMOS Transistor Array

A bit of a tricky business - very little space. We had to wrestle with the heat sink a bit so we could get the IC into place:

After installation, we tested the contacts to make sure all was well.

Screwing down the heat sinks

Installing the remaining ICs

again - we watched for their orientation (we made a mistake then fixed it - hense the slightly scuffed IC in the lower right corner)

Installing the LDRs

So - we put together all this stuff first so we'd be installing the LDRs last thing! By putting a bulb in one of the sockets, we determined that the bottom of the LDR should be 1" from the PCB:

We cut little 1in. pieces of heat-shrink to slide over the LDRs' leads. The heat shrink will space the LDRs accurately, serve as insulation, and also lend some structural integrity:

All shrunk up:

Mounting the PCB

Here's the hardware - 1/2" 6-32 screws, nuts, 1/4" spacer:

Ready to screw down:

Panel Connections

First - the stuff that's already on their wires:

Then the BYPASS switch - the NKK momentary switch in parallel with a N112A pedal jack:

The LED:

the anode goes to the right side of the plug; we'll use a red wire -
the cathode goes to the left; we'll use a black wire


we inserted the LED into the panel


heat shrink (shrunk?)


Finally, the Panel Mount E10 socket:

Finishing Up


The Lamps:



Set up / Testing

Use Notes


Bill and Will's Synth Main Page
Send eMail to Dragonfly Alley

The fine Print:
Use this site at your own risk.
We are self-proclaimed idiots and any use of this site and any materials presented herein should be taken with a grain of Kosher salt. If the info is useful - more's the better.  Bill and Will

© 2005-2011 all frilling rights reserved